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Open Intake - Hi Eff Furnace

I just moved into a home with a high efficiency furnace. There is an air intake that feeds into the top of the box. At one point, the previous owner removed the middle pvc portion of the vent so that the furnace is taking in the air from the basement instead of outside air. I feel I should replace the pvc since it's obvious that's how it was designed, but feel there must be a reason the previous owner did that.

Can you help me?


  • Brad White
    Brad White Member Posts: 2,398
    Forget the reasons that

    the previous owner did that. It does not matter and none of us would responsibly take a guess. (OK, maybe he needed stock for a plumbing repair- that is good as any! :)

    The furnace should be installed according to the manufacturer's recommendations. What you are dealing with is apparently the combustion air intake. It had better not be the exhaust.

    Without that full circuit connection it is possible that the combustion fan differential pressure switch no longer has a true reference. Also if open to the basement you could induce chlorides into the combustion air stream from laundry bleach or from paint stripper in a basement workshop. This will eat stainless steel over less time than you think.

    Install to specifications is my only real advice.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • MVP
    MVP Member Posts: 2

    ... He's already won! I've heard of many cases where maniacs break something or destroy something before they sell a home... My thought is that this old **** just cut out the pvc to make your life harder... Think about it rationally! Would you rather have FRESH air coming into a "FRESH AIR INTAKE" or stagnant-humid-musty-non circualting basement air??
  • Home Depot Employee
    Home Depot Employee Member Posts: 329
    Who knows

    Depending on the manufacturer and model, some units were manufactured as dedicated (requiring the two pipes) and, dual certified (your choice of running intake pipe).
    My thoughts are this:

    As mentioned earlier, why would a seller purposely sabotage the install with today’s tight sellers market with the possibility of holding up the transfer of deed during an inspection or final walk-thru? The thought of that is ludicrous.

    More than likely these are the reasons for the missing pipe. 1. During install they ran out of PVC and never finished. 2. Someone convinced the previous owner to remove the pipe to assist the air changes in the furnace area to provide fresh air via infiltration for reducing humidity or odors. (This happens all the time when replacing old inefficient equipment vented atmospherically into chimneys and after replacing suddenly the relative humidity spikes in the structure) 3. Finally, (My vote without seeing) the venting is incorrect and the unit only fires without complications with the intake pipe removed, allowing pressure switches and safeties to close/remain closed and burner to function.

    When given the option of two pipes on a furnace, they should be used. Thus, increasing efficiency, reducing infiltration and most importantly providing clean, uncontaminated air for combustion, prolonging the life of the unit’s components.
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    I agree

    I agree with 3#. I'll bet that if the intake is reconnected full length to the outdoor termination, the furnace will fail to light. Find the manual and check to see what size pipe of what length and using how many elbows are allowed. The other possibility is that the section that was cut out may not have been supported or pitched correctly and allowed moisture to "puddle" in it causing a pressure switch failure.
  • will smith_4
    will smith_4 Member Posts: 259
    Intake air

    John-While you lose some efficiency points on a two-pipe 90%er if you use indoor air for combustion air, assuming your system isn't in an enclosed space with insufficient air FOR combustion, there's no reason to worry about it as is. That being said, I've worked in some towns that require a fresh air feed for the mechanical room even if you have a standard 80%-to make sure that there is enough fresh air for combustion. Super tight houses do require outdoor air for combustion; at my house, the kids provide all the fresh air you could ever need (running in and out a million times a day)!
  • martin
    martin Member Posts: 144
    Air Intake

    Another reason for the removal could be the way the pipes are terminated outside. On a cold day if the intake can suck back any of the exhaust it will plug with ice crystals causing a no heat until it is cut thawed and maybe never put back togther.
  • Home Depot Employee_2

    You are correct plumbaria, but this happening is rare, unless extremely north (snowmobiling yet?)

    Even so still points back to improper venting.
  • Tony_23
    Tony_23 Member Posts: 1,033
    Not that rare : )

    There seems to be no shortage of people who can't read instructions and follow them.
  • bill_71
    bill_71 Member Posts: 46

    my two cents.......there was probably a bird or something stuck in the fresh air and his neighbor buddy came up with that quick and cheap fix instead of figuring out where exactly the bird was stuck
This discussion has been closed.