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MarketManager » Modeling HVAC Systems
Q: Why does decreasing Exhaust Fan CFM lead to a reduction in Unmet Heating Hours?


The Outside Air that MarketManager uses in its hourly energy calculations is the greater of:

  • the Outside Air % as determined by the Outside Air Schedule, the Economizer and the Min Outside Air Field in the System | Outs Air Data Form, and
  • Exhaust Air

From a conservation of mass standpoint, by increasing Exhaust Air, you are reducing Return Air, and thus increasing Outside Air%.

As you cut down on Exhaust CFM, you are cutting down on Outside Air as well, which reduces the heating energy required to heat the Outside Air, which could lead to a reduction in Unmet Heating Hours.

Two additional issues arise:

  1. The Exhaust Fan CFM from a Zone is the sum of the Exhaust Fan CFM and Equipment Exhaust flows if any.
  2. Why are there Unmet Hours in the first place? Unmet Hours can occur for a number of reasons, for example because MarketManager’s Design Calculations do not take into account Exhaust Air flows, while the Simulation does.

I can’t find the answer to your question ? Try checking our User Forums.

Otherwise feel free to contact our Tech Support staff at (805) 329-6565, or via email at helpdesk@abraxasenergy.com.

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