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Community and Q&A

High Efficiency Furnace in Ventilated Attic

zzvl | Posted in Energy Efficiency and Durability on

Hello! I have an american standard high efficiency direct vent condensating furnace. It is installed in an uninsulated, ventilated attic (the floor of the attic is heavily insulated, but the roof is not), though it shouldn’t have been…

Recently, there’s been a leak from the furnace (drain pan has water in it, ceiling of second floor (right below furnace) is damaged. I had two HVAC techs come to the house and say that the furnace is likely damaged because condensate froze in the winter. They recommended sealing my attic with insulation and then replacing the furnace or using a lower efficiency furnace (which I’d prefer not to do). I live in the boston area.

Reading the installation guide for my furnace, it says “the condensate drain should be installed with provisions to prevent winter freeze-up of the condensate drain line. Frozen condensate will block drains, resulting in furnace shutdown. If the drain line cannot be installed in a conditions space, then UL listed heat tape should be applied as required to prevent freezing. The heat tape should be rated at 5 or 6 watts per foot at 120 volts.” This seems to imply that heat tape would be enough to prevent future damage (and prevent me from paying to insulate the attic, which I’ve gotten quotes for at about $8k).  Right now there is only foam (pool-noodle) insulation around the condensation drain and both techs told me that probably wasn’t enough, which caused the condensation to freeze and crack my drain pain / furnace so that it leaks.

My questions are
1. Would heat tape (eg on the condensation pipe be enough to prevent freezing (without insulating the attic)?

2. Is it normal for a drain pan to have water in it at all? One of the HVAC techs said no and pushed a complete replacement of the furnace in addition to the insulation. The other HVAC tech suggested I could just replace the drain pan after insulating the attic. 

I’d also love to hear any other thoughts. 

Helpful videos of my space: – video showing my attic – video focused underneath furnace

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  1. walta100 | | #1

    Yes your drain line should have been heated.

    The condensate from the furnace should be collected in the furnace and empty into the heated drain line. You should have an emergency drain pan under the furnace with a separate drain to some place you would see the water making you aware the main drain is stopped.

    If you have water in the emergency pan all the time I would be concerned that when the pipe was frozen water backed up the drain into the furnace and froze and damaging the heat exchanger and making the furnace unsafe to use. Did you say you have 2 CO detectors in your home?

    In my opinion it is a very bad idea to have HVAC equipment in unconditioned spaces it is only slightly less bad idea to move the insulation to the roof line and condition the attic. Getting the equipment into the house where it belongs is almost impossible at this point so you are left picking between the bad options.


    1. zzvl | | #2

      thank you so much for your reply.

      We do have CO detectors in the house. The heat has been working fine, but there has been water the last few times I've gone up in the drain pan. I wonder if the drain from the pan is somehow clogged.

      It is a shame the builder put the furnace in the attic especially because the basement had plenty of space.

      I have 3 neighbors with the exact same set up. 1 of them had an issue a few years back with freezing, but the others have been ok. I wonder if that means adding the heat tape might be the best (still bad) solution.

  2. walta100 | | #3

    Do you have 2 separate drains?

    Sometime they fail to put a drain on the emergency pan and that is ok if a float switch on the pan disables the furnace. This photo looks like a float switch that may or may not be connected( red wires).

    Any water in the pan indicates a problem. If any water leaks out of the pan before the pan overflows indicates a defective pan. The pan should be level enough that the float activates before the pan overflows.

    Any damage to ceiling indicates you have 2 or more different faults that all need to be repaired and a viable home owner’s insurance claim.


  3. Expert Member
    Dana Dorsett | | #4

    Putting furnaces &/or AC in the attic in a Boston climate is truly insane. It creates ice damming problems for the roof, guarantees frozen condensate during cold snaps (unless taking all of these Mickey Mouse/Goofy mitigation measures), and punches holes in the building envelope (at the register boots & duct chases.)

    Before replacing any equipment, run this math:

    Since the ducts and furnace are in the attic a fuel-use load-calculation overstates the real load for just the house. It may be possible and worthwhile to replace the furnace with a right-sized heat pump or a right-sized gas furnace likely to be half the size of what's there currently, and put it in the basement rather than the attic.

    A right sized cold climate heat pump could still live in the attic, even though that's not as efficient as bringing it inside the building envelope, and the operating cost at Beantown gas & electricity prices is about the same as an minimum-legal 82% efficiency gas-burner (though more expensive than a right sized condensing gas burner.)

    Is this condo or townhouse? The very low loads of newer code-min town houses mean you could probably retrofit a suitcase sized Dettson Chinook C15-M-V tucked into the back of a closet or cabinet somewhere and still have plenty of margin for cold snaps. The Dettsons can even use ~2.25" diameter "ducts" that can be routed through non-structural stud. A fuel-use load calc would tell us how likely that would be. See:

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