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RoomAirModel exhaust temperature offset does not affect exhaust heat transfer?

asked 2023-11-13 15:45:29 -0600

Jamie Sullivan's avatar

updated 2024-01-04 16:17:40 -0600

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: image description

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: image description

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 ...
(more)
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answered 2023-12-11 02:51:45 -0600

Keigo's avatar

updated 2024-01-04 21:31:45 -0600

Are you using Balanced Exhaust Fraction Schedule Name?

I guess your zone has HVAC system other than Fan:ZoneExhaust.

  • If Balanced Exhaust Fraction Schedule Name is left blank or the schedule is 0, Fan:ZoneExhaust affects the air balance i.e., Return air flow rate = Supply air flow rate - Exhaust air flow rate (but 0 is the minimum of the return air flow rate).
  • If the schedule is 1, Fan:ZoneExhaust does not affect the air balance i.e., Return air flow rate = Supply air flow rate.

If your zone does not have HVAC system other than Fan:ZoneExhaust, where does the makeup air come from? From outdoor or from the adjacent zone? Anyway, I think you need to model the makeup air by ZoneInfiltration:DesignFlowRate, ZoneVentilation:DesignFlowRate, Airflow Network model, etc., otherwise, the zone air temperature would not change.


SUPPLLEMENT

The reason why I mentioned the make-up air is because my understanding is that exhaust air doesn't do anything to the zone heat balance as you mentioned as well.

Zone air heat balance algorithm in Engineering Reference does not account for exhaust air. Only Qsys (the energy provided from systems to the zone) affects the zone air heat balance.

"The third parameter can affect the zone air heat balance by altering the temperature of the air leaving the zone through exhaust fans." - I guess this is the only basis you rely on. I don't think this sentence is correct, or can we interpret the meaning of "can" as not always affecting the air heat balance but depending on the conditions? For example, if the exhaust air exchanges heat to the supplied fresh air (i.e., heat recovery), it affects the zone air heat balance.

I tested based on the ExampleFile UserDefinedRoomAirPatterns.idf and got the same result as yours i.e., the Output:Variable Zone Exhaust Air Heat Transfer [W] did not change with different Exhaust Air Offset settings. I think this is a bug. I couldn't find how the variable is calculated in Engineering Reference, but it should be Exhaust air mass flow rate [kg/s] x (Enthalpy of exhaust air [J/kg] - Enthalpy of outdoor air [J/kg]). If that's the case, Zone Exhaust Air Heat Transfer should change with different Exhaust Air Offset settings.

But even if the bug in the variable is fixed, the variable would not affect the zone air heat balance without heat recovery. In order to account for the exhaust air, I think the zone air heat balance algorithm needs to be updated.

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Mmm, I haven't been using the Balanced Exhaust fraction at all. Toggling it between 1 and 0 though doesn't have any effect on the model.

Fresh air ventilation in my model is provided by a combination of ZoneInfiltration:DesignFlowRate and ZoneVentilation:WindandStackOpenArea. I've then been using EMS scripts to read the ventilation rate and feed that into the zone exhaust. It's not a matter of the model not getting makeup air from outside to adjust the temperature here - that all works fine.

Jamie Sullivan's avatar Jamie Sullivan  ( 2024-01-03 20:07:27 -0600 )edit

Ok, you have already modelled the make-up air (fresh air ventilation).

What are your settings of RoomAirModelType?

I can't think of any other factors right away without your idf file.

Keigo's avatar Keigo  ( 2024-01-03 21:27:35 -0600 )edit

Thanks for the reply. I've edited in my room air model settings and exhaust fan setup above - they're fairly basic. I've poked around the inputs as much as I can without really having an effect, but there are some oddities in the outputs.

The exhaust air offset affects the exhaust inlet node temperature as would be 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 - though tbf it shouldn't be doing anything given this setup.

Jamie Sullivan's avatar Jamie Sullivan  ( 2024-01-04 16:20:32 -0600 )edit

@Jamie Sullivan I supplemented my answer a bit.

Keigo's avatar Keigo  ( 2024-01-04 21:32:46 -0600 )edit

Thanks! That makes sense yeah.

Jamie Sullivan's avatar Jamie Sullivan  ( 2024-01-08 14:51:02 -0600 )edit

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Asked: 2023-11-13 15:45:29 -0600

Seen: 696 times

Last updated: Jan 04