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Return Air Units are
widely used for direct fire space heating applications. They can be
supplied with a maximum 80% return/20% fresh air, per ANSI standards.
Other options include a 50/50 arrangement. “80/20” Units are configured
to modulate from the 80% return/20% fresh air to up to 100% outside or
fresh air. 50/50 units are generally designed to switch from 50% return
/ 50% fresh to 100% fresh, with no modulation between. All return air
should be brought in downstream of the burner and not recirculated
through the burner. This is to prevent buildup of waste products from
the combustion process within the building. It also prevents
contaminants originating within the building environment from being
passed through the direct fire burners. The combustion of unknown
contaminants can be a serious Safety or Indoor Air Quality problem, and
so must be avoided.
The way fan impellers (blades or rotors) are designed, the amount of
air they can move decreases as the pressure they are working against
increases.
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