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  1. #1
    Join Date
    Mar 2005
    Location
    RALEIGH
    Posts
    39

    Cool

    I was told by an old friend of mine that has been doing HVAC longer than me that you can place the coil on the return side of a furnace. I myself was always taught that they can only be installed on the supply side. Will it work properly if the coil is installed on the return side and if so in what circumstances or what kind of equpiment????

  2. #2
    If you think about RTU's it is done all the time.

    However, it's not a recommended solution to install issues. It can create more problems down the road. Just think about what that heat exchanger will be doing when water starts condensing on it!

    So always install the coil on the supply side.
    It's easier to blow threw a coil then, to pull threw one?

  3. #3
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,415
    Most if not all air handling units (heat pump or straight cool only) have the coils on the return. Never do that on a furnace though, it'll condense on the heat exchanger and accelerate it's death.

  4. #4
    Join Date
    Nov 2001
    Location
    east kansas
    Posts
    8,033
    And in violation of the Uniform Mechanical Code.

  5. #5
    Join Date
    Mar 2005
    Location
    RALEIGH
    Posts
    39
    Thanks guys thats exactly what I was thinking. I just wanted to make sure because from what I was told is that it can be done but from what I was taught is that it should never be done.

  6. #6
    Join Date
    May 2001
    Location
    Kansas City
    Posts
    2,729
    Coil always goes on supply side of fuel burning furnace's.
    Always below blower and heat strips on electric heat or no heat air handler.

  7. #7
    Join Date
    Feb 2005
    Location
    small island in the Pacific Ocean
    Posts
    558
    In Canada you can now put the coil in the return of a duel fuel system. In the past the thought was if it leaked and the burner was on it would send phosgene gas into the supply, for whatever reason the code has been changed. You can have a natural gas furnace and a heat pump, with the coil in the return, operate at the same time.

  8. #8
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    I can see how in heat mode a coil before the furnace would work fine, but the issues are the same in cooling mode with moisture on the heat exchanger. Most heat exchangers these days are luminized (don't know correct spelling) and I would think that should protect them somewhat from condensation damage. On an older furnce I am sure the exchanger would just rust like crazy.

  9. #9
    That condensation has to go somewhere. Even if the heat exchanger is aluminized or stainless steel, would you want water dripping inside your furnace?

    I don't!

  10. #10
    Join Date
    Aug 2004
    Location
    Apex, North Carolina, United States
    Posts
    57
    I've seen a coil on the return on a furnace, it took out the heat exchanger in a little over a year. Could this be one of the main problems with package units heat exchangers since they are all draw thru systems?

  11. #11
    It's been my experience that most of them have rusting issues.

  12. #12
    Join Date
    Jan 2001
    Posts
    7,913
    Packaged gas heating and cooling.....also see a lot of commercial air conditioining with gas duct heaters....

    Its hard to tell if the its the moisture from the ac or just the substandard quality of materials that causes the heat exchangers on these units to fail... One things for sure...when you see a package unit...it was designed by the manufacturer to be that way...its not some cobbled up mess that some slack jawed would be hvac engineer thought up to sell a job.....

    Ive always been taught that humidifiers on returns of gas furnaces were bad too....but I see them all the time around here...steam type humidifiers on returns....I always tell the unlucky recipient of that mess that its wrong and that they need to have that exchanger checked at least once a year because of the fact....but after seeing so many furnaces with heat exchangers that have actually lasted for over a decade plus with such a set up I gave up on shutting off the humidifiers and starting a bees nest of trouble telling the customer that the humidifier installation would probably get them killed........just document it and go on.... Ive seen them on new installs too....passed by the local building inspector

  13. #13
    Join Date
    Jun 2003
    Location
    Hell Hole Swamp
    Posts
    4,180
    I thought the condensation would form on the inside of the heat exchanger, not the outside, due to the flue being open to the outside air, why would a heat exchanger sweat with a coil in the return but not the blower or any other metal inside the cabinet? doesnt make sense

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