I have a Lennox high efficiency furnace the vents out the roof with plastic piping, both fresh air and exhaust. The fresh air pipe terminates above the roof with a U-shaped pipe at the end of it, I presume to keep out moisture. The exhaust pipe on the other hand just terminates with an open pipe facing skyward.

The problem we have is moisture and insects hit the open end , runs down inside the pipe until it collects in the exhaust blower in the furnace . We have to have a serviceman out approx. every 4 months to unclog the drains , and remove water from the blower.

Can we put a raincap or u-shaped piping on the end of the exhaust pipe? The serviceman says its just a problem with these furnaces that vent thru the roof.

  • Is replacing the dodgy venting an option? Plastic vent pipes on combustion appliances are a known hazard... Nov 20, 2019 at 2:40
  • 1
    Not true, TPE. Every high-efficiency furnace I've ever seen has PVC venting, and we know furnaces up here in the frozen north.
    – isherwood
    Nov 20, 2019 at 2:41
  • @isherwood -- I suspect that people get away with it more often for furnaces due to the HX efficiency drops with filter clogging being less than when a water heater scales up, but it's definitely a known hazard Nov 26, 2019 at 4:31

1 Answer 1


"These furnaces". LOL. It is a poor carpenter who blames his hammer.

I'd put a 45 degree elbow on it, pointing away from the intake duct and the roof (or any other structure). From that elbow add a short length of pipe with a 45 degree angle on it (or another 45 elbow), resulting in a vertical exit face orientation. Should keep 95% of the rain out without undue restriction.

You could also just add a 90 degree elbow, but that may not clear your roof as well. This question shows how vents protruding from walls are set up the same.

Disclaimer: I'm not an HVAC technician nor a code expert. I do have gobs of home construction and remodeling experience, as well as two decades of home ownership and improvement. This is what I'd do. YMMV.

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