[Equest-users] Increasing OA temp for specific AHUs

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Modelers,

We have relief/exhaust louvers located very close to the intake louvers in an existing mechanical space, and are trying to show the client how this kills their energy consumption. This lowers energy saved from the economizer during winter months, and also makes the unit work even harder in the summer.

Is there a way to raise the OA +20 deg over ambient, for the whole year, and for specific AHUs? I think this will be a decent estimate on the increased energy consumption from this scenario. Or maybe someone has modeled this a different way??

Any help is greatly appreciated. Thanks!

-Dana

Mr. Dana Etherington, E.I.T., LEED AP BD+C

Dana Etherington, CEM, LEED AP
email: dana.etherington@crbusa.com
cell: 617-583-3009

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Dana,

I am not aware of a way to increase OA temp in a model and isolate it to a specific AHU(s). You could change the weather file, but then you are drastically altering your envelope and ventilation loads for the entire building. Even if you could, I don't think it is reasonable to increase the OA temp uniformly, since the relief air will likely be the same temperature within a narrow range year round, while the OA temp has a much bigger range. If your building is in a heating-dominated climate, the short-circuiting of the relief air could actually reduce the ventilation heating load and save heating energy.

One thing you could try is to reduce the ventilation load in the model, since your situation is essentially returning more air to the system then the original system design. How much to reduce it is guesswork unless you're a wiz with CFD. For example, you could change your OA from 20% to 10% and see what the impact is. You could also adjust the temperature limits on the economizer.

Regards,

Bill

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That's a decent option. I'd make one additional suggestion: reduce your Max OA Fraction down by the same amount to account for the loss during economizer. In other words, if you're reducing your OA from 20% to 10% you'll probably need to reduce your Max OA Fraction from 1.0 to 0.9. Depending on the controls strategy, during the times of the year when the damper is properly modulating, the short circuiting should just open up the OA damper more to meet the mixed air temp, but the times when the economizer is fully open is when you need to account for the short circuiting.

But be aware that any changes are going to be, at best, an educated estimate because things like wind speed and direction will impact how much short circuiting there really is...

Eric O'Neill

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