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120k BTU or 100k BTU furnace?

4 years ago

Replacing my 23.5 year old Tempstar system (11 SEER 4 ton air conditioner, 90% efficient 125k BTU furnace). Main level is 2,900 sq', south facing, 9' ceiling with lots of very large double pane windows on all 4 sides. Lower level is 1,800 sq' finished basement (workshop and exercise room that are rarely used) that was finished 18 years ago, it is not exactly conditioned but the contractor put in supply line off the main trunk so there are 4 supply registers – the basement maintains 64 F temperature in the winter with the registers open, otherwise it will be more like 56 F. There is also a 1,100 sq' crawl space in the lower level. Only 1 person and no pets. House is located in western North Carolina where we have mild 4 seasons.

Got 3 quotes, a 5-ton single stage air conditioner and a 2-stage variable speed 96% efficient 100BTU furnace with ECM. The price is about the same. I asked 2 of the contractors why the replacement furnace output is smaller than my current furnace, 1 replied that my heating load is only 84k BTU and that my current 125k BTU furnace is over sized, the other contractor stated that my heating load is only 69k BTU, both said my current furnace short cycles. I don't know how they arrived at the calculation but both claimed to have done the load calculation, I believe the heating load calculation is based on maintaining 72 degree when it is 16 degree outside.

I don't have concern regarding the 5-ton air conditioner but I wonder if a 120k BTU 2-stage variable speed 96% efficient unit will be better. The spec on the Trane S9V2 – two stage ECM Variable Speed states that for the 100k BTU furnace the first stage output is 63,050 BTU and the second stage is 97,000 BTU; whereas for the 120k furnace the first stage output is 75,660 BTU and the second stage is 116,400. It is $1,000 more to have the 120k BTU furnace (the coil is different too as they quote a Trance 4tXCD010 with the 120k BTU furnace).

Heating is more important than cooling to me as it is not that hot in the mountains of western North Carolina. Should I go with the 100k BTU furnace or the 120k BTU furnace?

There is no “return” in the lower lever even though there are 4 supply registers. A dehumidifier keeps the humidity at < 50%. 1 quote states that I should install a “return” for an additional price. Do I need a “return” in the lower level?

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