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  1. #1

    Furnace size for 830 sq ft house

    I need to replace my old furnace and have received several quotes with widely varying furnace sizes suggested. Only two firms did a heat loss calculation, and of these, only one showed me the final result. Unfortunately, they are charging much more than everyone else. One firm that I am considering going with has suggested a 40,000 btu input/32,000 btu output furnace (80%/single stage). I am wondering if this size will be too small.

    My current furnace is 25 years old, 75,000 btu input and either 60 0r 65% output. At 65% output, I should be getting at most 49,000 btu heating the house. In my experience, this is way too much heat for my house, which is 830 square feet. I leave the furnace off at night. On the coldest nights, the house gets down into the 40's, and it only takes about 1.5 - 2 hours to heat the house up completely. On normal daysi in the winter, the furnace cycles on and off all the time, but I usually control it manually to avoid this.

    One contractor did a manual J calculation and said the calculation showed a heat loss of almost 40,000. Does that seem possible considering my experience with my current furnace? Does the manual J take sun exposure into consideration? I have massive south and west sun, and am in Colorado at 5000 feet.

    As a side note, has anyone out there had a furnace installed that was too small??

  2. #2
    Join Date
    Sep 2002
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    I don't know
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    Your current furnace probably has a rated output of 70-80%; AFUE reflects seasonal efficiency, not steady state efficiency.

    On the coldest nights, the house gets down into the 40's, and it only takes about 1.5 - 2 hours to heat the house up completely.
    Why do you leave it off over night? 40F return air can cause condensation to form in the heat exchanger. (most manufacturers specify that the return air shouldn't be lower than 55F)

    A 20-30F drop over several hours is huge - I would get an energy audit done. Provided that the furnace works, you might want to hold off on replacement and put the money towards building envelope upgrades.

    One firm that I am considering going with has suggested a 40,000 btu input/32,000 btu output furnace (80%/single stage). I am wondering if this size will be too small.
    Why only 80%? Is the furnace located in an attic or garage?
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  3. #3
    Join Date
    Jul 2008
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    Rochester NY
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    4,739
    Quote Originally Posted by blueandyellow View Post

    As a side note, has anyone out there had a furnace installed that was too small??
    Lots of people claim to have seen big foot, I think it's a myth.

    That is a very small house. What was your worst case heat load, like 20k? There is not a furnace I know of that goes that low, which means anything will be oversized even on the coldest days. Sounds like you've educated yourself and arrived at certain correct conclusions already. 2 stage heat pump might be a good option.

    Have you looked into mini split heat pumps? Do you have a/c now?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  4. #4
    Join Date
    Jul 2007
    Location
    Delaware
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    3,824
    a small one.
    Always here

  5. #5
    Quote Originally Posted by tedkidd View Post

    That is a very small house. What was your worst case heat load, like 20k? There is not a furnace I know of that goes that low, which means anything will be oversized even on the coldest days. Sounds like you've educated yourself and arrived at certain correct conclusions already. 2 stage heat pump might be a good option.

    Have you looked into mini split heat pumps? Do you have a/c now?
    I have no idea what my total heat loss would be. I guess the longest I've ever run my furnace would be 10 hours, so 10/24 times 49,000 btu gives us 20,312 btu. Hmmmmm.

    I do not have AC and don't need it. The swamp cooler works great here with our low humidity.

    What is a mini-split heat pump, and how might it help me?

    Thanks!

  6. #6
    Join Date
    Dec 2009
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    maroon lazyboy
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    i know a guy who went the cheap route with his new home hvac.

    when the system runs it sounds like the wind is howling outside.

    If someone is higher than the rest THERE IS USUALLY A REASON!!!!!!!!!!!!!!!!!!

  7. #7
    Join Date
    Sep 2002
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    What was the result of the load calc?

    The smallest unit available is around 40k; if you want long cycles, get a two stage furnace.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,331
    830 sq ft won't need a big furnace.

    The 2 that are higher priced, are probably higher because they do load calcs. And are doing it the right way.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,317
    My house is under 1000 sq ft, good windows, decent insulation. My furnace on low is about 33,000 BTU output and does fine at -5° out. Only use high for quick warmups and like evankoehnsays, sounds like wind howling through the vents. Should have gone 40K input but the supplier giving me a great deal doesn't sell that kind of furnace that small.

  10. #10
    Join Date
    Jul 2010
    Location
    nebraska
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    1,629
    House gets down in the 40's with the heat off over night? If that's correct then you have a lot more issues to deal with then just a new furnace.
    My house is the same size with newer windows and has a two stage 45K furnace. First stage kept the house at 72 even when it was -20F out . The heat pump can keep up with around 10K btu output when it's 9 degrees out. Just finished sealing penetrations and added another R25 in the attic so I bet it does even better this winter.

    Caulking is cheap. Windows are fairly easy DIY and don't cost too much. Insulation is cheap, less then $150 on sale to do mine. Cutting that load down is easy and inexpensive on a small home.

  11. #11
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    Sounds to me like oyu could improve the envelope, air seal then get a few really good space heaters since you don't like the heat on at night would be a much better option for you. Why anyone would opt to shut there furnace off while sleeping is beyond the imagination but to each his own !!

    800-sq.ft. is 3 really nice size rooms non the less being hoe small the space is, by air sealing, taking advantage of the solar gain during the daylight and insulating would probably drop your heat loss down to the low 20K BTU and with that amount of heat loss I'd go with a window type of heat pump the type you see in hotels where it provide good heat and nice AC when needed.

  12. #12
    Thanks for all of the comments. Funny thing is, if I'm buying a new furnace, there's really no point in insulating because my heating bills are so low anyway. Also, I don't think that the temp going down to *upper* 40's is so bad, considering it's only dropping from about 63 degrees to start. And as I mentioned, that's only on the coldest nights (2-3 nights per year), which would be around -10F with wind.

    I am intrigued by the idea of insulating and then forgetting the furnace altogether, but I can't imagine anyone would buy my house when it comes time to sell! Alternatively, maybe I should add a bedroom and bathroom on to my house to compensate for the large furnace.

    I received one bid for a 2-stage, with btu outputs of 35,000 low, 55,000 high. Sounds like, as many people have commented, that will be overkill and I'll almost never have to use the high stage.

  13. #13
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    Consider "marrying your Domestic Hot Water needs to your space heating needs with a Quietside unit. Retrofit your furnace with a HW coil and get the benefit of the higher efficiency
    http://www.quietside.com/Quietside/i...d=50&Itemid=97
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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