proper sizing of furnace - http://www.hvacmechanic.com/ Forums

Original message

unclesteve

68.58.62.178

"proper sizing of furnace" , posted Fri 31 Oct 11:21

I have a 20yr old 80,000 btu Trane BLD080F936B1 furnace I am about to replace. I've received several estimates from firms rated highly on Angie's List that do high volume in central Indiana. Their recommendations have ranged from Trane TDD060R9V3F-B to TDD080R936 furnaces. I'm concerned that the 60,000btu might be undersized for harsh Indiana winters (my house is a simple 1300sq ft ranch built 20yrs ago with all original windows/doors/etc).

Any thoughts whether 60,000btu is undersized? I like that vendor's bid but wondering if I should insist on an 80000btu unit? Any thoughts what the price diff might be?

Replies:

ampulman

68.84.238.104

"Re(1):proper sizing of furnace" , posted Fri 31 Oct 15:33

a 20 year old furnace with standing pilot and exhaust bonnet can run between 60 -70% efficient. This equates to a range of 48,000 - 56,000 BTU output.

The '60' furnace is a 2 stage variable speed unit, and has outputs of 31,200/48,000 BTUs; the '80' is a 2 stage unit with outpus of 41,600/64,000 BTUs. Usually, a difference of one size (in the same series) is relatively small.

Did any contractor do a heat calc? Considering the difference, one or both contractors are guessing. You can do one yourself. It will cost you $50. Go to HVAC-Calc.com.

Does your current furnace run (almost) constantly on the coldest day of the season. If not, then it's likely oversized.

AM

Freon

74.179.160.233

"Re(2):proper sizing of furnace" , posted Fri 31 Oct 15:46

Simple question... on the coldest of winter nights, does your existing furnace run almost constantly AND keep you warm. If yes, then it's the perfect size (net BTU output of 48,000 - 56,000 BTU ). If it cycles on and off frequently, it may be too big. If you are cold, it may be too small.