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I have about 900 finished square feet in my house, and I'll eventually be adding another 200 or so finished sq/ft in my basement. I'm looking to get my furnace replaced and have been quoted two options:

  • Carrier 60,000 BTU 92%
  • Carrier 40,000 BTU 95%

I'm currently renovating my house and looking for it to be as efficient as possible. Walls/trusses will be well-insulated and all the windows are being replaced with high-efficiency ones. I'd like to go with the more efficient furnace, but I'm not sure that it will be enough BTU's to sufficiently heat the place. I'm looking for an opinion from someone who knows a thing or two about this stuff.

Edit

I should probably mention that I'm from Wisconsin, where we've spent the last couple weeks in -20 degree (with windchill) weather!

There are two exterior doors, 9 windows (including an 8-foot bay window), and it's a single-story, single-family dwelling.

  • The most important piece of info you left out! Where are you? Can't answer this without that. (Also, what's wrong with the old one? Broken? Or just want more efficiency?) – Ariel Jan 13 '15 at 19:33
  • Next piece of info. Measure the size of the exterior walls, is this a row house? Multiple stories? How many windows (count them). How many doors? – Ariel Jan 13 '15 at 19:36
  • I edited my question, but to answer you here -- I'm from Wisconsin (it gets cold!). The old one is... old and inefficient, and I'm looking to get it replaced while I have everything torn out. – w.brian Jan 13 '15 at 19:36
  • Two exterior doors, 9 windows (including an 8-foot bay window), single-story. Not a row house, just a single-family dwelling. – w.brian Jan 13 '15 at 19:38
  • I can't give you the number (except by using one of the online tools that calculate it), but at least now you have the info here some would need to help you. – Ariel Jan 13 '15 at 19:51
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  1. The 40,000 BTU furnace is almost certainly sufficient. It doesn't take a gigantic amount of heat to heat an 1100 square foot house, even if the house is relatively poorly-insulated. If your goal is to improve your house's thermal envelope with more insulation and better windows, a 40,000 BTU furnace may even be oversized once you're done.

  2. To make sure, do a quick calculation yourself using http://www.loadcalc.net/. You can play with the variables to see what impact your decisions have on the ultimate heating loads. This tool isn't perfect, but will probably get you within 10-20% of the correct number, which is good enough for your purposes here.

  3. Since you're shooting for a well-insulated house, it would be a good idea to get a two-stage or modulating furnace that is capable of producing variable amounts of heat depending on the load. In the kind of house you're planning to have, one of these will be more comfortable, efficient, and longer-lasting compared to a single-stage furnace.

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