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  1. #1

    Hmm Lower BTU's recommended compared to my old system??

    I have a 25+ year old single stage gas furnace. As I'm getting estimates to replace it, contractors are recommending lowering the BTU from the original unit. My original furnace is a 100,000 BTU unit. Contractors are saying I need right around a 70,000 BTU furnace, especially if I get the 90% efficiency furnace. Thoughts??

  2. #2
    Join Date
    Apr 2008
    Location
    Alberta Canada
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    2,246
    20 Years ago we did not do load calculation and we usually errored on safe side and oversized equipment. Now a days technology has taught us to do load calculations and find out that systems can be alot smaller output. Efficiencies have alot to do with input/ output BTUs. If they are doing load calculation you should be able to rest assure it will be done right.
    Last edited by 21degrees; 08-02-2008 at 12:04 PM.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,311
    Common to have an over sized furnace.

    If they did a load calc, they would know for sure.
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  4. #4
    I've had three estimates. I've had one person that did a load calculation. Sears did the calc and they are the ones that recommended getting a 100,000 BTU unit the same as what I have. The independent contractors that did not do a load calculation but have probably worked on many systems in similar homes in my area recommend the lower BTU.

  5. #5
    Join Date
    Apr 2008
    Location
    Alberta Canada
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    2,246
    Load calculation are not perfect but better than assuming. I would get 1 more calculation done and do it right.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,311
    Sears makes more money off of a 100,000 then a 70 or 80,000.

    On your coldest nights, does/did your current furnace cycle on and off.
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  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,302
    Older furnaces had an output steady state around 80% of input. So your 100K would be putting out 80K on a cold morning. So let's say good old Sears puts in a 92% 100K. It's putting out 92K on a cold morning. You would have to be currently quite undersized to need this much more heating. Also to keep a big furnace off limit (which likes to fail open) and be efficient, it will want to move considerably more air than your old furnace. If it can't, it will be very noisy and possibly overheat and quit.

    Around here, if someone had a 125K or 137K oldie and we went 90+, we'd go 80-90K input and be fine.

  8. #8
    Join Date
    Jul 2008
    Posts
    53
    Did Sears show you the heat loss? That would be the first question. Also, the ductwork will affect it as well. Is it insulated? Is your floor insulated? These things will have a direct bearing on the amount of heat needed for your home.

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