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