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?
Best Answer
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.