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I'm determining if existing 2.5" PVC furnace vent pipes can be moved inside an existing soffit box in my garage ceiling. To do this the exhaust outlet pipe could keep it's existing grade from the terminal on the external wall to the furnace, but the inlet grade would be less.

From here:

All condensate formed in the vent must run back toward furnace for proper drainage. Install vent pipe with no less than a 1/4 inch per foot slope from furnace to vent terminal.

AFAIK condensation isn't a problem on the air inlet and I'd have bigger problems if water was dripping down the inlet into the furnace.

Does the grade of the air inlet need to conform to the same rules as the exhaust outlet?

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No , no slope is needed on the inlet. The exhaust is sloped to discharge or drain the moisture created in the combustion process. the inlet has no water issues. My inlet is open to the sky with a coaxial vent on the roof as the mfg recommends and there has never been a problem with that or others I have installed.

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I would slope it the same, as while it may not be condensate, there are other possible moisture sources (rain, snow) that could get in and a slope away from the device seems sensible to avoid issues.

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  • Thanks, but the slope of the outlet must be toward the furnace so condensate runs into the trap in the furnace then out to the drain pump. All installation examples just show the inlet and outlet running parallel from termination to the furnace, but I can’t see a good reason why this has to be true for the inlet, seems like it’s installed like that for convenience more than anything.
    – Geordie
    Dec 12 '20 at 15:51

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