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Fan:VariableVolume Minimum Flow Input

asked 2015-10-02 11:13:57 -0500

updated 2015-10-02 11:48:12 -0500

I have taught other EnergyPlus users and observed my own evidence that the Fan:VariableVolume minimum flow inputs, Fan Power Minimum Flow Fraction and Fan Power Minimum Air Flow Rate pertain only to the fan power calculation and do not impact that actual flow during operation. This is consistent with the documentation, but I recently discovered that there seems to be a contradiction when the fan is a return fan located before the outdoor air system.

Here are three idf files. Each has a single VAV air handler with a supply fan and a return fan. This one has no minimum on any fan. This one has a minimum fraction on the supply fan. Finally, this one has a minimum on the return fan, as pictured in the OpenStudio screen shot.

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If I graph the supply outlet node mass flow, the simulations with no minimum and a supply minimum stack right on top of each other, however the simulation that has a return fan with a minimum fraction of 0.75 is clearly flow limited. This seems in direct conflict with my expectations and the documentation. Can anyone justify this behavior or confirm that this is an EnergyPlus bug?

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Hey K, did you ever get/find an answer to this question? I've experienced the same link

NickC's avatar NickC  ( 2016-03-18 12:42:51 -0500 )edit

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answered 2016-03-23 20:56:56 -0500

updated 2016-03-23 22:10:36 -0500

I think this has more to do with fan heat than an issue with the program. When you limited the fan power at the return node of the OA mixer, you also limited the fan delta T, and therefore the response of the OA controller to the mixed air temperature set point.

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The supply air temperature hasn't changed significantly so it's not the TUs that are causing the change in air flow.

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Asked: 2015-10-02 11:13:57 -0500

Seen: 480 times

Last updated: Mar 23 '16