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Heat Exchanger and Outdoor Air Mixer

asked 2015-08-12 16:59:42 -0500

updated 2017-05-17 12:32:32 -0500

I have been using the "heatExchanger:AirToAir:SensibleAndLatent" object and the "OutdoorAir:Mixer" object in EnergyPlus and I have started to realised things have been going a little bit strange with this combination. When I look at the temperatures flowing through these two objects I get some odd results. I have a DOAS system and so the return air from the zones should not mix or come into contact with the supply air, other than through the heat exchanger. In EnergyPlus however if you have a heat exchanger then you must have an outdoor air mixer (I believe and am happy to be corrected on this). The temperature of the supply air coming into the heat exchanger and the air leaving the heat exchanger are as I would expect. However, when the air enters the outdoor air mixer from the heat exchanger it mixes with the return air from the zones and the temperatures leaving the OAM and into the AHU change, they go up in the winter and down in the summer, over stating how effective the heat exchanger actually is.

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My question is: How can I stop this from happening? Is there something in the controller than can stop the air mixing in the outdoor air mixer, is there something in the "heatExchanger:AirToAir:SensibleAndLatent" object to stop this from happening?

All and any help as always greatly appreciated.

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answered 2015-08-13 10:47:47 -0500

kwalkerman's avatar

It sounds like the total supply air flow for the AirLoopHVAC is higher than the outdoor air flow, thus mixing of some of the outdoor air with some of the return air. Have you checked the "Air System Outdoor Air Minimum Flow Fraction" output variable?

I have found that the way outdoor air is controlled in AirLoopHVAC is not very intuitive. If you are using variable volume zone air terminals, then the amount of air required at the air terminals drives the system flow. For most DOAS systems, this is the easiest approach, as it is easy to schedule the airflow fraction, or set a DesignSpecificationOutdoorAir object for use in demand control ventilation. In order to make sure that the AirLoopHVAC maintains 100% outdoor air, set the Controller:OutdoorAir Mininimum Limit Type to "Proportional Minimum" and the Minimum Fraction of Outdoor Air Schedule Name to a schedule that is always 1.

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Thanks @kwalkerman you were right, my flow rates were unbalanced and it was causing the problem. Thanks again! - edited from my previous comment

Annie Marston's avatar Annie Marston  ( 2015-08-13 11:56:12 -0500 )edit

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Asked: 2015-08-12 16:59:42 -0500

Seen: 777 times

Last updated: Aug 13 '15