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How to use constant supply air temperature in CAV in E+

asked 2015-08-12 11:54:02 -0600

wangh gravatar image

updated 2015-08-19 15:12:30 -0600

I would like to model a single zone CAV system with constant SA temp. It will cycle on and off to meet the load. I tried with adding single zone heating setpoint manager (min=max=95F) after Heating Coil and single zone cooling setpoint manager (min=max=55F) after Cooling coil. However, when I looked at the node temperature, it seems like the heating coil tries to heat the air to 95F first and then the cooling coil tries to cool it down to 55F afterwards. So both coils are working simultaneously and it always supply 55F to the room even though the room needs heating.

This is how my system set up

OA mixed with return air->Fan->HC->CC->Room

Also, how do I make the fan cycle on and off in a CAV system, does it do that automatically when no ventilation is required?

Thanks in advance

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answered 2015-08-12 12:44:38 -0600

updated 2015-08-12 13:38:52 -0600

There are several configurations possible for conditioning a single zone. A single zone system (ZoneHVAC:) or an air loop (AirloopHVAC:) can be used. Although they would be set up differently, with an air loop connected to a splitter and mixer, a main branch, and zone connections, while zone equipment would be connected directly to a zone, they work essentially the same.

Zone equipment would be used to meet a load, so controlling the equipment to a set point temperature is not an option. Airloop equipment can serve a single zone and certain configurations can be controlled using a temperature set point. The CoilSystem:Cooling:DX or place directly on the main branch a Coil:Cooling:Water object. Both of these can be used to control the coil using a set point manager. See example file 5ZoneAutoDXVAV for a DX cooling coil configuration and example file 5ZoneAirCooled for use of water coils.

If your coil set point temperatures at set at 95F heating, and 55F cooling, then yes, both coils will turn on to maintain their individual temperature set points. If you set both set point temperatures to the same value, only one coil should run. If you want to control both coils to different temperatures, then the only way to avoid both coils operating at the same time would be to use the availability schedules to turn the cooling coil off during the heating season, and the heating coil off during the cooling season.

Fan cycling is another issue. The set point based control is typically accomplished using a constant volume fan and this fan will not shut of unless the fan availability schedule turns the fan off, for example at night. If the system is needed to operate during the night, then a night cycle availability manager could be used to turn the system on to maintain a (setback) temperature.

There is also a new equipment model, namely AirloopHVAC:UnitarySystem, that can model any coil type and provide both load based and set point based control.

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Thanks for your detailed explanation. I read the E+ help, it says single zone heating setpoint manager will maintain the temperature when the controlled zone needs heating. And the same with cooling setpoint manager. But it seems they don't work as designed. I am hoping the program can automatically determine whether heating or cooling is needed and use the corresponding temp. I think this should be a very simple if then statement and just curious why E+ didn't implement it as it is the most common type of CAV system.

wangh gravatar image wangh  ( 2015-08-12 13:15:13 -0600 )edit

The problem with your original file is the use of heating/cooling set point managers that have "min=max=95F/min=max=55F". Don't set min/max and let the SPM calculate the temperatures. Try this on your file and report Output:Variable,YourCoilOutletNodeName,System Node Setpoint Temperature,Hourly; for the outlet nodes of both coils. You will see that when a heating load is present, the heating coil outlet node SP = some warm value while the cooling coil outlet node SP is some very small value (e.g., -99) and the cooling coil will not turn on.

rraustad gravatar image rraustad  ( 2015-08-12 13:35:59 -0600 )edit

Yes. If I set the minimum and maximum to different values, it will work. But the problem is the air temperature is not fixed at a constant for cooling or heating. This can also be achieved by a SPM:SingleZone:Reheat. So I don't really understand why E+ have the SPM:SZ:Heating and SPM:SZ:Cooling since they don't function as they intended to be. Or its just I didn't set them up right....

wangh gravatar image wangh  ( 2015-08-12 14:41:53 -0600 )edit

You did set them up correctly. These SPM's will calculate a temperature needed to meet a zone load. Since the zone load changes, so does the coil outlet node temperature set point. If you want a constant temperature off the coils, use SetPointManager:Schedule. Use the same SP for cooling and heating, otherwise they will fight each other unless the coil availability manager schedules one coil off when the other coil is active. Usually, the temperature SP for a heating coil is slightly lower than for the cooling coil. The zone terminal units pick up the remaining heating load.

rraustad gravatar image rraustad  ( 2015-08-12 14:47:33 -0600 )edit

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Asked: 2015-08-12 11:54:02 -0600

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Last updated: Aug 12 '15