26-Variable Volume and Temperature Systems (TDP-704).pdf

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Variable

Volume

and

Temperature

COMMERCIAL HVAC

SYSTEMS

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Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications.

Although TDP topics have been developed as stand-alone modules, there are logical group-ings of topics. The modules within each group begin at an introductory level and progress to ad-vanced levels. The breadth of this offering allows for customization into a complete HVAC cur-riculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

VVT is an economical, all-air zoned system that is ideal for many commercial jobs, espe-cially at a time when there is so much design emphasis being placed on high-quality air treatment, outdoor air ventilation, and room air circulation. VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the me-chanical contractor. Many manufacturers offer VVT-type systems. These systems are highly de-pendent on the control hardware and software used. This TDP uses the Carrier VVT system for all examples. The objective of this module is to define VVT, identify applications, compare it to alternative systems, and describe how it achieves zone temperature control.

The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing sys-tems. Judgment is required for application of this information to specific installations and design applications. Carrier is not re-sponsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design.

The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmit-ted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation.

Printed in Syracuse, NY CARRIER CORPORATION Carrier Parkway

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Introduction

VVT (variable volume and temperature) is an economical, all-air zoned system that is ideal for many commercial jobs, especially at a time when there is so much design emphasis being placed on high quality air treatment, outdoor air ventilation, and room air circulation. When a single heating/cooling unit is used, VVT works well for systems up to about 25 tons of total cool-ing capacity. Multiple systems make its application practical for much larger jobs.

This module defines VVT and describes how it achieves zone temperature control. Applica-tions for the system will be identified and VVT will be compared with alternative systems. Since the operation of the VVT system is under the direction of a complete, factory-packaged DDC (direct digital control) control system, various pre-programmed, operational sequences will be described so that the way it works will be clear. Guidelines for VVT system design are given so that the designer may focus on some of the unique aspects of the system.

Air conditioning design is all about solving building comfort needs to satisfy the occupants of that building. One of the buildings we will use to illustrate zoning and the use of VVT is this manufacturing office, which is a 60’ x 100’ single-story commercial construction attached to a small, air-conditioned

elec-tronics manufacturing and assembly factory. This is an owner-occupied office with relatively permanent partition arrangement and an expectation for a rea-sonably good level of com-fort. Occupants will be ex-posed to the indoor envi-ronment for long periods of time, so their comfort ex-pectation will tend to be high. In addition, they are sedentary, for the most part, which increases their sensitivity to variations in temperature, air distribu-tion and air stratificadistribu-tion.

Figure 1

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Commercial HVAC Systems 2

Figure 2

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Commercial HVAC Systems

3

The task of the air-conditioning system is to maintain comfort in the building by simultane-ously controlling space temperature, humidity, air motion, air purity, air quality, and mean radiant temperature. In our case, the system will be a VVT system. The layout shown in Figure 2 in-cludes many details for a real design. In a sense, this is your “map” for the information that is coming. It will help you to focus on the area of the system being addressed in virtually every por-tion of this module.

Take a few minutes to look the layout over, reading the designer’s system comments, which describe the VVT system designed for this job.

For this VVT system, a single heating/cooling, constant volume packaged rooftop unit pro-vides central heating or cooling capacity to the VVT boxes. Each box modulates its volume con-trol damper in response to the zone thermostat or sensor. Air not used by the zones is bypassed into the return air ceiling plenum. Thus, the zone airflow is variable but the rooftop airflow is relatively constant. This permits the use of standard constant volume equipment. Each box has a user-defined minimum cfm setting to ensure adequate room air circulation and outdoor air venti-lation in the zone regardless of zone load reduction. Typical minimum airflow settings vary from about 10 to 30 percent of design flow and are subject to local codes.

The VVT system is designed to provide all cooling capacity centrally and as much central heating as possible. When all zones require some degree of cooling, the unit remains in the cool-ing mode. When all zones require some degree of heatcool-ing, the unit remains in the heatcool-ing mode. However, when both heating and cooling loads occur at the same time, it becomes a time-share system. That is, its electronic controls determine the greatest need (heating or cooling) and they first satisfy that mode centrally. Then, once satisfied, it switches over to the opposite mode. The system can continue switching over from central cooling to central heating, back and forth, to satisfy all zones; thus, the concept of capacity time sharing.

Because zone 7 (interior zone) requires year-round cooling whenever occupied and lighted, the unit will need to remain in the cooling mode during most of its occupied cycle. Therefore, all perimeter zone damper units are equipped with a hot water supplementary heater. Electric heaters may be used instead. The supplementary heaters will pick up any zone heating load during the occupied cycle of operation if the central unit is in the cooling mode. The supplementary heaters will be off if the central unit is in the heating mode. The supplementary heaters are deactivated during the unoccupied cycle in both the heating and cooling modes. If a separate system is in-stalled in the zone with an unusual load pattern (zone 7), the energy efficiency of the system will be enhanced at the expense of a more costly installation.

Linear slot diffusers are used to keep cold primary air up on the ceiling at the reduced airflow occurring at partial cooling load. Conventional concentric, perforated, or curved-blade diffusers will create dumping of cold supply air on the occupants, causing poor room air mixing and tem-perature sensing, with the associated customer complaints. Director linear diffusers are used around the perimeter to enhance overhead heating. They contain a heat-sensitive element to change the direction of air diffusion to one-way when warm air is being delivered. That way, warm supply air washes the outside wall, as it should. Conventional, low-velocity, low-pressure sheet metal ductwork is used. It has a 1-in. duct wrap. Pre-insulated flex duct is used for limited lengths to make diffuser connections. All diffuser runouts include a round butterfly balancing damper. Observe local code limitations on flex duct use. The VVT boxes are sized to match the ductwork for ease of installation and fewest fittings.

