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NC-v4.1 EAp2:Minimum Energy Performance

Fan power

According to ASHRAE std. 90.1 2010, appendix G, section G3.1.2.9.1/ ASHRAE std. 90.1 2016, appendix G, section G3.1.2.8.1:
“If return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.”

Which means that, for example, a relief fan’s design air quantity will always be less than the supply fan air quantity (assuming minimum outdoor air > 0).

In the case where the economizer is fully open (100%) the air flow through the relief fan will have to be equal to the supply fan air quantity, thus operating above its design flow (PLRfan > 1). How should the fan power (Pfan) be calculated when the fan is operating above its design air quantity?

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Fri, 02/05/2021 - 18:58

Note that Pfan for the entire system is calculated based on the supply airflow only, so the relief airflow would have no impact on the fan power calculation. Pfan would then get distributed among the various fans in the same proportions as the Proposed design.

Mon, 02/08/2021 - 09:29

Hi Tyler and thank you for your answer.
If the simulation software models each fan separately then the design Pfan is distributed over all fans in the same proportions as the Proposed design. 
But then the actual power at each timestep is calculated at fan level based on the actual flow, a PLR curve and rated conditions. This is how I end up in the situation described above (PLR>1). Any advice?

Thu, 02/11/2021 - 19:32

Sounds like a question that might be best answered by the support team for your particular energy modeling software. I would consider reaching out to them.

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