Dual Maximum VAV Box Controls

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Dual Maximum

VAV Box Controls

Steven T. Taylor, PE FASHRAE

Taylor Engineering LLC

staylor@taylor‐engineering.com

ASHRAE Golden Gate Chapter

November 13, 2014

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Agenda



VAV Box basics

• Sizing VAV Boxes

• How low can you go?



VAV Box control logic

• Conventional VAV box control logic

• “Dual Maximum” control logic

• Non-complying dual maximum control logic

• Determining setpoints



How well does “Dual Maximum” logic work?

(3)

VAV Box Sizing



Oversized box

• Less pressure drop

• Less noise

• Higher box min → more reheat, higher fan energy

• Cost more



Undersized box

• Higher pressure drop

• More noise

(4)

VAV Box Sizing: LCC Analysis



Compared ∆TP from 0.3” to 0.8”



Low rise office building



Sensitivity Analysis

• Climate

• Loads

• operating schedules

• Utility rates

• Load calc’ (aggressive, conservative)

• 8 bit versus 10 bit A/D converter

• SAT (50°F-60°F), SAT reset, SP reset



RESULT: Size VAV Boxes for ~0.5” ∆TP

VP

SP

TP









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VAV Box Sizing: Sample Calc.

Nominal size Inlet dia. (in.) Outlet width (in.) Outlet height (in.) ∆SP (in. w.g.)* ∆VP (in. w.g.) ∆TP (in. w.g.) Max CFM Radiated NC* 4 4 12 8 0.08 0.42 0.50 230 21 5 5 12 8 0.15 0.35 0.50 333 20 6 6 12 8 0.24 0.25 0.49 425 21 7 7 12 10 0.25 0.25 0.50 580 20 8 8 12 10 0.33 0.17 0.50 675 22 9 9 14 13 0.27 0.23 0.50 930 17 10 10 14 13 0.32 0.18 0.50 1100 19 12 12 16 15 0.32 0.17 0.49 1560 19 14 14 20 18 0.31 0.19 0.50 2130 18 16 16 24 18 0.32 0.18 0.50 2730 22

*From selection software using ARI 885-95 and assuming inlet SP = 1.5 and outlet SP = 0.25

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What is the lowest minimum?

• Function of

–

DDC Controller: What’s the lowest

controllable velocity pressure (VP) signal?

• DDC velocity pressure transducer accuracy

• A/D converter resolution (bits)

–

VAV Box:

• Amplification factor, F

• Inlet size, D















4

2 min

min

D

FPM

CFM



F

VP

FPM

_min



4005

min

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Why Not Just Look in the VAV Box

Catalog?

Equates to ~0.03” minimum VP and about 30% of design CFM for typical box selections

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Why Not Just Look in the VAV Box

Catalog?

Still too high

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Controllable VP Minimum



Some manufacturers list the range in catalog



Some you have to ask



Most available from ASHRAE RP 1353 and PG&E

research projects (see references)

• Almost all controllers ± 10% at 0.003”

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Typical DDC Performance

Highly accurate down to about 50 CFM (0.003”)

-40 -30 -20 -10 0 10 20 30 0 100 200 300 400 500 600 700 800 Reference Flow [cfm] Fl o w E rr o r [c fm ]

Nailor VAV Box Titus VAV Box “A”

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Flow Probe Amplification

229 358 515 702 916 1160 1432 2062 3665 7000 2806 4 inch 5 inch 6 inch 7 inch 8 inch 9 inch 10 inch 12 inch 14 inch 16 inch 22 inch 10 100 1000 10000 0.01 0.1 1

Flow Probe Velocity Pressure Signal (Inches W.G.) C F M C F M @ O n e I n c h S i g n a l I n l e t S i z e 2

4005 













K

A

F

F = amplification factor

K = actual flow in CFM at flow probe output of 1.0” w.c.

A = is the nominal inlet area in ft2

K

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Typical Flow Probe Performance

V e lo c ity [ fp m ]

Flow Grid Pressure [iwc]

1 .5 .1 .05 .01 .005 .001 50 100 200 500 1000 2000

6 .

2 ~

slope

F



Specify ≥2.0

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Sample Controllable Minimum

F

VP

FPM

_min



4005

min

3 .

2

004 .

0 4005

170 

Box Inlet Diameter Maximum CFM at 0.5 in.w.g. pressure drop Minimum CFM at 0.004 in.w.g. sensor reading Minimum Ratio at Highest Maximum, % Minimum Ratio at lowest Maximum, % 6 425 33 7.8% ‐ 8 715 58 8.1% 13.6% 10 1,100 91 8.3% 12.7% 12 1,560 130 8.3% 11.8% 14 2,130 177 8.3% 11.3% 16 2,730 232 8.5% 10.9%















4

2 min min

D

FPM

CFM



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Q: Why do some buildings use 3 -5

times as much energy as others?