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Commercial HVAC Systems 4

The building occupants have comfort needs that the system is designed to solve. The system components provide heat transfer, filtration, ventilation, and air circulation capacity necessary to control the comfort conditions, like air temperature, humidity, cleanliness and distribution in the building spaces. In this

module, we will refer to the central equipment as the packaged air handler; air source; HVAC equip-ment; packaged unit, or rooftop unit.

Even though VVT systems typically use packaged rooftop units for their central air source and heating/cooling capacity, VVT can also be applied to a split system with a packaged air handler. The VPAC (vertical packaged air conditioner) is another good air source for VVT, since it tends to be applied

floor-by-floor for renovating existing buildings, where some zoning would be welcome. In es-sence, the VPAC is the indoor version of the rooftop unit, since it is a self-contained packaged air handler with all refrigeration cycle components included in one factory-assembled package. The only thing needed for the VPAC is a cooling tower to reject heat from the water leaving the wa-ter-cooled condenser at each unit. Air-cooled versions are also available, which reject condenser heat locally, through a wall, window, or by using a remote air-cooled condenser.

The VVT System

VVT stands for variable volume and temperature. VVT is provided with a complete factory-packaged control system designed to provide multiple zones of temperature control using a low cost, single zone, constant volume heating and cooling packaged rooftop unit, VPAC, or split sys-tem. Packaged rooftop units (RTUs) are most often used.

In the past, some manufacturers marketed a dump-box zone terminal that sent supply air that was not needed at the zone to the ceiling plenum return space. Systems using this kind of terminal were called VAV bypass systems. Carrier developed VVT, which uses a bypass concept, but does it at the air handler rather than at the space. It incorporates a complete, factory-designed DDC control system for the entire system instead of merely using dump-box terminals. Today VVT can be applied to air systems using either a ceiling return air plenum or a ducted return.

Figure 3

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VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. In addition to the central RTU, indoor package unit, or split sys-tem, the VVT components include the air source unit controller, bypass syssys-tem, zone dampers, zone and bypass controllers, space sensors, and necessary safeties to protect the system. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the mechanical contractor.

RTU EC ASUC

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ASUC = Air Source Unit Controller RTU = Roof Top Unit SPP = Static Pressure Pickup

BC Bypass Controller ZD

=

Zone Damper SW

=

Sensor Wiring- Twisted, Shielded Pair BO Bypass Damper SD = Supply Diffuser ZC = Zone Controller

CB Communication Bus - 3 Conductor Shielded RG = Return Grille ZS

=

Zone Sensor EC Economizer Controller SP = System Pilot ZH = Zone Heater LC Linkage Coordinator

PIZC = Pressure - Independent Zone Controller Figure 4

VVT System Schematic

When only heating or cooling capacity is needed, the system de-livers the appropriate amount of air to the zones. Using a time-sharing principle, when some zones need heat but others need cool-ing, the central HVAC unit alternates between providing central cool

-ing and central heat-ing. It satisfies the mode (heating or cooling) with the greatest need then switches to the opposite mode to satisfy the other zones. It does not provide a central source of cooling and heat-ing capacity simultaneously.

The complete, pre-engineered control system, comprised of microprocessor-based controls, electronic zone dampers and a bypass system allows the VVT system to deliver the appropriate temperature and quantity of air to the zone from a central heating and cooling source.

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VVT

is

Variable Volume

VVT is called variable volume because it delivers a variable volume of either cold or hot supply air to each zone as load dictates. However, it is important to remember that VVT uses a constant volume packaged unit that must maintain a relatively constant airflow at all times. The VVT system can track the load around the building, maintaining comfort and maintaining effi -ciency. Modulating zone dampers, with factory or field-installed zone controllers, are used to ad -just the volume of air delivered to each zone. Thus, the airflow sent to the individual zones varies over time to meet the changing loads in the zones caused by differences in solar exposure, usage or occupancy, and lighting patterns.

In addition, a bypass system is employed so that supply air, which is dampered down at the zones, is mixed with return air to keep the air volume entering the packaged air handler relatively constant.

VVT

is

Variable

Temperature

VVT is called variable temperature because the temperature of the air supplied by the central unit varies with time. Each zone gets the same temperature air at any point in time, but the tem-perature of supply air varies over time. In the heating or cooling mode, as supply air mixes with return air, the temperature entering the packaged unit

varies, causing the discharge air temperature to vary. Since most constant volume units have limited steps of capacity, the length of time the compressor is on will vary depending on the number of zones and the volume of airflow required for cooling. The same is true on heating, since limited stages of gas heat or limited steps of electric heat are provided. Therefore, supply air tem-peratures can vary widely during light loading condi-tions.

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What is Zoning?

When the design of comfort air-conditioning systems for commercial buildings is considered, the issue of temperature control zoning, or simply zoning, is sure to come up. A control zone is a whole building, a group of rooms, a single room or part of a room controlled by its own thermo-stat or temperature sensor (tstat/sensor). A zoned building is one that has more than one tstat/sensor maintaining its temperature. A zoned system has more than one tstat/sensor control-ling the areas it serves.

Zoned

Cooling Medium

System Basic System Types

Category Delivered to Zones

VVT (Variable Volume and Temperature) VAV {Variable Air Volume)

All-Air Air Multi-Zone

Double-Duct Terminal Reheat All-Water Water Chilled Water Fan Coil

Unit Ventilators

Direct Duct-free split systems

Refrigerant Refrigerant (Also called ductless, multisplit, multiplex, and cassette)

Air-Water Air and Water Conduit Induction

Supplementary Air Fan Coil Figure 5

Zoned systems have a central source of chilled air (all-air systems), chilled wa -ter (all-water systems), or chilled refrigerant (direct refrigerant systems), distrib -uted to several zones, each with its own tstat/sensor. Air-water systems distribute both chilled air and chilled water from a central source to the zones to do the cooling. VVT is an all-air zoned system. Other all-air systems include VAY (variable air volume), multizone, double-duct, and terminal reheat. Please refer

Zoned System Categories

to TDP-103, Concepts of Air Conditioning for a complete discussion on these systems.

