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How to deal with condensation in a cold attic furnace?

hers_rater | Posted in Mechanicals on

I am a HERS Rater and one of my builders said he received callbacks 15 homeowners on completed homes with 95% furnaces in unconditioned (cold) attics this past week. While I suggested they may have been not able to perform due to extreme cold conditions, we then got onto him going back to 80% furnaces, which I do not want to encourage. So here is my question:

Condensate line in attic furnaces – how to do them right without building a room or spray foaming the roof system. To keep them outside thermal envelope but not have a problem with condensate.

Researched ACCA, Building America, NREL – nothing useful here ….
Here are solutions I have come across from reading, talking to subs in the trade:

– 2″ drain line, gravity fed, into building envelope from attic, maybe a trap in wall system if needed (why bother with that?)

– insulate the condensate line in attic. I envision a 1 or 2 inch PVC drain with insulation

– use a single system, fed through chase from basement – get rid of second system

Not sure if there are summer considerations but winter concerns are prevalent here.

Here’s the real deal: HVAC subs want to keep doing what they know and never get a callback. 80% furnaces work for them.

Help a HERS Rater out so I can help my clients. Give me your best advice please.

Thanks in advance. Tom B

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  1. Expert Member
    Dana Dorsett | | #1

    Seriously, furnaces and ducts should NEVER be in an unconditioned attic above the insulation. (Neither should air conditioning ducts.)

    If that can't be changed and it's impossible to use a single system located in a basement or some other conditioned/semi-conditioned space, a small condensing boiler (located inside conditioned space) and a an insulated hydro-air air handler could be used (with the appropriate anti-freeze, not pure water) with the same ducts.

    Better yet, a condensing water heater set up as a combi-system, microzoning with room-by-room hydro-air coils can be easier to install than hacked separate duct system coming up from the basement, eg: (There are others.)

  2. hers_rater | | #2

    Thank you but that isn't my question.

    My question is with real constraints (a 95% furnace in an attic with the need for condensate drain, and potential for freezing), what is a best practice under these circumstances.

    My builders are struggling to maintain profitability in a competitive market.

    Thank you.

    1. tommay | | #10

      Well just like any drain line, the water should exit the drain and there should be no concern about freezing. If there is a condensate trap, insulate it or just remove it, since it probably just runs outside from an attic space.

  3. user-2310254 | | #3


    In line with what Dana's was suggesting, your best option is to move the HVAC to the basement. If the basement has code compliant insulation and air sealing, I would imagine the space would stay above freezing. (But it would be helpful to know your location.)

  4. Expert Member
    Dana Dorsett | | #4

    It wasn't clear whether this was for retrofit or new construction.

    The right answer for new construction would be:

    "- use a single system, fed through chase from basement - get rid of second system"

    80% furnaces are technically illegal it has to be at least 82%.

    At the actual heat loads of new IRC 2015 code-min houses, a right-sized hydro-air air handler married to a condensing water heater (used as a combi-appliance) located inside conditioned space is competitive with than a separate furnace & water heater.

    Almost all furnaces out there are ridiculously oversized for the space heating loads they serve, well over ASHRAE's recommended 1.4x oversize factor at the 99% (not 99.6%) outside design temperature. That's probably because many builders don't do design math pick a unit big enough to heat the biggest house in the development at an outside temperature of -50F and use it on all houses. The upcharge for that ridiculous oversizing is less than it costs to pay for the load analysis to right-size it. They could arguably do the same thing with hydro-air air handlers if they can be convinced that the loads are as low as they actually are.

  5. mackstann | | #5

    Pipe heating cable and pipe insulation seem like a reasonably easy fix. Anything electrical is of course subject to additional energy use and possible failure, but given that we are indirectly advising what sounds like a more cookie cutter builder and not a high end green builder, I don't think that "redesign and remodel the heating system of 15+ homes" is a realistic pitch...

  6. GBA Editor
    Martin Holladay | | #6

    My first reaction is the same as Dana's and Steve's: What builder in their right mind would put a condensing furnace in an unconditioned attic -- especially in a freezing climate, and especially since these appliances need a condensate drain?

    My second reaction is, "Call up the furnace manufacturers and find out about their installation requirements." If the manufacturers allow this type of installation (in a room subject to freezing temperatures), then follow the manufacturer's advice for preventing the drain from freezing.

    If the manufacturers don't allow this type of installation, then we're talking about a code violation.

    1. tommay | | #11

      I had a call this year for a condensate pump not pumping, ended up being a frozen condensate line from a furnace in a basement. The original installer ran the condensate line outside then down along the ground. Of course it froze. Just had to cut the line above grade a couple feet so all the water drained out.

  7. Mightymason | | #7

    I have 1932 original structure ranch no basement with boiler. I retrofit with 96 high e furnaces in attic space. Found really good heat tape and used pipe insulation. Never had a freezing condensate line

  8. Expert Member

    "My builders are struggling to maintain profitability in a competitive market."

    Missed that little gem the first time round. As though it is some kind of excuse.

    1. Expert Member
      BILL WICHERS | | #9

      Sounds like a recipe for corner cutting.

      I’d check everything else very carefully to see what else may have been done on the cheap. I’d be especially cautious with electrical and mechanical systems since those tend to be the most hazardous when installed cheaply.


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