GENERAL
Basic boiler connections for a three-zone hot water system are shown in Figure 47. In this system, two boilers are connected in parallel. Hot water from the top of the boilers moves to the air separator which removes any entrapped air from the water.
The expansion tank connected to the separator maintains pressure on the system. The tank is about half full of water under normal operating conditions. Air pressure in the tank keeps the system pressurized and allows the water to expand and contract as system water temperature varies. Water from the boiler moves through the separator to the three zone pumps, each of which is controlled by its own zone thermostat. In some systems, each zone may have a central pump and a valve. Return water from the zones returns to the boiler in the return line.
There several variations are possible with this type system but the process is the same. There is no minimum boiler water flow limit in this example.
The Dual Boiler Plant Control example following is a dual boiler plant with high-fire, low-fire controlled boilers, 145F minimum entering water temperature required prior to high-fire, water flow must be maintained when the boiler is enabled, and a secondary hot water reset schedule of 110F water at 55F OA temperature and 180F water at 5F OA temperature. The concepts adapt well for single or multiple boiler systems.
Fig. 47. Typical Piping for Multiple-Zone Heating System.
NOTE: The primary/secondary decoupler is sized for the full secondary flow, and like the chiller plant decoupler, should be a minimum of 6 pipe diameters in length.
Unlike the chiller decoupler, normal flow may be in either direction.
EXPANSION TANK MAKE-UP
WATER MANUAL AIR VENT
PRESSURE GAUGE RELIEF
VALVE PRESSURE
GAUGE
RELIEF VALVE
BOILER BOILER
DRAIN DRAIN
DRAIN AIR SEPERATOR
ZONE HWS
ZONE HWR ZONE HWR ZONE HWR ZONE HWS ZONE HWS
C2905 RETURN
LINE
Item No. Function
1 ON-OFF-AUTO function for secondary pumping system
2 ON-OFF-AUTO function for heating system 3 Selects lead boiler.
4 Heating system start point (Outside air temperature).
5,6 ON-OFF-AUTO function for primary pumps.
7,8 OFF-AUTO function for boilers.
9 Heating system stop point (Outside air
110 AUTO
180 55
5
HOT WATER TEMPERATURE RESET SCHEDULE
SELECTOR LOW LIMIT
SET POINT
DUAL BOILER PLANT CONTROL FUNCTIONAL
DESCRIPTION-Fig. 48. Dual Boiler Plant Control Graphic
12-14 Valve modulates to prevent entering water from dropping below low limit setpoint (145F).
15-18 Secondary water setpoint reset from OA.
19,20 Valve modulates to prevent entering water from dropping below low limit setpoint (145F).
21-23 Operator information.
24 Icon, selects the Boiler System Control dynamic display (Fig. 49).
25,26 Software signal selection functions, allows
BOILER SYSTEM CONTROL
A BLENDING VALVE ON EACH BOILER MODULATES IN THE RECIRCULATING POSITION TO PREVENT THE BOILER ENTERING WATER TEMPERATURE FROM DROPPING
BELOW DEGREES.
LEAD BOILER ( ) AND ITS ASSOCIATED PUMP START ANYTIME THE OUTSIDE AIR TEMPERATURE DROPS TO AND SHUTS DOWN ANYTIME THE OUTSIDE AIR TEMPERATURE RISES TO DEGREES. ANYTIME THE LEAD BOILER STARTS FROM THIS
OUTSIDE AIR TEMPERATURE SETTING, THE OTHER BOILER IS LOCKED OUT FOR MINUTES.
ANYTIME THE LEAD BOILER CONTROL VALVE IS COMMANDED FULL OPEN BY THE SECONDARY WATER TEMPERATURE CONTROL LOOP FOR MINUTES AND THE SECONDARY HOT
WATER SUPPLY TEMPERATURE IS MORE THAN DEGREES BELOW
IT'S SETPOINT, THE LAG BOILER AND ITS ASSOCIATED PUMP START. ANYTIME BOTH BOILERS ARE OPERATING AND THEIR CONTROL VALVES ARE LESS THAN PERCENT OPEN TO THE
SECONDARY RETURN, THE BOILER SYSTEM OPERATING LONGEST SHUTS DOWN.