Zoning is important in maintaining comfort conditions in air-conditioned buildings. However, rather than using a zoned system, like VVT, many commercial buildings are zoned using multiple constant volume, single zone systems, with a rooftop unit serving each zone. Tight construction budgets are the primary reason for this trend.

Inadequate to assure individual comfort!

Figure 6

Problems with Constant Volume, Single Zone Systems

In such cases, the packaged roof-top unit will control the space it serves based on the input it receives from one tstat/sensor located somewhere in the zone. Typically, comfort conditions will prevail in the space where the tstat/sensor is located. However, the remaining areas of the zone may be too hot or cold, resulting in occupant di s-comfort.

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To overcome uncomfortable conditions, VVT provides additional control zones at minimal

additional cost. A tstat/sensor and VVT damper unit added to each room will help to provide

comfort to all occupants. This allows the same kind of constant volume, single zone equipment to function on a zoned system without the additional equipment cost normally associated with a zoned system air handler.

Although zoning helps to ensure comfort, it does not come without cost. There is a tradeoff in zoning be-tween customer comfort and installed

system cost. More zones provide im-proved comfort for an additional cost. It is important to evaluate the value of

comfort that is added by zoning and the price the customer is willing to pay for it. There is usually a middle ground somewhere between inade-quate zoning and ultimate zoning that is acceptable.

Types

of

VVT

Jobs

With WT, a single-zone heating/cooling HVAC unit supplies a zoned system.

Typical Zone Sensor

Figure 7

rl. zoned system, like VVT, eliminates problems.

Traditionally, VVT has been used on rooftops and split system units under 30 tons capacity. The system is designed to provide a zoning solution in an equipment size range where other zon-ing options just are not nonnally available. While the VVT system theoretically could be used on

larger units, a number of issues in their application should be considered. Items like bypass size and control, or having large numbers of zones, some of which may have unique requirements, will create significant design issues. Larger systems are better done with VA V units or by divid-ing the space to use multiple smaller tonnage air source units usdivid-ing VVT. The system may also be applied on small units under 5 ton, but the cost is not normally justified. The most common size range for VVT systems is 71/z to 20 tons.

The Carrier VVT system can have up to 32 control zones on one air source unit. However, in most applications the number of zones is far smaller. It could be as little as one zone for a spe-cialized zone on a larger non-VVT system, but most often it is 5 to 16 zones on each air source unit. Systems below five zones are not usually economically feasible and multiple air source units may be a better solution. Having a large number of zones can also cause problems since it most likely indicates that a wide range of load conditions must be met with one constant volume unit. This approach greatly increases the chance that one zone will experience excessive temperature fluctuations. Having 6 to 12 zones is usually a good arrangement.

Jobs at 25 Tons or Less

The vast majority of VVT applications are designed with HVAC equipment smaller than 25 tons with 3 to 15 ton systems being the most common. When the total installed tonnage for the building does not exceed 25 tons, one packaged rooftop unit is the most common choice.

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Jobs Larger than 25 Tons

Since VVT is a time-sharing system, it is best to begin by dividing the building into large ar-eas that have similar load characteristics, with a VVT packaged unit serving each. Then zone

E each area served by a packaged unit

with VVT damper units, one per zone. In this way, buildings requiring over 400 tons of cooling capacity have been successfully air conditioned

us-I

0

Rental: Area 0

0::: ing VVT.

For instance, in this small strip shopping mall, the 95' x 155' rental area on the north side should be con-ditioned by two rooftops, each with around 25 tons cooling capacity, be-cause the area has a peak cooling load of around 48 tons. The present stock room could use a dedicated rooftop

unit; but, since this is rental property Figure 8

+

N 95'

1

90'

1

~ 0 _RTU1 _: 0 ... _{(25 tons>;} (/) 155' RTU3 (25 tons)

and future tenants may not continue Divide large spaces into HVAC unit areas. using this floor space as a stock room,

RTU2 (25 tons) RTU4 (25 tons)

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two equally sized rooftops (RTUl and RTU2) will standardize the design and provide future flexibility for tenant rearrangement. On the south, each rental space will require around 25 tons total cooling capacity, so the pattern set on the north side is continued for the south in choosing RTU3 and RTU4.

On larger buildings, when you are striving to use systems 25 tons and under, break the building down into smaller areas by providing separate

HVAC units for:

• Core versus perimeter areas

• East versus west perimeter areas with large glass exposure

Areas of different occupancy density and patterns

North versus south perimeter with large glass areas

• Top floor (with roof) versus in- Figure 9

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termediate floor Now zone each area using VVT.

• Areas occupied by different ten-ants

• Separate large lighting zones

• Areas with special exhaust requirements

• Areas with special ventilation or other air quality standards

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Even though VVT can handle a diversity of load patterns, the designer's goal for a larger building is to break it down into areas 25 tons or less, which stabilize VVT system operation by

minimizing load differences within each system.

Once the building has been divided into areas that can be conditioned by rooftops, indoor

package, or split systems; then zone each air source area by using VVT zone damper units. For

instance, the south store, supplied by RTU3, was

di-vided into south and west perimeter zones to track so -lar variances on those exposures. The display area along the hallway is a separate zone because of its in-tense lighting load. The core area is divided into 2 zones but could be handled by one. This layout re-quires 5 zones, with a VVT damper unit in the supply

ductwork feeding each zone. Similar zoning work has been done in the north rental area.

Retrofitting Existing Systems with VVT

The retrofit of existing HV AC systems with VVT system components has grown in popular-ity. Generic system compatibility of DDC controls has improved so that the application of VVT

upgrade components has become easier than in times past.

