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Vent heights for a two pipe sidewall

donovanii Member Posts: 1
edited February 2019 in Gas Heating
Hi, Its been difficult to find a discussion online on accumulated snow and vent height. Do both pipes need to be over 30 inches which is what I have on one pipe. The other, exhaust pipe is just a foot off the ground. Two vent systems aren't mentioned but maybe implied here. I'm going with thirty inches for Des Moines, Iowa. These are two inch pipes. But, ok, for two pipes which pipe needs to be higher the intake or exhaust? Here's a photo attached of what I have. The intake pipe plugged with frost when it gets to -9 and below probably because of the warm moist exhaust rising. Of course the the furnace shuts off when it's starved for air because the intake pipe is plugged up. Guess which is the intake pipe? The taller 2" pipe! I think these are reversed. What do you guys think? And is that low pipe too short, I mean should it be around 30 inches or taller? Thank you.


  • Steamhead
    Steamhead Member Posts: 16,272
    edited February 2019
    What is the deepest recorded snow accumulation in that area?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    HVACNUT Member Posts: 5,259
    They are reversed. The exhaust must be above the combustion air for the reason you described but more importantly its causing cross contamination and will kill the HX.
    I'm sure the appliance came with an installation and operating manual. They always show specific parameters for venting their equipment, meeting or exceeding national code.
    Have it corrected, serviced and checked with a digital combustion analyzer.
  • captainco
    captainco Member Posts: 767
    Flue gas is mostly CO2. CO2 is heavier than air and even heavier when it gets cold. CO is heavier than air below 30 degrees. Moisture is heavier than air especially below freezing. So should the flue be higher or lower than the intake? Seems crazy that no one knows that!

    It is also ridiculous to bring in outside air for combustion air when temperatures can drop that low. The colder the air the poorer the combustion. At NCI we recommend you add a Tee indoors on the intake to temper the air and eliminate problems if the outside pipe clogs. The flue pipe should be high but then it will be hard for the moisture to get out and it can freeze. Therefore it is better to use a swing Tee for the flue so the flue gases go up and the moisture goes down. Drilling holes in the 90 degree elbow can help the flue drain.