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What is the differences between the Makeup Air and Fresh Air or Outside Air (OSA) for space?

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404 views asked Apr 20, 2015 by Joe Mahini

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Outside Air need to be provided per ASHRAE 62.1 for commercial and high-rise residential (over 75 foot height), and ASHRAE 62.2 for low-rise residential for indoor air quality (IAQ). See “How much outside air (fresh air) need to be provided for each space?” in this site.

Make up air is for replacing the space’s exhausted air to keep the air balance in space, which can either be provided from the outside or from the adjacent space, depends on the application. For example, restrooms need to be exhausted by 6 air-change or 50 CFM per odor generated fixture based on ASHRAE, no air can be returned to the AC system from the restrooms and air just needs to be exhausted to outside. For saving energy, instead of supplying air from the conditioned air to restrooms to provide cooling or heating, the good practice is to provide makeup air from the adjacent conditioned spaces. In this case the Makeup Air is not an Outside Air.

Another example is electrical room (with no transformer), which can provide air from the adjacent conditioned space for ventilation (the air temperature in space is always below 74°F). The air from the electrical room also cannot return to the AC system.

For electrical room with transformer (usually over 75 KVA) which dissipates a significant heat to the room, the “cooling only” AC unit need to be provided.

answered Apr 20, 2015 by Joe Mahini

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