For instance, the southeast corner of the strip shopping mall was laid out on a modular basis for the smallest

rentable retail area, which is 70 ft x

40 ft (2800 sq ft). A 12Yz-ton constant

volume, single zone heating/ cooling

rooftop unit was installed to satisfy each zone. But, due to perimeter ver-sus core load differences, as well as

varying lighting and usage patterns, the new renter of the southeast mod-ule, now a separate store, wants to add

zoning. This is no problem with the flexibility ofVVT.

As long as the existing unit is deemed reusable by the engineer, owner or contractor, and is capable of

deliver-ing required airflows and capacity to the newly created zones, VVT retrofit

is an excellent approach.

70'

•Typical 12-% ton PAC-5000 cfin

• Main ducts line sheet metal

•Branch supply ducts insulated flex (16' round)

• Supply diffusers= round ceiling (16")

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• Return glilles not shown

• Ceiling cavity used as return air ptenum

Figure 10

Southeast Corner, as Originally Designed

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Specify manual, locking balancing damper in each supply branch (typical)

Specify rectangular ducts with 1-in. acoustical

insulation RA

Straight flex runs Flex maximum positive and

negative static pressure = 2.0 in.wg

Figure 11

Southeast Corner, Constant Volume, Single Zone System

5-piece metal elbow @diffuser (insulate outside)

All rectangular ducts , _ _ _ _ _ __.~galvanized sheet metal,

properly reinforced and supported

Typical 16-in. round ceiling diffuser

Conical sheet metal takeoff for runout connection and installation of balancing damper

The existing lower pressure class ductwork can be reused for the new VVT layout wherever its size and configuration is correct. In this case, the designer and installer of the original single zone system made the conversion to VVT easy by following good 2-in. pressure class wg duct design practices.

To change the system over to VVT, a VVT damper with a zone controller is added to each runout duct. Even though less than 6 zones would work pretty well, the overall cost to modify the duct layout would more than offset any VVT terminal savings. Round 16-in. damper units easily install in each existing branch (runout) duct. Branch balancing dampers are removed, since the VVT damper unit is now in control of supply air quantity to each diffus~r, and there is only one diffuser fed by each damper unit.

The supply diffusers are replaced because cone-type diffusers are one of the worst culprits for dumping cold, primary air at reduced airflows. VA V-qualified linear slot diffusers are best but would require radical room air distribution redesign. Instead, high quality, square, multidirec-tional, anodized aluminum ( 4-way) louver diffusers are chosen. The louvers are adjustable. Even though these will not hold the cold air up on the ceiling at partial load as well as VA V-qualified linear slots, they should be adequate in this application due to the consistent high lighting load and the level of activity of the people, who are up on their feet, moving around.

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Where a ceiling plenum return is used, a bypass damper with bypass controller must be added

in the main supply duct, as in this example. When a ducted return system is used, a bypass

damper must be inserted between the existing unit main supply duct and the return main. Be sure

to follow the manufacturer's bypass design recommendations. It is a key element in the stability

of VVT system operation. The bypass damper assembly with bypass controller, includes a static pressure sensor. So a static pressure pickup (SPP) is installed in the supply duct just upstream of

the first branch duct. Tubing sends the static pressure back to the sensor at the bypass controller. 24" x 24" adjustable ~---iTypical Zone Sensor

louver diffuser (wall mount)

(typical)

System Pilot

Typical wiring between zone sensor and damper

Figure 12

Southeast Corner, Retrofitted for VVT System

Communication bus

e > -- ----1!-fi 20 AWG 3-wire shielded

'•···•••••••H····

rT'",._...-ri Typical zone sensor

(ceiling mount)

Bypass damper assembly and static pressure pick-up

with tubing

The majority of VVT projects use new HVAC units and ductwork. This happens when the

age of the system being replaced is more than about 10 years and also when the changes to the

existing system are too extensive to reuse the existing air system. Caution should be exercised to_ avoid compromising zoning when reusing existing ductwork. Control zone location should be based on building zoning needs and not solely on existing duct runs. It will cost more in the end

to fix a zoning problem than to do it right in the first place.

An air source unit controller is installed in the rooftop unit and communication bus wiring in-terconnects all the controllers. Each zone controller is connected to its temperature sensor by sen-sor w1nng.

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Figure 13

Rooftop Retrofitted with VVT Air Source Controller

The VVT zone controllers and

by-pass controller will communicate with other brands of existing heating/cooling equipment when an appropriate field-installed air source unit controller is

installed. Carrier's PremierLink

con-troller will satisfy this need, as shown

in this rooftop VVT retrofit.

Existing HVAC units and

associ-ated ductwork retrofitted with VVT

dampers, controllers, and room tstats/

sensors can be an effective and

afford-able upgrade in comfort for existing systems where only one zone currently

exists or for applications in which

oc-cupant comfort is inadequate due to the

original system zoning design.

VVT

versus

Other Systems

One of the most important jobs of the HVAC designer is to pick the right system for the

building. Unfortunately, the right system sometimes remains unknown until another system has

been installed and is performing inadequately.

In many areas of the country, air-conditioning systems must deliver as much satisfaction in

the heating mode as in the cooling mode.

Consequently, systems are well

ac-cepted that can deliver heating and

cool-ing capacity to any zone, on demand, like

VVT does when it is applied correctly.

This is also one of the strengths of PT AC

(packaged terminal air conditioner)

sys-tems; standard (non-VVT) water source

heat pump systems; duct-free split

sys-tems; and chilled water fan coil systems.

VVT cannot deliver heating or

cool-ing simultaneously from the central

source to any zone, on demand, when the

central HVAC unit is in the heating

mode, because it relies on a central

source for cooling capacity instead of

producing it at the zones, as these other

systems do.