THE BOILER BLENDING VALVES MODULATE (SUBJECT TO THEIR LOW LIMIT
CONTROL) TO PRODUCE SECONDARY WATER TEMPERATURES FROM TO DEGREES AS THE OUTSIDE AIR TEMPERATURE DROPS FROM TO DEGREES.
145
1
60 65
60
5
5
40
110 180
55 5
M15052
Features
1. Full flow through operating boilers.
2. Minimum temperature limit on boiler entering water.
3. Variable flow secondary system with full boiler flow.
4. Automatic boiler staging
5. User friendly monitoring and adjustment.
Conditions For Successful Operation
1. Control network, software, and programming to advise heating plant controller of secondary fan and water flow demands.
2. Interlock and control wiring coordinated with the boiler manufacturer.
3. Control in accord with boiler manufacturers recommendations.
4. Proper setpoint and parameter project specific settings.
Fig. 49. Boiler System Control Dynamic Display.
Specification
The heating plant shall operate under automatic control anytime the secondary pump ON-OFF-AUTO function is not “OFF”, subject to a heating system ON-OFF-AUTO software function.
The lead boiler, as determined by a software lead boiler selection function, shall be enabled anytime the date is between October first and May first, the OA temperature drops below 60F for greater than 30 minutes, and an AHU is calling for heat. Each boiler primary pump shall have a software ON-OFF-AUTO function, and each boiler shall have a software AUTO-OFF function. The heating plant shall be disabled anytime the OA temperature rises to 65F for greater than 1 minute and after May 1.
Anytime the boiler plant is enabled, the lead boiler primary pump shall start and as flow is proven, the boiler shall fire under its factory controls to maintain 180F. If the lead boiler status does not change to “on”, or if flow is not proven within 5 minutes, the lag boiler shall be enabled.
During boiler operation, a three way blending valve shall position to place the boiler flow in a recirculating mode until the water entering the boiler exceeds a low limit value of 145F, at which time the blending valve shall modulate to maintain the secondary water temperature between 110F and 180F as the OA temperature varies from 55F to 5F.
The lag boiler shall be locked out from operation for 60 minutes after the lead boiler starts. Thereafter, anytime one boiler control valve is commanded full open by the secondary temperature control loop for greater than 5 minutes and the secondary water temperature is a temperature less than 5F below the secondary water temperature setpoint, the “off” (lag ) boiler pump shall start, and upon proving flow, the “off” boiler shall be enabled to fire under its factory controls to maintain 180F.
The just-started boiler blending valve shall be controlled by an entering water 145F temperature low limit sensor and setpoint similar to the lead boiler, and subsequently, in unison with the other boiler blending valve to maintain the reset secondary hot water temperature.
Anytime both boilers are operating and their control valves are less than 40% open to the secondary return line, the boiler and pump that has run longest shall shut down.
MODULAR BOILERS
Modular boilers provide heat over a large range of loads and avoid standby and other losses associated with operating large boilers at small loads. Figure 50 shows a primary-secondary piping arrangement where each modular boiler has its own pump. The boiler pump is on when the boiler is on.
Boilers that are off have no flow and are allowed to cool.
Each boiler that is on operates at or near full capacity. Avoiding intermittent operation prevents losses up the stack or to the surrounding area when the boiler is off.
Normal control of modular boilers cycles one of the on-line boilers to maintain water temperature in the supply main to meet load requirements. The supply main control sensor cycles the boilers in sequence. If the load increases beyond the capacity of the boilers on-line, an additional boiler is started. The lead (cycling) boiler can be rotated on a daily or weekly basis to equalize wear among all boilers or when using digital control, the program can start the boiler that has been off the longest.
HOT WATER SUPPLY SENSOR
PRIMARY PUMP
MODULAR BOILERS SECONDARY
PUMPS HWS
HWR
HOT WATER RETURN SENSOR
5 4 3 2 1
C2906
Fig. 50. Typical Primary-Secondary Piping for Modular Boilers.