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# Bypass of HeatExchanger:AirToAir:SensibleAndLatent based on temperature

Hello,

I am trying to implement a bypass to HeatExchanger:AirToAir:SensibleAndLatent which is coupled to the outdoor. The intention is to simulate Mechanical Ventilation with Heat Recovery.

The problem is that during summer, we might have for example 20C outdoor air, but the return temperature can be 25C, as a result of internal and solar gains. In this case, heat exchange between outdoor and return streams is not desirable -since the former becomes warmer and increases cooling demand.

1. I have tried availability managers but they turn off the entire ventilation system.
2. I have rejected the option of a economizer since the ventilation mass flow rate should remain constant.
3. Controller:OutdoorAir has an option for bypassing HEx but it is based on humidity (if I understand well).
4. Setting the option of the heat exchanger "Supply air outlet temperature control" to Yes, kind of works but it obeys my setpoint manager for cooling. I did not figure out how to provide independent criteria to the bypass.

Another question is: My setpoint manager activates the cooling system when supply temperature is higher than 25. But at the moment, it does that throughout the year - while 27C can be beneficial in the winter. How can I set this criterion as valid only for summer months?

Thank you very much. Comments are very welcome on how to convey my questions more effectively.

Have a nice day

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It sounds as if you have tried all the options available and it works at times while not working at other times. You need a way to vary the HX outlet node set point to achieve the desired result for cooling, heating and economizing. I am sure you could accomplish this using the Energy Management System program. Without using EMS maybe one of the other set point managers could get you closer to an answer? Maybe SetpointManager:Scheduled with different set points across the year or SetpointManager:OutdoorAirResetwhere the set point changes with outdoor temperature?

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Thank you very much for your immediate response!

It kind of surprises me that the heat exchanger cannot be bypassed according to temperature. This makes it impossible to simulate a Passivhaus (in which Mechanical Ventilation with Heat Recovery is an essential component) at warm climates - where bypass is very important to reduce cooling load and overheating during hot months

( 2019-03-14 11:38:12 -0500 )edit

Another thought is to set up an OA economizer. From the IO Ref for this HX: When an air-side economizer is also being modeled for this air system, the heat exchanger is deactivated during economizer operation. So if the OA and HX air flow was for example 0.05 and during economizer operation it was 0.050000001, then when economizer was active the HX would be bypassed with no real change in OA flow. There are a lot of options for economizer control.

( 2019-03-14 12:03:48 -0500 )edit