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  1. #1

    New Gas furnace - need sizing help

    I am a homeowner located in the Pacific NW. I need a new gas furnace for my home. One HVAC guy I spoke with is recommending a 75K BTU unit, and another is recommending 90K. In both cases, it's a 90+ multistage/multispeed furnace, so the effective BTU recommendation differs by 15K.

    From the best that I can tell by doing my own calculation, my heat loss is just about 70K BTU at the coldest outside temperature. The guy who recommended a 90K unit said that I'd be "safe" with the bigger unit running on its low speed, and the guy who recommended the smaller one said that I'd be better off and more efficient to run a 75K closer to the max design capacity, since it would be cycling less often. It's rare in this part of the country to be truly at the ends of the temperature range, which are 24 and 84F. We're usually 15 degrees or so inside of that year-round.

    The 90K guy also said that he can tweak the unit to use less gas if I was concerned about cost, but I don't understand why I'd buy a V8 to run on 7 cylinders. I also got an earful about CFM, and how the blower on the 90K unit would work out better for me.

    Every time I read about buying a furnace, homeowners are told to be careful to right-size their equipment, but every installer I spoke with seemed very casual about it, and some of these places are big, well-regarded outfits. None of them did an on-site calculation, or even took a look at the rest of the house.

    What should I know as a homeowner about sizing, or is it just not that big a deal?
    Last edited by caterpillar; 07-14-2008 at 12:41 AM.

  2. #2
    Join Date
    Jul 2008
    Location
    Alabama
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    658
    Have a qualified company come out and run load calculation!

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,753
    What did you use to do your own load calc.

    Turning down the gas pressure on that 90 can make it use more gas.
    Newer furnaces require more air flow then the older ones did. So too large a blower can make your duct system noisey.

    What size is your current furnace, and is it a 80 or 90% efficienct furnace.

    You can screen contractors over the phone. If they don't or won't do a load calc, they do need to come out.

    WAG, teh guy recomending the 70,000 is probably right.
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  4. #4
    Quote Originally Posted by superd77 View Post
    Have a qualified company come out and run load calculation!
    Thanks, but that's easier said than done. As I mentioned, I talked to many "qualified" companies, and none of them felt it was necessary. They just sized up the basement and gave me a figure.

    I am not asking how to hire a contractor, but what I should know about the importance (or not) of sizing.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,753
    Proper sizing prevents.
    Short cycles that add wear and tear to the new equipment, shortning its life span.
    Higher then needed heat/cooling bills.
    Noisey duct systems.
    Over shooting of the thermostat set point.
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  6. #6
    Quote Originally Posted by beenthere View Post
    What did you use to do your own load calc.

    Turning down the gas pressure on that 90 can make it use more gas.
    Newer furnaces require more air flow then the older ones did. So too large a blower can make your duct system noisey.

    What size is your current furnace, and is it a 80 or 90% efficienct furnace.

    You can screen contractors over the phone. If they don't or won't do a load calc, they do need to come out.

    WAG, teh guy recomending the 70,000 is probably right.
    I used a formula that the building department provides. It's been written just for our area. Its number seems to be consistent with the equipment that my neighbors in similar houses have installed. Also, my physicist/engineer neighbor just installed an 80K unit in his house, which is slightly bigger than mine (2000 square feet), and claims it's overkill. He measures and analyzes everything.

    My current furnace is an 80% (on a good day), 22 years old, and rated at 105K BTU. It cycles about once or twice an hour on all but the coldest days.

  7. #7
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    I would go smaller

    I would rather undersize my furnace alittle than oversize. I once under sized a house buy about 18000 btus output, a mistake in load calculation did not realize till they phoned me back for a/c. I ask the customer how things were and he said is gas bills were awsome and they were comfortable. I was shocked. The best way is the right way, but when all else fails go smaller for efficiency. Just my opinion.
    Do it right the first time.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    You didn't mention what area your in.
    Around here, many 2000 sq ft houses can be heated with a 60,000 BTU 90%. And on the coldest days, it will still maintain the set temp.
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  9. #9
    Quote Originally Posted by beenthere View Post
    You didn't mention what area your in.
    Around here, many 2000 sq ft houses can be heated with a 60,000 BTU 90%. And on the coldest days, it will still maintain the set temp.
    I did. Pacific NW. The other thing worth mentioning is that my house lacks any insulation and has single pane windows (typical for my area). This will get corrected eventually, but I am nowhere near as tight as a new house.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,753
    Being from Lanc PA.
    I don't know if Pacific NW is one state, or a couple states.

    I still think you'll find teh 70 is probably the better size for your house.
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  11. #11
    Join Date
    Mar 2008
    Posts
    87

    Question for 21degrees

    Just wondering 21Degrees, you meant 18,000BTU and not 1,800BTU correct? Just curious what the original load calc called for. I am having a system installed next week and the load calc came in at 51,000BTU heating and 18,000BTU cooling. I have been a little concerned if I ever add an addition to my 1250SQ house that the 60,000 96.7% Trane furnance and 2 Ton A/C may not be enough in Norhtern, NY. But, it sounds like I would have some wiggle room. Any comments are welcome. Thanks

    Teddy

  12. #12
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    Hey caterpillar, you need a proper load calc done....bar-none. Those city forms aren't that accurate.


    If you are in the Seattle area give me a call, I can be of great assistance to you.

    Info in profile.

  13. #13
    Join Date
    Sep 2002
    Posts
    11,536

    beenthere -

    So why isn't this an important consideration with a dual fuel system?

    PHM
    ---------------


    Quote Originally Posted by beenthere View Post
    Proper sizing prevents: Short cycles that add wear and tear to the new equipment, shortening it's life span. . . . . .
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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