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Condensation in Attic HVAC System

JabersteinJaberstein Member Posts: 19
Trying to determine the best solution of this problem. My house has 2 HVAC units, one in the basement and one in the attic. The basement HVAC unit mostly heats the 2 story home as it has a 2 story foyer and family room and being so open the basement unit provides the heat for both stories. We use set back thermostats and the attic HVAC system almost never comes on in the winter for heat as the basement unit maintains the thermostat settings of both systems.
The attic HVAC system and ductwork has been sealed and insulated but during the winter months as the warm air migrates into the attic ductwork, furnace and A/C coil, in time condensation forms inside the ductwork and unit.
Considering that the unit almost never runs in the winter in time the system and ductwork temperature will match the attic temperature even though they are well insulated.
So thoughts are maybe put a strip electrical heater in the HVAC unit and cycle it on a thermostat, considered putting a return air damper that would close when the unit is off but unless 100% air tight, I don't think this would work.
Also as I have some access from the basement to the attic maybe run small diameter duct from the basement unit to attic HVAC unit to provide heat into that unit.
If I would have thought of it, I would have put thermal traps in at every diffuser and return air in the attic HVAC system and most likely would not have this problem.
Also if I ever build another home, I will installed sheets of insulated foam sheets on the 2nd floor ceiling before the drywall ceiling as it would have a much better insulting effect than insulating the attic. They do this on the exterior of the home but I have never seen it done to the 2nd floor ceiling even though it would be the same principal.


  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 1,492
    You putting some heat from the basement unit up there would likely cure it. Also you could get magnetic covers for all of your ceiling ductwork, like those used to print vehicle signs on in parades. Install these after you've run the attic unit on heat for a good hour or so. Then turn off the attic unit disconnect with a note attached to remove the magnets when its returned to service.

    If you build another home dont put HVAC in the attic, unless you insulate the roof deck and make the attic part of the thermal envelope.
    Master electrician specialising in boiler and burner controls, multiple fuel systems, radiant system controls, building controls, and universal refrigeration tech.
  • Jamie HallJamie Hall Member Posts: 10,383
    By far the simplest way to control condensation In that attic system -- which will ruin it, even if it doesn't create a mold problem -- is to set your thermostats so that it does, in fact, run in heating mode from time to time -- at least daily. Are you using setbacks? If so, consider setting the attic unit to come on half an hour before the basement unit, for example.

    The question of whether to insulate the attic from outside, using properly installed insulation in the roof, or to insulate the attic from the upper floor, using insulation in the attic floor/upper floor ceiling -- is an interesting one, and open to debate -- but as a rule, if there is any equipment or storage in the attic, my opinion is that the roof should be insulated.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ratioratio Member Posts: 1,955
    Some modern thermostats have a 'circulation' mode for the fan that runs the fan 15-20 minutes out of every hour. That might be enough to keep the ductwork above the dew point.
  • JabersteinJaberstein Member Posts: 19
    Thanks for all the ideas. This is a Ryan home and they are designed to have the 2nd flr units in the attic, they even have 5 special roof truss that make the opening for the equipment room. But the design doesn't make the attic part of the thermal envelope, although one could change that.
    There are many homes in this (Pennsylvania) area built that way with just a fiberglass box discharge and many flex ducts feeding the different 2nd flr rooms. I would think they have the same problem.
    In my case I sized and installed all hard metal duct, sealed it and insulated it. If the house wasn't such an open floor plan I'm sure the 2nd flr unit would come on in the winter, but the fuel is propane so we try to conserve.
    I have been running the fan so get things dried out and the thermostats have a circulation mode might be an easy answer.
    Thanks again
  • Jamie HallJamie Hall Member Posts: 10,383
    What I would be concerned about -- very concerned -- is that you get some circulation through that ductwork on a regular -- daily -- basis -- of air which has relatively low humidity. This means warmed, if not actually heated. A circulation mode might do -- but I'd still rather see the heat turn on and circulate at least once a day. This is not going to cost you extra propane; the heat goes into the house anyway, whichever HVAC system is running, so this isn't a conservation factor.

    What it will do is sharply reduce, if not prevent, mold from growing thin those ducts, which it otherwise will. This you do NOT want. Duct cleaning doesn't come cheap, and mold problems can be very expensive, never mind hard on your health.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • pecmsgpecmsg Member Posts: 720
    Depending on environmental conditions indoors it takes several days to weeks to develop enough condensation to pool.

    You need to run that system at least once a day. 10 - 15 minuets can make all the difference!
  • ChrisJChrisJ Member Posts: 9,666
    Running the heat isn't going to do anything for the return side that simply running the blower won't do.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • HVACNUTHVACNUT Member Posts: 2,120
    Good call on getting rid of the flex ducts. Prime spots for condensate to settle. It went from a spider system to a robot system. I wonder where the border line is for using fiber board vs sheet metal. Very rare on L.I.
    Like the others said, you have to run the heat. I've seen many horizontal furnaces and A/H's get pretty nasty with mold.
    Upper level new construction here uses spray foam in the roof rafters and no soffit vents. I'm comfy up there in August.
    A dehumidification system can be installed in the attic and ducted in to the HVAC. Automatic circulation too.
    Or even an ERV.
    Theres also a product they're using called AeroSeal. Not DIY. No hands on but saw a live demo. Supposed to seal holes up to the size of a nickel in the ductwork.
    Also. Is this a condensing furnace? You dont want water freezing in the traps either.
  • ChrisJChrisJ Member Posts: 9,666
    Maybe someone asked and I missed it.
    But, what RH are you maintaining in the home?

    Do you ever get condensation on your windows? There is a limit to how much humidity you want in a home relative to the outdoor temperature.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • JabersteinJaberstein Member Posts: 19
    Thanks again for all the suggestions. Some additional points, the furnace is a condensing furnace and the exterior casing has
    1 1/2" duct board on it. Because it is a condensing furnace the condensate line and traps are heat trace and cycled by the thermostat. Also I do have a ERV system connected to the system but have chosen not to use it. Why, because they put a cow farm across the street and I no longer want to bring in outside air. I also have a wood burning fireplace that heats 250 cfm's of outside air but because of the cow farm I now use house air for that intake.
    The humidity is kept at around 40% in the home and I have a humidifier on the basement HVAC system that I never use.
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