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Thread: What should hot air temp be for forced air heat ?

  1. #1
    DIY Junior Member clownfish's Avatar
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    Default What should hot air temp be for forced air heat ?

    I have a trane xv80 forced hot air system (gas fired). The "hot air" blowing out is only a little warm at best and less so the further I get from the blower. I'll put a thermometer on it when I get home, but what ballpark reading should I expect?
    The unit is in the basement, two returns (one on each floor besides basement). The unit runs for what I think is a long time, maybe 30-40 mins (or more) to go from 62-67 deg (25-35 deg outside). It always seems to be on.
    This is my first winter in this house (2000 sq ft) so not sure what to expect here...
    Also, the steel ductwork is not insulated, should it be? there is also a humidifier on the unit, not sure if that has an effect.

    Thanks for any input here.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    It would help if the ductwork was insulated, especially if it runs in unconditioned space. Does the house also have an a/c? The fan speed may be set too high for heat (but okay for cooling). Do you have a CO detector? Always a good idea with fuel burning appliances in the home. The design delta T (rise in temp) range should be listed in the furnace's manual. If you don't have yours, you might find it on-line. That would tell you what is normal. You'd want to check the outlet temp at the furnace, since the length and location of the ductwork could affect the reading a lot. Taking a long time to raise the temp is not necessarily a bad thing - it indicates that the furnace may not be severely oversized, like in many older (and newer, too) homes. A furnace that runs nearly constantly and maintains the desired temp will produce the most comfortable dwelling. If it is very close to the heat required in the house, it may not have enough reserve to set it back at night, or you'll see that long recovery time.

    A humidifier can cool the output some...evaporative cooling and all. But, normally, it isn't a big factor. Generally, a properly humidified house will feel more comfortable at a cooler temp than a dry one since you won't sweat and cool yourself as much.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    The XV80 is a 2-stage unit with a variable speed blower, and SHOULD be running cooler/longer than a typical bang-bang gas furnace. Most 2-stage units (I'm not sure about the exact specifics on the XV80 controls) will step up both the flame and the blower speed if the thermostat isn't satisfied in something like 12-15 minutes, but always starts up in low-fire low blower speed mode. Does your air flow seem to speed up after it's been running for 15+ minutes?

    The ductwork should be both mastic-sealed on the seams & joints (or FSK- taped with 2" aluminum tape, though that isn't as reliable) AND insulated to at least R6 (inside of conditioned/semi-conditioned space), or R8+ if the ducts run in a vented crawlspace or attic. Clearly it's more important to insulate the supply ducts than the returns, but both need to be sealed. In conditioned space you can skip insulating the returns though.

    Thre are about 10 different models with different high/low fire BTU output numbers, which will make quite a difference in how long it takes to recover from a 5-degree setback. But 30-40 minutes when there's a 50% heat load isn't unreasonably long- it may be on the short side if the unit is perfectly "right-sized" for your coldest-temp conditions. But recovery time from setback is often a better measure of the thermal mass of the house than anything. Read off the whole model number (probably TUD********* ) on the name plate, or find the BTU input numbers on a plate somewhere- it's in there. Then with your zip code and description of the house's window size/type and insulation levels it's possible to take a WAG at what your design condition heat load would be.

    Exit temps at the registers at high-fire high blower speed should always be above 100F (body temp), but at low-fire & low blower speed tepid temps in the 90s are usually acceptable, since there's less induced "wind chill" from the air flow. Most ~80-85% AFUE furnaces will have output air temps in excess of 125F right at the furnace's heat exchanger at high fire, with 110F+ at the registers if the duct design & implementation was decent. At low fire they're milking it for highest efficiency (==lowest exhaust stack temp), but that doesn't always mean super-low air temps.

    The variable speed blowers are a nice feature, and use a fraction of the electricity of single-speed split capacitor type blower motors.

    Humidifiers are mold-inducers (and if never cleaned mold SPREADERS). In most of the lower 48 wintertime humidity under 30% (dry, uncomfortable, staticky) are an indication of too high a ventilation rate, which is most instances means your house leaks air like a sieve. Adding humidity to the air then causes higher rates of condensation and mold in the walls/ceiling/attics along the exfiltration path. The "right" solution is to tighten up the place, not add humidity. It's nearly impossible to retrofit tighten up a pre-1990 house (and many newer homes) to the point where indoor air quality become problematic, unless you smoke, cook without running exhaust ventilation, and never run bathroom exhausts, etc.

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