Question-and-Answer Resource for the Building Energy Modeling Community
Get s tarted with the Help page

# Economizer changes supply air set point and zone set point temperature

Using UnitarySystem, controller:outdoorair, and setpointmanager:MixedAir to model a constant volume single zone system serving a single zone. With NoEconomizer selected the cooling coil operates to meet the loop outlet set point and the connected zone is maintained at the cooling setpoint. Enabling the economizer causes the loop outlet set point is several degrees lower and the connected zone is maintained at the heating set point. I've looked at other input decks and it seems like the setPointmanager:mixedair is set up the way others have done. Is there an obvious missing manager? How should I go about debugging this issue?

Update: The heating coil is not on. The only difference in the run input is the outdoor air controller has FixedDryBulb economizer in one case and no economizer in the other. The zone set point as reported by system node setpoint temperature is the same in both runs. A difference appears at the supply equipment outlet setpoint. In the NoEconomizer case this is set at a temperature that maintains the zone cooling setpoint, in the Economizer case it is lower and maintains the space temperature at the heating set point. If the heating set point is lowered then the economizer opens up. The input seems pretty standard and a few others have looked at it to no avail. Seems like some sort of interface issue between controller:outdoorair and unitarysystem but it could be I'm just confused by the various controllers.

Update2: Before the fans turn on, the space temperature is above the cooling set point. Once the system starts, if no economizer is present and the system is at minimum air, the DX cooling is required to maintain the cooling set point. I don't understand why a modulating economizer would force the zone temperature below not only the cooling set point but the heating set point which is 5F lower, and why it would modulate to maintain the heating set point rather than the cooling set point. In a real system, the cooling signal would drop and the economizer shut down. Why would an economizer be controlled differently from the compressor which faithfully maintains the cooling set point? Is there a control that will limit this? Sorry if this is dense, it does feel like I'm missing some basic item in the controls.

edit retag close merge delete

This sounds like its working correctly, describe more specific if you think otherwise. For the no economizer case, the set point will be what is required to meet the cooling load. If economizer is active, free cooling can force the zone temperature below the heating set point. In this case, the set point will be the heating set point not the cooling set point since you would not want to heat all the way up to the cooling set point (Note: E+ used to heat up to the cooling set point but was revised to only heat up to the heating set point).

( 2017-01-19 14:18:18 -0500 )edit

If the heating set point is lowered then the economizer opens up.

This one relates to the mixed air temperature set point at the OA mixing box mixed air node. If the heating SP is lowered, a lower mixed air SP is calculated to meet the reduced load. This in turn allows the economizer to open to provide a lower mixed air temperature.

( 2017-01-19 14:22:26 -0500 )edit

Update 2: What type of set point manager are you referencing with the mixed air set point manager?

SetpointManager:MixedAir,
Air Loop Outlet Node,    !- Reference Setpoint Node Name

SetpointManager:SingleZone:Reheat,
Air Loop Outlet Node;    !- Setpoint Node or NodeList Name

( 2017-01-19 15:22:13 -0500 )edit

SetpointManager:MixedAir controlling the mixed air setpoint node with a reference at the Supply equipment outlet node, and SetpointManager:SingleZone:Reheat with Room, !- Control Zone Name Room Air Node, !- Zone Node Name Room ATU Inlet Node, !- Zone Inlet Node Name

( 2017-01-19 16:02:47 -0500 )edit

Now compare the mixed air node temperature to the set point temperature with and without economizer. If the mixed air temp is below the mixed air temp SP, the OA flow should be at the minimum, the zones may be overcooled, and reheat coils will maintain the heating SP when necessary. If mixed air temp is equal to or greater than the mixed air node SP, the zones should not require reheat.

( 2017-01-20 07:21:50 -0500 )edit

Sort by » oldest newest most voted

Sounds like the economizer is providing sufficient cooling to meet the cooling load and then some. In this case, the zone heating temperature set point will be maintained. Check the outdoor air flow rate, mixed air temperature, and cooling/heating coil outlet air set point temperatures with respect to each coils inlet air temperature. I would suspect the heating coil is active at this time since the zone heating temperature is maintained. If this is not the case, check the set point managers serving each coil to see what set point temperatures are placed at each coil's outlet node.

more