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  1. #1

    Replacing gas furnace, how big?

    I'm replacing an old 1966 200,000 Btu input furnace with a 95% efficient furnace to get the tax credit. So far I have one estimate and it has generated a few questions. The dealer is recommending a 90,000 Btu output Amana AMH95 two stage burner with multispeed blower.

    Questions:

    1) I did a manual J calculation and also back calculated heat loss based on 2008 gas consumption, 60% efficiency and 2008 Heating Degree Days for my area (SE Michigan). Answer was 32,000 Btu / hour heat loss. Assuming this calculation is correct, how big should I go on the furnace? Dealer says he only uses house square footage to size the furnace, independent of insulation, air infiltration etc. He also said that since it is a two stage, normal sizing rules don't apply.

    2) Since the basement ceiling is drywalled, it would be a bear to rewire the thermostat, so I think I'm stuck with a single stage thermostat. The dealer quote says I can have any brand of furnace I want for the same price with the same basic specifications (95%, two stage burner, multispeed). Does being stuck with a one stage thermostat limit my choices of other equivalent furnaces?

    Thanks in advance for your advice and feedback.

  2. #2
    Join Date
    Feb 2003
    Location
    pittsburgh,pa
    Posts
    727
    Did your contractor do a manual j or equivalent. There are other options for tstat wiring you could relocate or go wireless etc. First find a contractor to give you a good system evaluation. Square feet for sizing is not correct.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,758
    If he says 2 stage furnaces get sized differently, get another contractor.
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    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Nov 2008
    Location
    Colorado
    Posts
    2,687
    At the top of the page is a $50 load calc for homeowners called HVAC Calc. A proper load calc should have yeilded the size needed. Finished basements can be a bear, but we have run new stat wire through a nearby return in similar cases.

  5. #5
    Join Date
    Aug 2002
    Location
    Southold.calm
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    4,850
    There’s more to consider then just the size of the heater!
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  6. #6
    Join Date
    Feb 2009
    Posts
    131
    i was gonna take a look at your link but for some reason it wont open.

  7. #7
    Thanks for the replies so far. I did use a Manual J calculator and that is how I came up with the 32,000 btu /hour loss for the house. As I indicated, back calculating from last year's gas usage verified this number. But aren't furnaces generally sized with a safety factor to take into account especially cold periods? For instance I believe the Manual J calculator assumes 5 degrees F as a baseline, but the loss would be greater on a -10 degree day, which we get occasionally here.

    I was disappointed that this contractor did not even offer to do their own Manual J, as they are highly recommended by our home handyman radio show guy, our local building inspector and have been in business for 40+ years with a 70 truck fleet.

    I appreciate the suggestions for thermostat options. I didn't realize that a wireless option existed, but it might be the easier solution.

    Thanks again.

  8. #8
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,350
    I'm not a pro, but when watching the demonstration of HVAC-Calc heat gain/loss calculator, it suggested a 15-25% safety factor to add to the heat loss. Still, that's putting you at 40k btu.

    Sizing by square footage is never the right way to do it.

  9. #9
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,802
    Make who ever do a heat loss calc or move on to the next. As far as the thermostat, ask about the Honeywell wireless RedLink series. A little pricier but it will practically vacuum your house it's apps are that versatile.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  10. #10
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,347
    Find a competent contractor that will perform the Man J calculation. They should have no problem showing it to you and leaving a copy when you sign the sales agreement. Very important this is done correctly.

    As stated there are many options for the T'stat. In addition to wireless H/W VP IAQ uses only 3 wires between the wall unit and interface module.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,758
    Quote Originally Posted by Travelover View Post
    I was disappointed that this contractor did not even offer to do their own Manual J, as they are highly recommended by our home handyman radio show guy,



    Thanks again.
    Thats probably a PAID endorsement.

    You don't size for the occasional coldest temps.

    A a 40,000 BTU input 90% would give you 36,000BTUs output, A 50,000 BTU input 90% would give you 45,000BTUs output.
    That would cover you on those few colder days you get every couple years.
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  12. #12
    Join Date
    Feb 2009
    Location
    Arnold, Mo
    Posts
    13

    Get a good estimator

    Travelover, A good estimator will know how to size a system for your home and your area instinctively, but he/she will always check.
    Most 2 stage equipment will work off of a single stage t'stat. Carrier has 1 system that only needs 4 wires.
    I suggest that you call a couple of manufactures, ask who they consider is the best 3 dealers in your area and go from there.
    In my opinion 2 stage variable speed( D.C. drive ) is the way to go. Do yourself a favor and add 10 yr. labor warranty.

  13. #13
    Thanks all. Contractor #2 is coming tomorrow - I'm sure I'll have more questions.

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