Commercial HVAC Systems

• Multiple rooftops, indoor package, or

split systems

• PTAC (Packaged Terminal Air Conditioners)

• WSHP (Water Source Heat Pumps)

• Duct-free split systems

-• Room fan coils (chilled water)

• VAV (Variable Air Volume)

Figure 14

Zoning System Alternatives to VVT

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VVT

Advantages

However, VVT offers many advantages over these other systems.

PTAC, standard water source heat pumps, duct-free splits, or chilled water room fan coils are typically used when small "repetitive spaces" are required or for room fan coils when a central source of cooling is preferred.

These alternatives to VVT have difficulty meeting good indoor air quality standards de-manded by ASHRAE Standard 62. Considering the way these systems attempt to meet the filtra-tion and ventilation needs of the building, it makes sense that an all-air system, like VVT, does a far better job.

For instance, if chilled water fan coils, water source heat pumps, most duct-free split systems, or PTACs are used for zoning, then the ventilation system must be addressed. Unlike VVT, a

dedicated ventilation system, at substantial added expense, will usually be required for these other systems in order to provide ASHRAE Standard 62 verifiable quantities of properly filtered and dehumidified outdoor air.

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Plenum 6-ton fan coil

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• Poor filtration

Typical rectangular ceiling diffuser

• No controllable positive ventilation

-

60'

0 a

@

• Dedicated ventilation system required to meet ASHRAE 62

Figure 15

Chilled Water Fan Coil or PTrlC

Typical chilled water fan coil

orPTAC

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• Poor filtration

• No ventilation - perimeter • Poor ventilation - interior

Typical In-Ceiling duct-free fan coil. Horizontal discharge. Multi-Zone split system.

@

Typical duct-free fan coil, high wall

mount

60'

• Dedicated ventilation system required to meet ASH RAE 62

Figure 16

Duct-Free DX Split System

While PTAC units offer optional ventilation from the outside, the maximum rated capacity is about 35 cfm and the fan compartment is not sealed from the space. This means that outdoor air can infiltrate directly into the space without being filtered or treated by the unit at all. When in-door pressure exceeds outdoor pressure, the indoor air will exfiltrate out the ventilation opening and ventilation ceases altogether. So the direction and quantity of air moving through the unit's

ventilation opening is not verifiable, is highly variable, may bypass the filter and coils, and is sub

-ject to wind direction, speed, and building height.

These same problems exist when using floor-mounted or ceiling-mounted chilled water fan coils located around the building perimeter with an optional outdoor air wall sleeve and damper.

INSIDE OUTSIDE

Figure 17

Ventilation - PTAC (left) and Room Fan Coil (right)

INSIDE

(f):::::::::::::::::::::i''''\ '''l Floor mount with optional

OA wall sleeve

Fan compartment : ... not sealed from s ace WaterSu I

._Water Return

OUTSIDE

<1fl6@+1

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In contrast, an all-air system, like VVT, will provide good quality, verifiable ventilation to

replenish oxygen and dilute pollutants in the space. In most cases, it can also provide betkr air

filtration than these other systems, since the filters on a central packaged unit are more efficient.

However, once a dedicated ventilation system is added to the other systems, the filtration can be

excellent on those systems, but at substantial added cost.

An all-air system, like VVT, can use an outdoor air economizer to provide free or

reduced-cost cooling when off-peak outdoor air conditions are acceptable.

Since the zones hit their peak cooling load at different times, the capacity of the VVT unit

need only be as large as the coincident peak or "block" load of the area served; that is, less than

the sum of the zone peak cooling loads. By contrast, the installed tonnage of PTAC and water

source heat pumps is greater than VVT. It is the sum of the zone peak loads. A load estimating

program, like Carrier's E20-II Block Load or HAP (Hourly Analysis Program) will show the

dif-ference.

VA V System Comparisons

There are several VA V systems that offer comparable zoning to VVT. However, when

multi-ple VVT systems are used to condition a building, VVT often competes with VA V because both

are all-air systems providing similar performance benefits in filtration, ventilation, air

distribu-tion, aesthetics, and quiet operation in the conditioned space. VA V's traditional advantage is pre-cise, small zone temperature control

when heating and cooling demands

WT

occur simultaneously. Both systems . Lower installed cost

provide the energy-saving benefits of · No ATC contractor

an outdoor air economizer and block

load diversity through the use of a

single HVAC unit to condition all the

zones it serves.

• :S. 2-in. wg duct design • WT-qualified dampers and

diffusers

• Standard HVAC machine • Retrofrt easy

VVT has several benefits over · No fan control • Few power connections • Minimal fan energy savings • Poorer part load latent

VA V. The most significant advantage is its lower installed cost in smaller

tonnage sizes. When VA V is

de-signed using temperature controls

supplied by a manufacturer other than Figure 18

the HVAC equipment manufacturer, VVTversus VAV

an ATC (automatic temperature

con-VAV

• Higher installed cost

• A TC contractor usually required • ~ 3-in. wg duct design

• VAV terminals and diffusers • VAV HVAC machine • Retrofit more difficult • Fan control

• Many power connections with fan-powered mixing boxes • Maximum fan energy savings • Better part load latent

trol) contractor will usually be required. When VA Vis designed using on-board, factory-installed

controls, it is more likely that the need for an ATC contractor may be minimized or even elimi

-nated, if the installing HV AC contractor is able to install and configure the control system

com-pletely. Eliminating the ATC contractor means single-source responsibility for the installing

con-tractor, which streamlines job scheduling and usually reduces installed cost.

VVT on the other hand, always comes as a factory-integrated system of product and DDC

controls. The configuration is easier and has less variation than for a VA V system, so it is more

likely that the cost and scheduling inconveniences of the separate ATC contract can be

elimi-nated.

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Turn to the Experts·

(21)

Standard, lower pressure class ductwork (~ 2-in. wg) is used on VVT. This is not true on

VA V. Since VA V uses no bypass, the supply air system pressure can increase at partial cooling load. This requires a more expensive schedule of reinforcement for rectangular supply ductwork used on main ducts. Spiral round or flat oval mains are an alternative. Duct joints must be tightly

sealed to avoid objectionable noise and leakage. Generally speaking, the design and installation of VVT supply duct systt:ms is easier, without specialized methods or materials being required.

