Can two HVAC systems in the same zone have different setpoints?

asked 2016-05-18 18:02:24 -0600

updated 2016-05-19 07:42:37 -0600

I'd like to use two HVAC systems in one zone.

The first one is a "normal" system responsible for internal temperature, for instance a ZoneHVAC:Baseboard:RadiantConvective:Water.

The second one is an ZoneHVAC:IdealLoadsAirSystem. Its job is to supply air into the zone with given temperature, humidity ratio and massflow specified by an EMS program and its actuators.

The problem is: an EMS Actuator can override the IdealLoadsAir Massflow as far as it's on. When no cooling or heating is needed, the IdealLoadsAirSystem is off regarles of my helpless little EMS Actuator. The oly way I could make the IdealLoadsAirSystem operate all the time is by specifying an extreme setpoint, e.g -50 °C. It's fine by the IdealLoadsAirSystem because the EMS Actuator overrides its supply temperature, but this setpoint messes up the operation of the other "normal" system. (the other system can be anything else, not just a convective system. I need cooling and heating too in the zones)

Is there a way to specify a different Thermostat setpoint for the IdealLoadsAir other than the one for the "normal" system. As far as I know I can only have one thermostat in a single zone, and the IdealLoadsAirSystem reads the thermostat of the zone it's supplying to. (is this correct?) Maybe I'm looking for something like: "Controlling Zone or Thermostat Location" to set up a dummy zone somewhere with extreme setpoints. AirLoopHVAC:UnitarySystem has this thing. But I don't think that IdealLoadsAir has something similar. Or is there a simpler way to turn on the IdealLoadsAir all the time?

thanks,

EDIT:

Probably I didn't formulate the question very well. Anyway, I Implemented, what rraustad advised, at least I can show the problem on a graph:

image description

In short, I want the green line (massflow of IdealLoadsAir hardset to 10 kg/s by EMS Actuator) to be flat 10 kg/s, regardless of the internal temperature. In the meantime the other systems should work properly (this excludes the use of the -50 °C setpoint)

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One of the E+ developers may correct me, but I'm not sure you can use the ideal loads object like this. As I understand it, it will use as much fictitious district heating or cooling as needed to maintain the setpoint in its assigned zone. Adding another system in the same zone will likely drive that utilization up even further if the setpoints don't match.

ljbrackney ( 2016-05-18 20:55:18 -0600 )edit

IdealLoadsAir is usually used for load calculations, but i think it's just like any other ZoneHVAC system, only simpler, thus easier to use when you have to set it up by hand. In my case the "normal" system comes from DesignBuilder, then I put the IdealLoadsAir directly to the idf file.

adambgnr ( 2016-05-19 08:17:20 -0600 )edit

If you know the supply temp, hum ratio, and flow, could you not just calculate the load using the current zone temperature and use that as a internal load to the zone instead of using the ideal loads system?

Rohini ( 2016-05-19 09:40:19 -0600 )edit

Maybe, but I don't use IdealLoadsAir here for load calcuIations. I just use it as a very simple AHU that can be easily conrolled by EMS actuators.

adambgnr ( 2016-05-19 09:55:33 -0600 )edit

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ZoneHVAC:EquipmentList, ZONE 1 EQUIPMENT, !- Name ZoneHVAC:Baseboard:RadiantConvective:Water, ZONE 1 Baseboards, !- Zone Equipment 1 Name 1, !- Zone Equipment 1 Cooling Sequence 1, !- Zone Equipment 1 Heating or No-Load Sequence ZoneHVAC:IdealLoadsAirSystem, !- Zone Equipment 1 Object Type ZONE 1 Ideal Loads, !- Zone Equipment 1 Name 2, !- Zone Equipment 1 Cooling Sequence 2; !- Zone Equipment 1 Heating or No-Load Sequence

Comments

This is not an uncommon situation. I am just finishing off a project that has a water source VRF system for ventilation via an ERV, heating and cooling. The perimeter zones also have ceiling mounted radiation. There is a single temperature sensor controlling the VRF fan coil and the rad panel in each zone. The rad panel is controlled to 1°C below the heating set point, the VRF fan coil is controlled to meet the heating set point. Both systems come on when the space temperature drops more than 1°C below the heating set point. I didn't use EnergyPlus - I'm stuck in the last century.

Chris Jones ( 2016-05-19 07:38:09 -0600 )edit

@rraustad, I edited the question, with a graph, to be more specific. I think my problem is that the thermostat pulls the plug of the IdealLoadsAir completely, thus the EMS actuator can't actuate.

adambgnr ( 2016-05-19 07:47:15 -0600 )edit

Well, I can't think of an easy way to keep the Ideal Loads system on even when there is no load. Since you are already using EMS, I would change the cooling SP to -50, and then control both systems with EMS. The other system will be turned on when zone temp > 25, and ideal loads system will be on all the time. @MJWitte, is there a way to keep the ideal loads system on even when the thermostat says there is no load?

rraustad ( 2016-05-19 08:08:09 -0600 )edit

What if I use humidity control? Since I already EMS-override the IdealLoadsAir humidity ratio, it won't have any efect on the system. Would a 100% rel hum setpoint keep the IdealLoadsAir on? I'll look into it

adambgnr ( 2016-05-19 08:36:29 -0600 )edit

Looks like the humidity control workaround works. I added a humidistat with 100% hum & dehum setpoint (0 & 0%works as well), and set Humidification and Dehumidification Control Type to "Humidistat" in the IdealLoadsAir. This keeps it on at all times. This will do for now. Thanks for the help!

adambgnr ( 2016-05-19 09:37:22 -0600 )edit

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Can two HVAC systems in the same zone have different setpoints?

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Can two HVAC systems in the same zone have different setpoints?