Fan Power Limitation Pressure Drop Adjustment for "fully ducted" return or exhaust system in ASHRAE 90.1

I would like to know how to interpret "Return or exhaust systems required by code or accreditation standards to be fully ducted" when considering Fan Power Limitation Pressure Drop Adjustment in ASHRAE 90.1.

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The Standard 90.1 User's Manual explains it as follows, but it is still not clear to me.

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Can anyone advise if the pressure drop for fully ducted return or exhaust system (125Pa) should be considered for the following 3 fan systems (A) to (C)?

In my opinion, 125Pa should be added in baseline model when the fan system of proposed model is (A), and it should not be added when the proposed model is (B), but I'm not sure whether it should be added when the proposed model is (C).

(C) is a fan system in which the outdoor air supplied to rooms is finally exhausted from restrooms via corridors. I think it's a very common system.

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If I understand your sketches correctly, "A" & "C" have return duct which starts at the ceiling return grille and runs all the way to the return fan. These are considered "fully ducted" and should use the 125Pa additional pressure rise for the return fan. "B" draws air from a plenum and does not qualify for the additional 125Pa.

Your exhaust fan configuration is novel, but the principal is that the extra duct from the main duct / shaft to the return grille requires more fan power, so "a" & "C" appear to be the correct systems for applying the adder.

Note the exception: if the return system is a combination of fully ducted and plenum return, the adder may not be used.

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Jim Dirkes

answered 2021-08-01 19:21:55 -0500

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My concern with "C" was that the exhaust air passing through the corridor might be considered Not fully ducted. Thank you for your opinion.

I only have experience of MEP design in Asia, so I'm not familiar with HVAC systems in other countries, but I'm surprised that "C" is novel. If the exhaust air is directly taken from the room where the outoor air is supplied like "A", where does the make-up air for the restroom exhaust air come from? Additional OA is needed to balance with the restroom exhaust, which is not energy saving. This is not the intention of my original question, but just curious

Keigo (2021-08-06 15:41:00 -0500) edit

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Fan Power Limitation Pressure Drop Adjustment for "fully ducted" return or exhaust system in ASHRAE 90.1

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Fan Power Limitation Pressure Drop Adjustment for "fully ducted" return or exhaust system in ASHRAE 90.1