The return air system design is about the same for VVT and VA V systems. The supply terminals for VA V

systems are designed with acoustical

attenuation in mind. When the VA V flow control device is located at the diffuser, it is called an integral dif-fuser terminal. This is a sophisticated diffuser. When the flow control

de-vice is in a tt:rminal remote from the diffusers, like a fan-powered mixing box or a single-duct damper box, the box includes the necessary leakage prevention and acoustical treatment.

Unlike VA V, VVT does not throt- Fan-Powered VAV Mixing Box

tle the fan airflow at partial loads, but Figure 19

rather uses a bypass damper that

lim-its duct pressure so there is limited VAV supply terminals are more complex.

potential for a buildup of static

pres-Integral Diffuser VAV Terminal

sure in the VVT ductwork. A low pressure VVT damper and diffuser can be selected using low pressure ductwork and less specialized diffusers. However, the diffuser style must avoid dumping

cold supply air at reduced airflow in order to avoid discomfort and inaccurate zone temperature sensing. Qualified linear slot diffusers work best. Cone-type round or square diffusers are less capable of holding the air up near the ceiling at partial load during the cooling season.

A significant difference between VA V and VVT systems is the central HVAC equipment. VVT is able to use a standard, constant volume, single zone unit because the air volume never goes below the manufacturer's minimum recommended limit. On the other hand, VA V requires a

variable volume qualified unit, which is substantially more expensive than a

standard unit because it must have fan control (typically a VFD), controls to maintain discharge air temperature,

and more steps of part load capacity. Therefore, VVT systems can utilize

existing constant volume air sources, while VA V systems cannot. The VA V retrofit requires such extensive modi-fications to the central machinery that it is usually impractical to attempt the change. The central equipment is usu-ally replaced when going to VA V.

Figure 20

V AV air handlers have added components.

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Without VA V modifications to the central unit, if applied on a VA V system, the central unit can experience unstable operating conditions and frosting of the evaporator coil, causing nuisance tripouts on equipment safety limits as well as liquid refrigerant floodback, which will hann or destroy the compressor. In addition, the VA V fan will require a VFD (variable frequency drive), inlet guide vanes, or other static pressure control device to optimize fan energy savings and limit static pressure buildup at pa1tial cooling loads. VVT systems require no fan control device since it uses a bypass damper.

When comparing VVT with a VA V system that uses fan-powered mixing boxes, VVT re-quires less line voltage electrical power wiring and fewer points of connection. The VVT electri-cal distribution system will cost less to design and install. Each VA V fan-powered mixing box has a small electric motor that requires line voltage electrical power wiring for each zone. VVT damper units require only 24 volts for the damper motor. Both systems require some type of con-trol connection at each zone. Fan-powered mixing boxes may be used for VVT. When this is done, the electrical distribution cost benefit over VA V no longer exists.

The same electrical distribution system cost savings exists when comparing VVT to PTAC, water source heat pump, duct-free split, and chilled water room fan coil systems, since each of these systems have a line voltage motor in each zone. The PTAC also contains the compressor, so it has the greatest requirement for electrical power distribution to each zone of all systems con-sidered so far. However, the PTAC unit includes controls, so there is no additional cost for con -trol wiring as on the rest of the systems.

Another main benefit VA V has over VVT is its part load energy efficiency. This comes from fan energy savings at par-tial load and no mixing of hot and cold airstreams with the by-pass, which is something VVT cannot achieve. Generally speaking, the bigger the job, the higher the monthly power bill and the more important fan energy becomes to the owner.

VVT versus

Multiple Units

VVT is a cost-effective solution to zoning, especially when applied on commercial applications that require 5 or more zones. An alternative, against which VVT is measured, is zoning the building using constant volume, si n-gle zone equipment, one per zone, typically rooftops. Multiple packaged units used in this way are typically more cost-effective when providing 4 or less zones of control in the build-mg.

WT is the right choice when:

Above four zones, VVT reduces installation cost by minimizing the number of HV AC units required and

OR

Figure 21

• 5 or more zones may be necessary • Centralizing and reducing

unit maintenance is desirable

- Filter changes

- Belt adjustments - Economizer adjustments

- Yearly maintenance costs

• Reducing installed cost by minimizing:

- HVAC units

- Power supplies and disconnects

- Duct system

- RiQging, piping and wiring - Roof penetrations

- Roof curbs

VVT versus Multiple Constant Volume, Single Zone Units

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(23)

power connections, electrical disconnects, control wiring, piping, and ducting.

VVT also keeps the heating and cooling equipment maintenance centralized and at a point outside the occupied spaces of the building. This means that the number of maintenance points is reduced and that scheduling of maintenance or repair does not disrupt the occupants. Both of these features are attractive to customers who are thinking in terms of the life cycle cost of the system and the indoor environmental quality of the building spaces.

Another advantage of VVT is in applications where multiple, small zones of control are re-quired and separate units are not available in sizes small enough for each zone. Combining the small zones creates a total capacity requirement in a size range that can be met by a single central unit with VVT control.

Zoning the Building for VVT

In defining zones, one should be logical and prudent in their selection since increasing the number of zones adds to the installed system cost. Hence, the designer should be careful and not give in to the urge to give

every room a sensor.

Building zones are typi

-cally required because of differences in the following

factors from one area of the building to another:

• Space usage

• Glass exposure (ft2 and %wall area)

• Glass orientation (e.g. N, E, S, W, etc.)

