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  1. #1
    Join Date
    Apr 2006
    Posts
    2
    I have a small house (~900 sq. ft.) in the Cincinnati area, and am considering a new HVAC system. My current furnace is a 120,000 BTU GE unit that is at least 25 years old. I've been told it is way too big for the application. Temperature swings are very noticeable.

    One vendor quoted single stage and two stage 70,000 BTU replacement units (Bryant 310 and 312). Another vendor told me the two stage unit wouldn't do me much good with such a small house.

    I've done a lot of reading in this forum over the last week, but haven't been able to find a thread discussing single vs. two stage furnaces in a small house. Any thoughts would be greatly appreciated!

  2. #2
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,149
    sw_ohio

    I think the question of single stg vs two stage depends more on your budget and comfort requirements rather than size of home. I also would strongly suggest that a heat load calculation be performed. I question whether a 70K BTU furnace might be oversized.

    Depending upon any installation issues, you should only consider 90+ AFUE condensing furnaces. Personally I would not want a 2 stg furnace without a var spd blower.

    IMO

  3. #3

    2 stage variable speed good for smaller houses.

    I have a 1200 sf house in Denver. I have a 2 stage / variable speed furnace (60,000 btu input). I did not request a 2 stage thermostat when I got the furnace in 2004 (budget). The installer wasn't familiar with 2 stage. He forgot to install a "jumper wire". Therefore, my furnace never runs in high mode. The variable speed is very quiet and there are NO temperature swings because of 2 stage.

    The only time it was a problem was when the outdoor temp was -11. Indoor temp dropped from 71 to 65 in 6 hours. The jumper wire or 2 stage thermostat will correct this. The 2 stage thermostat is best.

    I would have saved money in the long run with a 90% efficiency furnace. Instead of getting regular AC, I am scheduled for a heat pump install this Friday 15 seer/ 8 hspf. My gas furnace will be back up heat for when its too cold for the heat pump.

    The trial version of HVAC-Calc is great (red tab near top of HVAC-Talk). Lots of great info on HVAC Talk.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,934
    Don't oversize!!!!! I have a 950 sq ft home, I went 80K 80% because smallest 2 stage variable I could get at the time. I could disable high and heat the place except during a freak -27 morning we had right after I installed the sucker. So in effect, I have an expensive single stage furnace although I bought it as much for the variable speed blower as anything. Someone needs to check carefully. I also wonder if 70K is too big and you'll spend all your time on low defeating the purpose.

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    Get a load calc done. If your house has any insulation at all you will probably find 45,000 is plenty but again you need to know and the only way you'll know is to have a load calc done.

  6. #6
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by sw_ohio
    Another vendor told me the two stage unit wouldn't do me much good with such a small house.
    That doesn't make much sense. If anything, a two stage (if properly sized) makes more sense for a small house because the comfort benefits of the lower stage (less noise, smaller temperature swings) will be more noticeable.

    You're looking at a 70K unit, which may be too big (you can get smaller two-stages with non-variable speed blowers), but surely for the same maximum output furnace you're better off having a lower ouput capability for days when it is less cold.

    When getting any furnace, it's critical to size them correctly, but a 2-stage furnace is wonderful when the 2nd stage (output on high) is properly matched to the home, not the low stage.

    In a nutshell, a single-speed furnace will always cycle on and off (with temperature swings and efficiency losses) any time the heat loss is less than the overall output of the furnace. Since most furnaces are oversized, this means you'll always get cycling unless it is very cold.

    A two-stage furnace will run continuously any time the heat load exceeds the low stage output -- it switches from low to high and back as needed. This provides more even warmth, more efficient operation, and less wear and tear on the furnace from startup cycles.


  7. #7
    Join Date
    Dec 2002
    Location
    North Central Ohio
    Posts
    230
    I have a 2 stage variable speed furnace and would not trade it for anything. The house is 1600 sq ft and 60k on high heat and 45k on low. This thing hardly ever comes off low heat, on when coming out of setback or extreme cold snap. I live in North Central Ohio. You can also use the timer on the control bd. to activate 2 nd stage and would not need the jumper or a 2 stage tsat but the Honeywell vision pro is a good tstat if needed. If your bugdet allows I would advise you on this purchase.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    Stop sleeping with the windows open, and you won't need a 70,000 furnace.

    Do the load calc, and get the right size.

    You won't regret 2 stage.
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  9. #9
    Join Date
    Apr 2006
    Posts
    2
    Thanks to everyone for the feedback. Sounds like two stage is the way to go. I'll be sure to perform a load calc to make sure it is sized correctly.

    This forum has been a tremendous help!

  10. #10
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    I agree with most of the guys, the 70k will be way oversized.

    I have a 1700 sq ft home and we got a 2 stage Trane XV90 60k on a 2 stage VP, and been VERY happy with the comfort of it.

    You may need around 45k, and Rheem has a two stage that small. It's only on a standard PSC motor.

  11. #11
    Join Date
    Jun 2002
    Posts
    460
    Be sure to get a two-stage thermostat if you get a two-stage furnace. Otherwise, much of the two-stage benefit is lost since the furnace will switch to high whether you need it or not after a certain amount of time (and then switch off when it might have continued purring along on low).

    White-Rogers has a good one that is reasonably priced. 1F81-261.

  12. #12
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by mayguy
    You may need around 45k, and Rheem has a two stage that small. It's only on a standard PSC motor.
    Trane has an 80% 2-stage that only has 40K input on high, 24K on low, for those who really need a small furnace. Still a standard PSC blower.

  13. #13
    Join Date
    Mar 2004
    Posts
    1,042
    We don't KNOW your load calc results, but based on the square footage I can tell you that you will have a hard time finding a two stage furnace that's suitably small, especially if you want variable speed.

    Unfortunately the manufacturers know that, by volume, almost all of the super-small furnaces end up in apartments, so they tend to only make those sizes in the low-end models. That leaves those of us with small houses and condos scrounging for something nice that's still a reasonable size. If you want something above the simple-and-basic, not every brand will have a candidate that's an appropriate size.

    The closer you get to matching your furnace size to your load calculation, the steadier your temperature control will be. Starting with 120,000 BTU, though, you have a huge amount of improvement available to you by slashing the size of that beast.

    If you do want two stages and variable speed, Rheem/Ruud has a 45k input 80% efficient unit, and Carrier has a 40k input 96% efficient unit. Many brands only go down to 60-70k as their smallest size with a variable speed blower.

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