floor vent air temps

Discussion in 'HVAC Heating & Cooling' started by jeremytl, Oct 20, 2009.

  1. jeremytl

    jeremytl Scientist

    durham, nc
    Hi all,
    This is our 4th winter in this house and I'd like to learn how I can make my heating system run a little more efficient. The house was built in '65. It is a one story ranch at about 1,350 square feet. The crawl space underneath, where the furnace operates, ranges from 2' to 3.5 '. The air ducts are the old square galvanized ducts approximately 18" wide and 12" high. The furnace uses propane.

    We had the floors insulated last year, it helped a little. The air coming out does not seem to be as warm as I would expect. I haven't put a thermometer down in one of the vents yet but it barely feels warm. The house heats ok, it seems.

    What temp should the air coming out of the vents be and can those air ducts under the house be insulated?
  2. jimbo

    jimbo Plumber

    San Diego
    For sure the ducts should be insulated. You are loosing a tremendous amount of heat off those uninsulated ducts
  3. jeremytl

    jeremytl Scientist

    durham, nc

    I assume there are companies that insulate them? When I had the floors insulated, I asked the contractor what he thought about insulating the ducts and he seemed very disinterested.

    What do you think would be a ballpark price for one of the companies to charge? Again, its a 3 bedroom house with a whopping total of 7 floor vents.
  4. Dana

    Dana In the trades

    Before insulating the ducts, seal the joints & seams with duct mastic. (If that's too messy for ya, FSK tape is usually good enough, if the ducts are shiny-clean.) "Typical" duct leakage in one CA statewide survey was ~25% (fully a quarter of the air was going somewhere other than intended, and if they're in the crawl, it's outside of conditioned space, and a complete loss.)

    Insulating over unsealed ducts does has very little impact on the leakage area- they really need to be tightened up first. Sealing alone might improve both temperature and flow enough to feel the difference.

    Depending on the duct configuration, it might be easier to seal & insulate the crawl rather than insulate the ducts. (It usually requires laying down a ground vapor barrier and closing off any vent grills, and adding a tiny amount of ventilation of conditioned air as well, then using rigid plastic or spray foam against the crawlspace walls.)

    Guesstimating a contractor's cost for the duct job is impossible. Some crawl space jobs are major pain, others fairly straightforward, and the installed price difference could vary by as much an order of magnitude. If it makes a difference, have them quote sealing all ducts, but only insulating the supply ducts. You'll still be losing some heat, but the comfort factor will be much-improved. (The loss would still be cut by well over half, since the supply ducts run typically 50F+ hotter than the returns.)

    Exit air temps under 105F tend to feel lukewarm, under 100F (body temp) feels cold. Most find 110-130F pleasant, but over 140F is often referred to as "scorched air" by folks pushing hydronic or "hydro-air" systems (=hydronic boiler + coil in air handler.) Geothermal heating systems often have sub-100F temps, which requires placing the ducts where they won't actually blow on the occupants- it's definitely not a warm, or even lukewarm breeze, more like wind-chill factor.
  5. SteveW

    SteveW DIY Senior Member

    Omaha, NE
    Even though you tell us the furnace is propane - just want to make sure - you don't have a heat pump by any chance, do you?
  6. Runs with bison

    Runs with bison Member

    Agreed, seal the ducts first. Until relatively recently, duct sealing has been very haphazard. All the old type seams appear to be leaky to me. Make sure the register boxes fastened to the duct runs are sealed too. Often the crude metal cut outs have large gaps in them.

    Make sure the air handler/filter box are well sealed too. Mine was leaking copious volumes of conditioned air where it joined the return below and the supply above.

    I've skipped the mastic and have been using clear GE silicon II on every exposed seam. I think I've gone through 7 tubes so far.

    While most of my leakage was back into conditioned space it was causing "short circuits" back to the center of the home and the HVAC. The corners that really needed the air weren't getting it, despite fiddling with the register dampers.

    As for insulating the main trunks, I'm not sure what a good material would be. I used some R3 stick on material for 6' of large supply duct in the utility room. Between this, replacing some bare round duct with insulated line, sealing the many large gaps, etc. the delta T in the utility room is ~10 F less than what I measured when I started this a year ago (both heating and cooling seasons.)
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