I was applying a vertical temperature gradient using the RoomAir:ConstantGradient inputs, and looking to exhaust warmer air out of the zone. According the the reference manual, having a temperature offset on the exhaust should affect the zone temperatures: "The third parameter can affect the zone air heat balance by altering the temperature of the air leaving the zone through exhaust fans".
When I tried to apply this using Fan:ZoneExhaust, there was no effect from the temperature offset. Looking at the outputs the fan is successfully removing air and heat from the zone, and if I look at the system node temperature for the exhaust node, it is indeed warmer than the zone air temperature as intended:
However, this does not translate into any change in the Exhaust Air Heat Transfer, or the zone air temperature. Their values stay the same regardless of what I set the exhaust temperature offset to:
Wondering if this might be an issue with the Fan:ZoneExhaust object, I also tried ZoneHVAC:OutdoorAirUnit, to the same result. I can change the exhaust node temperature, but this doesn't affect anything else.
I know that usually exhaust air doesn't do anything to the zone heat balance because if you're just removing air at the zone temperature then nothing about the zone temperature needs to change. But if you're removing air at a different temperature to the zone average, then this should affect matters, and the reference manual seems to say that this should be the case. So I was wondering what's going on here? Is this how the RoomAirModel is supposed to be working with the temperature offsets? Are the impacts of the room air model here purely on something like HVAC energy use, and it doesn't really affect the zone temperature or the heat transferred through the exhaust?
Edit: adding room air settings for detail. Inputs are mostly just basic examples - I've tried adjusting return, exhaust, and temperature gradient values and they don't have any impact on the actual zone temperatures and heat losses. Also included how the exhaust fan was hooked up to the zone in case that's relevant. In terms of observations, the exhaust air offset affects the exhaust inlet node temperature as expected, but the exhaust outlet node temperature isn't affected the same way, and is closer to the zone air temperature. Return air offset affects the zone air node temperature, but not the actual zone air temperature.
!- =========== ALL OBJECTS IN CLASS: ROOMAIRMODELTYPE ===========
RoomAirModelType,
Unit5_livingkitchen_airmodel, !- Name
Unit5_livingkitchen, !- Zone Name
UserDefined, !- Room-Air Modeling Type
Direct; !- Air Temperature Coupling Strategy
!- =========== ALL OBJECTS IN CLASS: ROOMAIR:TEMPERATUREPATTERN:USERDEFINED ===========
RoomAir:TemperaturePattern:UserDefined,
Unit5_livingkitchen_temppattern, !- Name
Unit5_livingkitchen, !- Zone Name
, !- Availability Schedule Name
VertTemp_sch; !- Pattern Control Schedule Name
!- =========== ALL OBJECTS IN CLASS: ROOMAIR:TEMPERATUREPATTERN:CONSTANTGRADIENT ===========
RoomAir:TemperaturePattern:ConstantGradient,
Unit5_livingkitchen_temppattern, !- Room Air Temperature Pattern Constant Gradient Name
123, !- Control Integer for Pattern Control Schedule Name
0, !- Thermostat Offset {deltaC}
0, !- Return Air Offset {deltaC}
2, !- Exhaust Air Offset {deltaC}
2; !- Temperature Gradient {K/m}
ZoneHVAC:EquipmentList,
Unit5_livingkitchen_EquipmentList, !- Name
SequentialLoad, !- Load Distribution Scheme
Fan:ZoneExhaust, !- Zone Equipment 1 Object Type
Unit5_livingkitchen_ExhaustFan, !- Zone Equipment 1 Name
1, !- Zone Equipment 1 Cooling Sequence
1, !- Zone Equipment 1 Heating or No-Load Sequence
, !- Zone Equipment 1 Sequential Cooling Fraction Schedule Name
; !- Zone Equipment 1 Sequential Heating Fraction Schedule Name
ZoneHVAC:EquipmentConnections,
Unit5_livingkitchen, !- Zone Name
Unit5_livingkitchen_EquipmentList, !- Zone Conditioning Equipment List Name
, !- Zone Air Inlet Node or NodeList Name
Unit5_livingkitchen_Exhaust_Inlet, !- Zone Air Exhaust Node or NodeList Name
Unit5_livingkitchen_air_node; !- Zone Air Node Name
Fan:ZoneExhaust,
Unit5_livingkitchen_ExhaustFan, !- Name
Always On, !- Availability Schedule Name
0.7, !- Fan Total Efficiency
91, !- Pressure Rise {Pa}
1, !- Maximum Flow Rate {m3/s}
Unit5_livingkitchen_Exhaust_Inlet, !- Air Inlet Node Name
Unit5_livingkitchen_Exhaust_Outlet, !- Air Outlet Node Name
Fans, !- End-Use Subcategory
Unit5_livingkitchen_ventrate; !- Flow Fraction Schedule Name
