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Constant Mechanical Ventilation with AirLoopHVAC:Unitary:Furnace:HeatCool

asked 2019-08-21 15:52:02 -0500

camlawrence's avatar

I'm working on a model of a 4 zone single family home. I'm trying to get a constant outdoor air flow/mechanical ventilation rate but running into difficulties. Currently the model uses AirLoopHVAC:Unitary:Furnace:HeatCool which uses fan:onoff. The system operates by cycling the fan on and off at a constant volume in sync with the heating and cooling coils to meet a setpoint temperature in the control zone. A controller:outdoorair object specifies a fixed minimum and maximum outdoor airflow rate but it seems this outdoor air is only being supplied while the furnace fan is operating.

It seems like it should be a simple task but I can't get a constant mechanical ventilation rate, is there an easy way to do this? Will a different fan that cycles between outdoor air flow rate and mixed air flow rate work and how would I implement that.

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What kind of a mechanical ventilation system are you trying to model? Bathroom exhaust fan? ERV/HRV? Something else?

shorowit's avatar shorowit  ( 2019-08-23 13:38:05 -0500 )edit

I'm trying to model an ERV

camlawrence's avatar camlawrence  ( 2019-08-26 15:37:36 -0500 )edit

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answered 2019-08-27 20:44:51 -0500

AirLoopHVAC:Unitary:Furnace:HeatCool with a fan operating mode of cycling fan will turn on and off proportional to load. Just like the AC system in your home. Question: when the AC system fan in your home turns off does the ERV still have air flow? A possible solution is to use constant fan mode either during the day or 24/7. In southern U.S. climates the extra fan power typically exceeds recovered energy.

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answered 2019-08-27 18:03:08 -0500

updated 2019-08-27 18:03:59 -0500

The typical solution to modeling a residential ERV is to use the ZoneHVAC:EnergyRecoveryVentilator object. The advantages of this object are that it's easy to work with, incorporates the heat exchanger, can easily be scheduled with constant airflow, and automatically interfaces with outdoor air. The disadvantage is that it can't handle supply air to one zone and exhaust air from a different zone (if that's what you are trying to accomplish with your 4-zone model). Perhaps you can use this object in conjunction with ZoneMixing objects to approximate this effect.

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Thanks for the response. My model already has an air loop that services the zones, can I still add the zonehvac object? what would that look like? Do I need to remove the outdoor air connection from the existing air loop?

camlawrence's avatar camlawrence  ( 2019-09-03 13:22:41 -0500 )edit

Yes, you can still add the ZoneHVAC object, you just need to attach it to a single thermal zone. You'll also want it to operate before the airloop system in the ZoneHVAC:EquipmentList object, so that your HVAC unitary system meets the heating/cooling load that includes the ERV. And yes, you'd then want to remove the outdoor air connection from the existing air loop.

shorowit's avatar shorowit  ( 2019-09-03 15:50:02 -0500 )edit

answered 2019-08-26 16:37:06 -0500

Do you have a DesignSpecification:OutdoorAir object for each zone? These should be referenced in the Sizing:Zone objects.

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I've already added those objects, seems to be delivering the required outdoor air but only during furnace operation

camlawrence's avatar camlawrence  ( 2019-09-03 13:23:32 -0500 )edit

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Asked: 2019-08-21 15:52:02 -0500

Seen: 272 times

Last updated: Aug 27 '19