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Thread: running a high ef. furnace at a low temp.

  1. #1
    DIY Junior Member
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    Default running a high ef. furnace at a low temp.

    i was told by a furnace installer from a reputable company that a high
    efficiancy furnace should not be run at a thermostat setting below 65
    degrees or so. he said it would damage the furnace. i am considering
    a new furnace, but i like to keep my house cooler than that. i would
    like some professional opinions.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I'd ask the manufacturer.

    Note that the smaller the delta T (difference in temperature) between outside nd inside, the smaller the heat loss and therefore the size of furnace required to keep the house at the desired set point. So, the colder you keep the house, the smaller the furnace required, even for the same house. A furnace would not need to run as long if it was sized for a higher temperature, and short cycles are less efficient. Just like running your car around town on numerous short trips when it doesn't have time to fully warm up. So, depending on how much oversized the furnace really is, it might be an issue. Now, if it was sized for that temperature, it would likely be little different than one sized for the higher temp.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by handywolf View Post
    it would damage the furnace.
    You have a link to your make & model of furnace? I'd like to hear this by e-mail from the horse's mouth.

  4. #4
    Senior Robin Hood Guy Ian Gills's Avatar
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    Cooler than 65? Wow. Ever considered moving into the fridge?

  5. #5
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by handywolf View Post
    i was told by a furnace installer from a reputable company that a high
    efficiancy furnace should not be run at a thermostat setting below 65
    degrees or so. he said it would damage the furnace. i am considering
    a new furnace, but i like to keep my house cooler than that. i would
    like some professional opinions.
    I would say that your thermostat has NOTHING to do with how hot your furnace runs. This is a very common misconception though.

    Your thermostat is merely a switch and tells the furnace to turn on or off. The furnace has no clue what-so-ever what the temp of the thermostat is set at.

    As to why running a furnace in cold conditions like that would be hard on it I don't know... unless like pointed out -- the furnace is short cycling.

    Now if it's a HE furnace that would mean you have plastic venting and it's likely to be a condensing direct vent unit.... so you don't have to worry about venting corrosion.

  6. #6
    DIY Senior Member Rich B's Avatar
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    I installed a Rheem high efficiency nat gas furnace 4 years ago. It is a condensing model with a 2 stage burner and is a hot air system. I routinely turn it down to 60 or lower when I am not home to save $$$. I have seen no problems doing this. I believe the heat exchanger is stainless steel and there is no issue with a chimney because as was mentioned it is PVC now and vents out the side of the house. I saw about a 20%+ reduction in gas bills the first season after it was installed. Nat gas prices really went up the last couple years and I am happy I did this install. I am anxious to see my latest gas bill usage as we were supposed to see a price cut. It has been pretty cold and fuel useage in an older poorly insulated house is high. Last year was easily the highest costs ever for gas.....

  7. #7
    DIY Senior Member zl700's Avatar
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    What are you guys talking about?
    The room temp setting has nothing to do with longentivity, in fact the lower the room temp setting the longer it will last because of fewer cycles.

    All furnaces have a minimum and maximum temp rise and as long as the unit selection, ductwork and install allows the heating blower speed to be set within those perameters the unit doesn't care what the return temps are.

  8. #8
    DIY Senior Member Rich B's Avatar
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    Just as a side note.....I did check the temp rise difference on my unit after it was in and working and it was right in the middle of the acceptable range.

  9. #9
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    Default thanks

    thanks for your info so far. to make it a little clearer, i still
    have my old furnace - a williamson gasaver II, 1983 - and really
    should get a new high ef. one but am trying to clear up this
    question first. reading this forum has been very helpful.

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