Sitting here looking over proposals for my two new units, getting ready to pull the rope. I got to thinking (which can be dangerous) about the downstairs furnace sizing.
The existing downstairs furnace is an OLD 80,000 BTU Lennox (Natural Gas) from the early to mid 1970's. My understanding is that it's probably between 65-70% efficient, based on its age. Of you split that range (67.5), the old beast is outputting 54000 BTU. We had no complaints about the heat level from the old furnace, so for the sake of discussion let's assume that 54,000BTU output is the reasonably correct size.
All three of the bidding contractors specified an 80,000 BTU replacement furnace. I've settled on the Trane XV95, which according to AHRI is 97% AFUE and outputs 78,000 BTU.
Looking at these in terms of BTU Output (and for the moment ignoring that the XV95 is 2-stage), isn't the 80,000 BTU XV95 seriously oversized? The first stage in the 80K model is rated at 49,500BTU out. It seems to me that the 80,000BTU m0odel would rarely (if ever) use the second stage.
The 60,000 BTU XV95, on the other hand, is rated for 37,7000/58,000 BTU output. Much closer match.
And before the question is asked, I've already asked my guy to go back to his calculations and tell me what size the math thinks the furnace should be. On the other hand, since the 60,000 BTU is the smallest in the line, there's nowhere else to go.
Am I missing something here? Does my logic make sense? Should I downsize this to the 60K model?