• Occupant schedule • People density

• Lighting control zone

• Lighting level

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Zone Number Figure 22 © 60' ®

• Perimeter versus core

exposure 60 'x 100 _Zone_Numbers'A1anufacturing Office Floor Plan with Partitions, Dimensions, and

• Roof exposure or none

• Occupant responsibility (authority in building management)

• Wall exposure (area and orientation)

• Tenant variations (schedule, preference, billing) • Special ventilation needs ./ • Special exhaust needs

• Special air cleanliness requirements

•+•

Commercial HVA C Systems ®

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(24)

As you can see, there are many factors that could drive the designer to make an area a sepa -rate zone of control. So, to simplify the zoning decision, the designer must identify the

differ-ences that exist and prioritize the importance of them.

As zoning decisions are made by the designer, the floor plan is an indispensable tool. The

zone boundaries should be drawn on the floor plan as decisions are made.

In order to economize on system cost, similar rooms should be grouped together to form a single control zone.

Group together areas with a similar solar exposure. First, separate the core from the perimeter

areas. Then separate perimeter areas based on orientation. Along the perimeter of the building,

Percents of zone room sensible heat Each mark = 1 %

solar load usually constitutes about 45

percent of the peak room sensible cooling

load for a standard, single-story office.

Since this load is most significant, it is

important to group exposures with

differ-ent peak times separately. Also be sensi

-tive to shadow lines along a single expo

-sure, which are cast by neighboring

buildings, covered walkways, or trees. A

shadow makes east, south or

west-exposed glass and walls perform as if it were facing north.

Figure 23 The manufacturing office demo

n-Typical Perimeter Zone Sensible Cooling Load Components strates typical peak cooling load times for various exposures. Coupled with the large impact of solar load, the peak times show why the east, southeast comer, south, southwest comer, and west office spaces were placed on separate control zones. An additional consideration

was that the southeast and southwest comer offices an~ occupied by company management

per-sonnel and that the building is owner-occupied, with a higher expectation for comfort and a

greater willingness to pay for it than would be the case for a speculative (rental) building of the same design.

Transmission, the

con-ductive heat transfer across

all external barriers caused

by the difference in tempera

-ture from outside to inside,

is significant for perimeter zones. However, it is not a consideration in zoning be -cause the outside air tern -perature and inside air t em-perature are uniform all around the building. Only

differences in loads and load

patterns stimulate a zoning consideration. Load calcula-tion methods often combine

·---- - - -1 1 0 0 ' -Engineering ul, 4 p.m. 6 people M

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General Office (core) Jul, 4 p.m. 30 people

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Sep, 3 p.m.

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Figure 24

Zones, Owner-Occupied, Fixed Partitions

Conference Room Jul, 9a.m. 12 people

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Manager Sep, 10 a.m. 2 people @ 60'

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(25)

the effects of solar on walls and roofs with the transmission. Wall and roof solar loads are impor-tant in zoning decisions since the mass and exposure will impact the load.

The perimeter zone pie chart (shown in Fig. 23) also shows that lights and equipment (com-puter) are a significant part of the zone load picture. It constitutes about 25 percent of the sensible cooling load in sunlit perimeter spaces. If a person in one office turns off the lights and computer and leaves, but the person in the adjacent office on the same exposure continues working, the temperature difference between the two may become unacceptable if they are in the same control zone because there is only one tstat/sensor to respond to the two circumstances. Differences greater than 3-5° Fare usually considered too uncomfortable.

In the cooling mode, the difference in dry bulb temperature between the air coming out the diffuser and the room temperature is 15-20° F, say 16.5° F (with the supply diffuser outlet perature at 58.5° F and the room

tem-perature at 75° F). This 58.5° F cold air warms to room temperature as it absorbs the sensible loads entering the room. So if lights and equipment (computer) constitute about 25 per-cent of the room sensible heat added to the space, then about 25 percent of the warming of the supply air can be attributed to lights and equipment. With a difference of 16.5° F at the supply diffuser, this amounts to about 4° F (16.5° F x 25%). If that load is present in one space in the zone but absent from the other, then under the these circumstances, the difference in temperature between the 2 spaces will be about 4° F. This 4° F difference is

Roof ZD . ,zc CommunTcaiion : ~ - -Bus; Balancing Damper Figure 25 T0 = 58.5° F • ~~~--....,.-~~-=<

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TRM= 79°

F

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Lights and computer on

@

Zone Temperature Difference when Zoning is Compromised

somewhat reduced by the mass of air in the room, the barriers around the room, and the time it takes to reach equilibrium, however it may still be considered unacceptable.

Based on the pie chart and the zoning work done so far, it makes sense that core areas and north-facing areas should not be plagued by variations in solar loads. Therefore, their load pat-terns should be more similar than

pe-1imeter areas. However, the pie chart for a typical core area with roof shows a potential problem with this reason-ing. Since solar and transmission loads make up less of the load in the core than in the perimeter areas, the people, lighting, and equipment loads have a more significant impact in the core than the perimeter areas. There-fore, if the occupancy, and especially the lighting patterns, are the same

throughout the core, then the core can Figure 26

be treated as one zone of control.

Percents of zone room sensible heat Each mark = 1 %

However, if floor-to-ceiling partitions Typical Core Zone Sensible Cooling Load Components (with roof)

(26)

are installed, and lighting and occupancy patterns vary within the core, then zoning will be de-manded even more in the core than in the perimeter areas of the building. If partitions and light switches are added to the core in the future, VVT is a system that has the flexibility to add zones of control as needed, with minimal system changes.

A core area on a floor without roof overhead is even more sensitive to occupancy and lighting variations because the lights, equipment and people comprise the entire room sensible cooling load. In those cases, lighting may contribute up to about 80 percent of the room sensible cooling load.

The impact of people and lights required the use of occupancy zoning in our example for zone 1, the conference room. Even though the peak time caused by exposure is identical to Office 2, the potential load variation caused by people and lights in the conference room prevents plac-ing these two areas on the same

con-trol zone. The magnitude of the load caused by occupancy and lighting can be seen from the pie chart shown.

