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  1. #1
    Join Date
    Nov 2005


    I have a 35 year old G10 Lennox counterflow furnace (110K BTU capacity) that I am looking to replace with a more efficient single speed unit. I have had 3 Lennox dealers appraise my heating needs and between them have received 2 different recommendations regarding required furnace capacity. One dealer states emphatically that I need to replace the G10 with a unit rated at 70K BTU. The other 2 dealers recommend the same unit but with a rated capacity of 90K BTU. We are talking about a furnace with a 92.1% AFUE replacing one that has a 60-70% AFUE.

    When I questioned the dealer who quoted me the lower capacity furnace, he stated that because my duct work is buried in a concrete slab floor, and the ducts may not be the optimal size, the higher capacity furnace would generate more heat than could be effectvely delivered, thereby causing the furnace to shut down prematurely and hence not operate efficiently. When I questioned one of the dealers who wants to provide the 90K BTU model regarding the nature of the ductwork, he did not see any problem.

    Can anyone offer thoughts regarding the factors I should be taking into account in making a decision regarding furnace capacity?

    -- Bill Zide

  2. #2
    Join Date
    Mar 2004
    SE Michigan
    I would go with the dealer that provides you with a manual J load calc. Also, try and go with a better blower package. sometimes those slab ducts dont move air as well as they should. Make sure the job gets inspected.
    “Now the freaks are on television, the freaks are in the movies. And it’s no longer the sideshow, it’s the whole show. The colorful circus and the clowns and the elephants, for all intents and purposes, are gone, and we’re dealing only with the freaks.” - Jonathan Winters

  3. #3
    Join Date
    Nov 2000
    Waco, Texas, USA
    Zidecar it would be helpful if you clarified whether the btu is input or output.

    Correct me if I am wrong....
    In your old furnace you have 110 btu input at 70% AFUE which gives you 77,000 btu output.

    The 70,000 btu input at 92% AFUE gives you 64,400 btu output.

    The 90,000 btu input at 92% gives you 82,800 btu output.

    One question is will your duct system handle the extra heat without overheating the furnace?

    Another concern is how much heat is needed for your home to keep you at a specific indoor temp with a specific outdoor temp?

    A problem with your set up is since the supply ducts are under a concrete slab you are pretty much stuck with what you have be it right or wrong. The contractor can't view your supply plenum until the equipment is removed (assuming you have cooling). He can however take your floor registers off and measure the diameter of each duct run. Not the best method but it is about all he can do with the situation he has to work with.

    Can you measure all your supply duct diameters and report back? How about return duct dimensions? Maybe the supply is large but the return intake is too small. In that case you could add some fresh air and go with the larger btu furnace. Then upgrade to two stage heat so most of the time you can run in low fire. You can sleep better at night without choking everytime the furnace kicks on.

    How did your old system perform on the coldest days? If you were comfortable then there is no need to put in a bigger furnace and risk ruining it with a duct system that can't keep it cool enough to operate within its safe temperature range.

    [Edited by Steve Wiggins on 11-29-2005 at 07:36 AM]
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  4. #4
    Join Date
    Nov 2005
    Steve - thanks for providing some insight. Here's the additional info that you requested. Perhaps this can help clarify my situation.

    The BTU ratings are for the input side. The current furnace also lists a "bonnet capacity" of 80 KBTU. I also misrepresented the furnace model - it is a G8 model, not G10. By way of additional information, the area to be heated is the bottom floor of a two story home. I have a separate system proving H/AC to the upstairs. The total square footage of the bottom floor is 1300 sq ft. Total air space being heated is ~ 10.5K cubic feet including a 12 ft cathedral ceiling (furthest from the furnace) for 40% of that sq footage.

    The air return for the downstairs furnace is an ~8"x9" rectangular duct measured at its far end. It feeds into a larger return near the furnace. The second floor furnace is adjacent to the downstairs furnace where they share the air return duct.

    The one supply duct that I have access to has a circular diameter of 5" coming out of the floor. I presume all of the floor ducts also have this diameter.

    I use a setback thermostat to turn off the furnace at night after 11 PM. It does not re-engage the furnace until 6 AM the next morning. On cold days (20 degrees F) here in Central NJ, the temp will drop 4-5 degrees overnight at the thermostat location (inner central wall in line with the furnace).

    You mention upgrading to a two stage heating unit. Do you mean a 2 speed fan or 2 stages of heat exchange? If the former, I have two concerns. The first is that the current thermostat wiring is 4 wire and it would need to be upgraded to 6 wire. This is not easy to do because of where the thermostat is located and the fact that I am on a slab. The second concern is will the variable speed get the area heated up as quickly as a single speed unit in the morning following a 7 hour setback interval. I am also not certain that the additional increase in cost will be worth the small increase in AFUE. Further clarification/insight in this area would also be appreciated.

    As for comfort levels, on the coldest days we are comfortable in all of the rooms except the living room & dining room. These 2 rooms share the cathedral ceiling and are the farthest from the furnace. However, We do not use these rooms on a regular basis. When they are used, it is because we are entertaining guests and they often contribute to the heating process. As such we can live with this situation.

    -- Bill Zide

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