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  1. #14
    Join Date
    Feb 2010
    Location
    Calgary, Alberta
    Posts
    1,134
    A double furnace install is common in Calgary and most builders oversize on new homes here so people don't complain of being cold. It has been known to get to -45*C in the winter here.... Recently.
    If you put 2 45's in I doubt they'll shut off ever, when we get those cold snaps.
    I agree the 90 is too big, I would look at something in the 60-70mbtu range for the main and a 45 for the top floor. 2 stage and ECM for sure.
    A 2 stage 70 will start at 40-44,000btu on low fire to keep your costs down and you'll have that extra capacity when needed.
    Make sure they run enough wires so you can actually control the 2nd stage!

  2. #15
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Maybe this will get popular for AC too.... install a 2 stage 2 ton unit and a 3 ton unit when the design calls for 5 tons and you can go from about 1.5-5 tons in up to 5 stages. (1.5, 2, 3, 4.5, 5) Who needs a stinking inverter drive! Then again, you could also just throw in a 2 circuit coil and use a single 5 ton air handler and keep costs reasonable. Hmmm... maybe on my downstairs, rather than a 2.5 ton single stage unit, I put in a pair of 1.5 ton units and to try it out. It would run on 1st stage almost continously for a good part of the summer.

    Another thought, I suppose you could use a single AC unit with 2 coils as well. I wonder if you could take for example a 4 ton 2 stage unit, split 2 linesets, then install 2.5 ton coils co they could run invididually on low stage, or both on high stage.

  3. #16
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,756
    Quote Originally Posted by syndicated View Post
    If you put 2 45's in I doubt they'll shut off ever, when we get those cold snaps.
    You seem to be straddling a line here. Do you think the fact they'll never shut of is a good or a bad thing? I can't tell.
    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. #17
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,434
    (2) 45's will use about $1/hr at average gas costs. Are you OK with a $720 gas bill caused by a cold snap or would you be willing to bundle up for a month? Bigger furnaces of course would cost even MORE to run !!!

  5. #18
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,756
    Good point. Back into it.

    $720 running flat out, ever have a bill even 1/2 that?
    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.

  6. #19
    Join Date
    Jun 2011
    Location
    Seattle WA
    Posts
    1,245
    I just pulled up a load calc I did on a 2100 sqf house that was built in 1988 and changed the outdoor design temp to 0 degrees and the load for the whole house came out to 57,276 BTU. 72,341 BTU for -20 outdoor design temp. Although your home is obviously different I'd bet the farm that 1 90,000 BTU furnace would be too big, much less two of them

    Edit* House had R38 in the ceiling, R19 in the crawlspace, R13 in the walls, double pane vinyl frame windows
    America; first we fight for our freedom,
    then we make laws to take it away.

    -Alfred E Newman

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