why the autosized design air flow rate differ so much in different floor?

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It's almost surely your first floor slab. The default configuration and U-value for these constructions are usually very poor.

-James

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Thanks James.
But I'm afraid it is not that reason. I set the ground floor as adiabatic but the design air flow is still high, just very little lower compared to the previous one.

It's almost surely your first floor slab. The default configuration and U-value for these constructions are usually very poor.

-James

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Five to ten times? Check your occupancy carefully. It sounds like
outside air to me. You either have no occupants on the second floor or
ten times the occupants on the first floor.

Robert Wichert P.Eng. LEED AP

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I think I have known one of the reasons( if more than one).
I set a self-defined wall in the Wizard data edit mode, using the R value definition. However, the R value automatically became U value under Detailed data edit mode. So the roof heat conduction is very large. Besides, I set a ceiling in the second floor which exhaust the hot air out, so the roof conduction load in the second floor is zero. These settings caused the first floor load much larger than the second floor.

Five to ten times? Check your occupancy carefully. It sounds like outside air to me. You either have no occupants on the second floor or ten times the occupants on the first floor.

Robert Wichert P.Eng. LEED AP

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An extension of Robert?s advice: Check your first floor?s inputs for increased infiltration loads the second floor does not have. This behavior may be normal, considering doors/vestibules/windows.

I don?t think I?m totally following your exhaust strategy, but another general caution per your word choice: You may not want to have a ?roof? on your first floor. If you ultimately do have a roof (exterior) surface sandwiched between your floors (easy mistake if you defined each floor as a separate shell), you have possibly nearly doubled your roof loads for the building. Much further discussion on this and how to handled inter-floor heat transfer in the archives, but planes sandwiched between shells should generally be either interior surfaces of some sort (air/framed/adiabatic), or deleted.

~Nick

[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.

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