0 500 1,000 1,500 2,000 2,500

Mar-02 Apr-02 May-02 Jun-02 Jul-02

B o il e r In p u t ( B tu /s f) Site #1 Site #2

A: Reheat

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Conventional VAV Box Minimum

Setpoint



No less than larger of:

• Minimum ventilation rate

 Per Title 24

• Controller minimum

 Not an issue for conventional logic – see discussion above

• Limit “dumping”

 Not an issue – see RP-1515 results below

• Limit stratification

 No more than 20°F above space temperature (~≤95°F) per Standard 90.1



No more than larger of:

• 30% of cooling maximum

 Per Title 24/Standard 90.1. Now only allowed for non-DDC controls

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Title 24 VAV Control Requirement

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Dual Maximum Control

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Dual Maximum Logic in Action

Start heating,

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Sort of Dual Maximum Control

(Found in some configurable controllers)

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Sort of Dual Maximum Control

(Found in some configurable controllers)

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VAV Box Dual Maximum Control Setpoints

per Title 24 and Standard 90.1



Minimum:

1. No less than larger of:

 Minimum ventilation rate for the zone  Controller minimum

2. No more than 20% of cooling

maximum



Heating Maximum:

1. No less than larger of:

 Minimum

 Limit stratification - ≤20°F SAT above space temperature (≤~90°F to 95°F)

2. No more than 50% of cooling maximum



Always use Option 1 above

• Do not use code maximum just because it’s legal!

• Avoid using %-of-cooling-maximum setpoints – boxes are

usually oversized!

COOL MIN HEAT VR‐101 12 1035 135 260 VR‐102 10 810 90 230 VR‐103 6 210 50 50 MARK INLET SIZE DESIGN CFM

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What about Standard 62.1 Multiple

Spaces compliance?

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How Well Does “Dual Max”

Logic Actually Work?

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RP 1515, "Thermal and Air Quality Acceptability in Buildings that Reduce Energy by

Reducing Minimum Airflow from Overhead Diffusers"

Objectives



Measure energy savings & validate simulations



Identify comfort issues that may occur at low flow

Funding



California Energy Commission - PIER



ASHRAE



UC Berkeley - Center for the Built Environment

Research Team



UC Berkeley



Taylor Engineering



Price Industries

Method



Field Study in 7 buildings



Background survey



“Right now” survey matched to zone trends



Energy monitoring



Laboratory Study

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Yahoo! Sunnyvale Campus



_{1073 Zones}



_{3700 Occupants}

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800 Ferry Building



22 Zones



Perforated Diffuser

with blades in face

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Measured flow fractions: Yahoo campus

0 20 40 60 80 100 0. 00 0. 05 0. 1 0 0. 15

Warm Season All Occupied Hours

Flow Fraction [%] D ens it y Low Minimum 30% Minimum 0 20 40 60 80 100 0. 00 0. 05 0. 1 0 0. 15

Cool Season All Occupied Hours

Flow Fraction [%] D ens it y Low Minimum 30% Minimum

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Occupant Comfort Survey



Questions

•

When the logic changed from conventional (High minimum) to Dual Maximum (Low minimum):

 Did occupants become less comfortable?

 Was their sense of air movement stronger due to “dumping”?



Approach

•

3 surveys:

 Yahoo warm and cool seasons  800 Ferry building warm season

•

3 – 4 weeks of surveying each season

•

Switched between high and low minimum operation in the middle of each survey period

•

Surveys administered 3 times/day

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% Dissatisfied people HIGH Conventional LOW Dual Maximum 800 Ferry Building 27.3% 12.5% Yahoo! cool season 8.7% 9.4% Yahoo! warm season 20.1% 10.3% 249 1408 681 463 1793 766 0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0%

800 Ferry Building Yahoo! Cool season Yahoo! Warm season

% dissa tis fied of people

"How satisfied are you with the temperature in your workspace?“

HIGH min flow rate LOW min flow rate

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Thermal sensation distribution

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Loads are very low!

0.2 0.5 1 CFM/FT

2

_{at 20˚F}

∆T

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What happens when load is less than

airflow minimum?

Actual Required CFM

Conventional Logic

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Air temperature

(800 Ferry building)

09/ 22 09/ 23 09/ 26 09/ 27 09/ 28 09/ 29 09/ 30 10/ 03 10/ 04 10/ 05 10/ 06 10/ 07 10/ 10 10/ 11 10/ 12 10/ 13 10/ 14 10/ 17 10/ 18 10/ 19 10/ 20 60 65 70 75 80 85 Dates z one t em p er at ur e [ F ] 1.4 2 1.8 1.5 1.6 1.5 1.4 1.8 1.6 1.5 2 0.89 1.6 1.5 1.1 1.9 1.1 1.4 1.3 1.2 2.1 72 72 71 71 72 71 71 71 70 71 71 72 73 74 74 73 74 74 75 75 73 14 27 23 35 45 36 35 41 47 34 65 23 30 51 45 64 25 37 67 27 48 sd Mean N

HIGH minimum flow rate

LOW minimum flow rate

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How about Drafts?

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Price Lab Tests

Perforated diffusers with blades in the neck, Cooling Mode.

Results:

• Negligible impact on ADPI – all near 1

• Negligible impact on ACE – all near 1

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Dual Maximum VAV Box Logic

Summary



Title 24 requires Dual Maximum logic for VAV zones with DDC



Setpoints

• Set minimum and heating-maximum setpoints as low as possible

 Minimum allowed by controls is seldom a factor if box properly sized  Do not use maximum allowed by T-24 just because it is legal



Cost impact

• Requires discharge temperature sensor

 This is nice for diagnostics anyway

• Usually requires programmable zone controller

 Configurable controller programming should soon catch up



Performance

• Thermal comfort improves with low flow operation

• Dumping & draft are not an issue at low flow

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