Replacement Furnace size


Old 09-03-09, 07:44 AM
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Replacement Furnace size

I am getting ready to replace my 25 year old oil furnace with a new natural gas furnace. The old furnace is 90,000 BTU output and I am sure not very efficient. Will a 80,000 BTU 95% efficient gas furnace be sufficient?
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Old 09-03-09, 10:00 AM
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In years past, many furnaces were oversized, as it was inexpensive to err on the safe side and oversize the furnace. If your HVAC contractor has done a Manual J calculation and determined that 80,000 is sufficient for your house, then yes, it should be fine. Make sure he does the calculation and does not simply replace your old furnace with the closest size to your old furnace that he stocks.

For what its worth, when my old 100,000 BTUH output furnace was replaced, it was replaced with a 64,000 BTUH output two stage furnace, and the smaller size has been more than adequate.
Old 09-03-09, 11:12 AM
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Make sure you get a DC variable speed fan. You will not regret it in spite of a slightly higher cost is it not standard for the furnace you are considering. It was standard for my smaller 80% replacement unit (bad heat exchanger in the old). In the automatic position when heating, it runs constantly at a very low speed to provide comfort and uniformity.

I went with the 80% instead of the 95% because of installation problems for intake and venting in a townhouse that was completely finished on the upper level. My previous heating bills were not high, but the costs with the newer 80% unit are lower with more comfort. Despite -25F temps, the only time I can get the second burner stage to kick in is when I have a very low setback in the evening and a much, much higher morning temp set and the temp rise is not fast enough and it comes on for a couple of minutes, then cuts way back for the rest of the day despite the frigid outdoor temps. I do have a lot of masonry and a minimum of wood/fiberglass.

Surprising, my AC costs have even gone down, with the new furnace even though the same old compressor and coil were used.

Get a calculation and not a guess. Too big is not as efficient.

Old 09-03-09, 12:25 PM
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Sounds like I should hire a pro to make an eval. May cost me a little more up front but save me in the long run.
Old 09-04-09, 06:40 PM
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Furnace size

Hiring a 'pro' does not ensure a proper heat loss calculation & furnace sizing. Here's a link to a web site where for about $50 you can download & run the calculation for yourself. I've used this program & been quite satisfied with the result. It could very well be the best $50 you spend on the project.
HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations
Old 09-06-09, 06:38 PM
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I'm with Grady.. At least you got numbers to compair to what they may have..
Old 09-09-09, 06:41 PM
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to concretemasonry

It may not be so surprising that your A/C costs have dropped, since the new furnace fan is probably much more efficient (lower wattage) than the old one.
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