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  1. #1
    Join Date
    Jun 2002
    Location
    Ottawa, Ontario
    Posts
    6

    high efficiency furnace hoar frost problem

    We bought an american std high-efficiency furnace a couple of years ago. Every winter it stops working at some point due to buildup of hoar frost on the inside of the cold air inlet pipe. We've had technicians come out to look at this and they've said that the intake and outlet are positioned backwards - that the hot/moist air should not be exiting down below because it rises and then the moist air freezes on the cold air intake as it is being recycled into the furnace.
    The office at the company who we bought from disagrees each time with the tech and says the vent install is correct. (Of course that doesn't change the fact that every winter hoar frost blocks the intake pipe, so basically we don't have a reliable working furnace.)

    It would make sense to me, assuming that the total distance and bends in the pipes permit, to raise the hot exhaust significantly and also to raise the cold inlet (but lower than the hot exit), since without the hot air exiting below, blockage due to snow buildup would become a concern. But, perhaps there's some reason this wouldn't work or be permitted due to codes? I don't know. At any rate, I'm anxious to figure out some sort of resolution to the symptom, whether the theory as to the cause is correct or not.

    I've attached a photograph of the actual pipes as they exit from our home, annotated as to which is the hot out and which is the cold in.

    Can anyone offer any advice as to what needs to be done to fix our hoar frost problem?
    Attached Images Attached Images  

  2. #2
    Get a copy of the installation manual.


    See page 11 and 12 of this manual. Your air in / out setup looks reversed from this setup. The correct slope of the air in pipe and air out pipe matters also.


    http://www.thermopride.com/manuals/mg-1006.pdf

  3. #3
    Join Date
    Jul 2008
    Location
    Alabama
    Posts
    658
    its definitly piped wrong. they should be 24" apart and the inlet should be on the bottom. i can see why you are having this problem with the ice. the inlet should be 12" from the outlet too.

  4. #4
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    The installing contractor needs to read the installation instructions, they way they have the pipes terminated is completely wrong, and they need to fix it.

    What is the model number of the furnace.
    If more government is the answer, then it's a really stupid question.

  5. #5
    Join Date
    Jun 2002
    Location
    Ottawa, Ontario
    Posts
    6
    Hi, Thanks to everyone for the confirmation that the vents are not installed right.

    The model is American Standard AUY100R9V4W6.

  6. #6
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    What on Earth is hoar frost?

  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Installation instructions:
    http://www.markbeiser.com/HVAC/docum...UI-install.pdf

    Page 17.
    If more government is the answer, then it's a really stupid question.

  8. #8
    Join Date
    Mar 2003
    Posts
    7,321
    Hoar Frost: (N.) a buildup of moisture in the solid state is caused by streetwalkers to be too fridgid. ex. My wife was really great when we married, but now she is full of nothing but hoar frost.

  9. #9
    Join Date
    Jun 2002
    Location
    Ottawa, Ontario
    Posts
    6
    Hoar frost forms when warmer air with water vapor in it hits a colder surface. Ice crystals form by a process of sublimation -- going from gas to solid directly without passing through a liquid state. It tends to form largish white crystals. In my case, the moist air from the vent gets drawn into the intake and past the very cold ends of the intake pipe, where the vapor is deposited and forms ice crystals that end up blocking the intake such that the furnace no longer functions.

    Some of it that's near the end of the intake can be cleared out manually, but not all of it can be extricated which means that in addition to the air going into the furnace being moist, eventually the hoar frost melts and goes into the intake as liquid water. Hopefully that would not have done any damage to the furnace (anyone know whether that would be a big concern -- I mean I would assume it's not something you would want but is it really bad or just not a good idea?).

    Eric

  10. #10
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by eajohnson View Post
    Hoar frost forms when warmer air with water vapor in it hits a colder surface. Ice crystals form by a process of sublimation -- going from gas to solid directly without passing through a liquid state. It tends to form largish white crystals. In my case, the moist air from the vent gets drawn into the intake and past the very cold ends of the intake pipe, where the vapor is deposited and forms ice crystals that end up blocking the intake such that the furnace no longer functions.

    Some of it that's near the end of the intake can be cleared out manually, but not all of it can be extricated which means that in addition to the air going into the furnace being moist, eventually the hoar frost melts and goes into the intake as liquid water. Hopefully that would not have done any damage to the furnace (anyone know whether that would be a big concern -- I mean I would assume it's not something you would want but is it really bad or just not a good idea?).

    Eric
    Thanks for the vocabulary expansion!

    Water in the furnace should have no ill effect - it is actually there by design in the vent pipe, and both the vent and fresh air intake should pitch back to the furnace. This is so any water that does form or is introduced gravity drains back through the furnace and into the condensate pump (or whatever is recieving the drain.)

    In short - the internals are well suited to deal with moisture / water, and no harm is likely.

    Obviously, as you know, your installation needs to be fixed by the nstaller because of the - hoar frost blocking teh air intake. That's a fun word to use.

  11. #11
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
    Posts
    769
    Quote Originally Posted by flange View Post
    Hoar Frost: (N.) a buildup of moisture in the solid state is caused by streetwalkers to be too fridgid. ex. My wife was really great when we married, but now she is full of nothing but hoar frost.
    lmao.

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,923
    Yep. Its piped wrong.

    Ask the office of the installing ontractor to show you in the install manual, were it says its proper.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  13. #13
    Join Date
    Sep 2008
    Posts
    33
    I would also make sure that the pvc meets the page 15installation instructions from MB's post as well.

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