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Hi everyone,

I'd like to hear your ideas and thoughts about the differences between DOAS and MUA's, and when each should be used.

It's clear to me that a DOAS generally supplies oudoor air to AHU's, fan coils, VRF evaporators, heat pumps, etc. In turn, a DOAS helps to cool, dehumidify the outdoor air, and in this way the indoor equipment requires less sensible and/or latent capacity.
But what about the MUA's? Are they still just a support system? can they heat a space completely with outdoor air alone? Do they contribute to the pressurization of spaces?

I look forward to your comments and ideas. Thanks.


Semantics. They fulfil the same function in that they bring in OA and pre-treat it before either directly discharging into the space, or discharging into an AHU for further treatment.

where I live most people who say MUA mean a unit that filters and heats air to discharge into a garage. But they also come with cooling option, so really, they are DOAS.... A regular AHU also can be a DOAS. You just have to design it for the conditions.


DOAS usually has more options and is more sophisticated with controls, sometimes energy recovery, power exhaust, etc. MUA usually just has filters, cooling section, heating section and no return. But like HVAC-Novice said, you could technically call either one the other name and it wouldn't really matter. Technically, a makeup air unit should be making up air from a required exhaust source; that's why it usually doesn't have return or exhaust duct.

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