In addition to people and lighting variances, the special, intermittent ventilation need for a conference room full of people also demands a separate zone of control. VVT offers features that will accommodate the need for increased ventilation to this

important area of the building. Figure 27

Solar

(30%)

Lights (25%)

Percents of zone room

sensible heat

Each mark = 1 %

Conference Room Sensible Cooling Load Components

Basic Sequence of Operation

VVT is demand-oriented and responds to individual zone loads (see Figure 24). The load conditions in the space control the equipment capacity through a sharing of consolidated

informa-tion and modes of operainforma-tion between system components over a communications network. This system coordination, or "linkage," is intelligence that resides at the linkage coordinator and

-within the air source controller, located at the rooftop unit. The linkage coordinator, chosen at the time of system configuration, is located at a zone damper. The linkage coordinator is responsible for operation of the VVT system and sends out one set of inputs to the air source controller that is responsible for ef1icient and reliable operation of the air source equipment. Before we discuss actual operational sequences, we need to define two important concepts: linkage and pressure dependent/pressure independent.

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Turn to the Experts-

(27)

Linkage

Linkage simply refers to the process through which data is exchanged between the unit con-trollers at the air terminals, bypass controller, and the air source controller. The process "links"

the VVT damper terminals, bypass damper and air source to form a coordinated system. Linkage

allows the air source to operate efficiently and reliably while responding to and satisfying the

changing conditions in the zones. Linkage also allows the zone terminals to respond properly to

changes in the air source, so the feedback is mutual. A linkage coordinator, located at a zone

VVT damper, coordinates the flow of data between the air

source unit controller and

VVT zone unit controllers.

The linkage coordinator is

one of the zone controllers selected when the system is

configured. It will poll the requirements of all the other

zone controllers and send the air source controller one set

of integrated requirements

representative of weighted requirements of all the zones. The air source controller can

then respond as if all the zones were one weighted

av-erage zone.

Zone Controller

Zone space temperature Occupied heat and cool set points Unoccupied heat and cool set points

Occupancy status Damper position Demand

Damper size co, Level (optional)

%RH (optional)

Bypass Controller

Air Source Controller Air source linkage table

Air source update flag Operating mode

Supply air temperature

Start bias

AS linkage tables

Coordinator zone's address & bus

Computed occupied & unoccupied heal/cool set points

RED= INPUTS BLUE

=

OUTPUTS Figure 28

Linkage - Flow chart showing the information passed back and forth between the controllers used on a VVT system.

The information exchanged between the linkage coordinator, bypass controller, and air source

controller flows both ways. As you can see, a substantial amount of information is exchanged. In fact, this electronic dialogue between the VVT unit controllers is the means by which the system is controlled in a comfortable, coordinated, energy-efficient manner.

For further information about linkage, please refer to the, Controls, Level 2: DDC Network-ing TDP.

Pressure

Dependent (PD)

versus

Pressure Independent

(PI)

The zone controller, which is used at each VVT zone damper terminal, has traditionally been a pressure dependent device. However, by choosing an optional zone controller for selected zone dampers, those zones may be made pressure independent.

The term pressure dependent means that as the static pressure in the supply duct changes, the airflow volume through a given damper opening also changes. Therefore, the zone airflow is de-pendent upon the zone's supply duct static pressure at any damper position. With a pressure de

-pendent control strategy, zone damper position is modulated to maintain a zone temperature set

point, without regard for duct static pressure. The speed of the control loop response is assumed to be quick enough that duct static pressure variances will not disrupt the space temperature in

any significant way. This usually proves sufficient for jobs that do not have a specific airflow

re-quirement.

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Pressure independent means that the airflow volume at the zone remains constant even though the supply duct static pressure changes. With a pressure independent control strategy,

zone damper position is modulated to maintain a desired airflow set point based on maintaining

comfort in the condi-tioned space. Where zone

airflow must be main-tained constantly at some

level for the sake of venti-lation or room air motion, pressure independent con

-trol is recommended.

PRESSURE DEPENDENT

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Zone Sensor I , \_ Zone

Pressure Dependent Zone Controller • Responds to a temperature variance • Controls damper position based on

zone demand

• Corrects for cfm variance caused by static pressure variance by responding to zone temperature variance

• Most typical

• Lower installed cost

• cfm varies with duct static pressure

• No extra sensors Figure 29

Zone _/ , I Zone Sensor I

Controller L ... <D _J

Pressure Independent Zone Controller

• Responds to a temperature variance

• Controls damper position based on zone demand

• Anticipates and compensates for the cfm variance caused by static pressure variance

• Least typical

• Higher installed cost

• elm constant with varying duct static pressure

• Pressure sensor required at each zone

Call for Heat/Cool

and Equipment

Mode

VVT Zone Controller - Pressure Dependent versus Pressure Independent

In order to understand how the DDC controls manage the VVT system, what follows

de-scribes one manufacturer's approach to various situations that require a change of mode.

A VVT system's mode of operation can be heating, cooling, or ventilation. The mode is de-termined based upon whether there is a call for heating capacity (zone 4), cooling capacity (zone

RG

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=

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=

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=

0.0 ZS D

=

1.5 htg

SP

=

Zone temperature set point

ZT = Actual zone temperature

D

=

Demand (htg or clg)

Figure 30

VVT Zone Capacity Demand

2) or no call at all (zone

3). The demand a zone has for capacity is de-termined automatically, once per minute on a continuing basis, by the linkage coordinator as it-gathers demand infor-mation from itself and all the other zone con-trollers. Each controller calculates its cooling or heating demand as the difference between the mode set point and the actual zone space tern-perature.

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26-Variable Volume and Temperature Systems (TDP-704).pdf

Variable

Volume

and

Temperature

COMMERCIAL HVAC

SYSTEMS

Table of Contents

Introduction

The VVT System

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