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How inefficient is my 40 year old gas furnace?

16 years ago

Hello -

When we purchased our home 5 years ago we were told the furnace was original to our 1971 home. I thought I have been putting off replacing it for too long already, but I have read many posts which support the idea of either keeping an older furnace or just replacing it with an 80% model. The reasoning being that they are simpler, less prone to failure, and tend to last much longer than the more expensive, more complicated, high efficiency models.

I suspect my furnace has run it's useful life, so my issue is probably just whether or not to get a HE model or not. I would like to get some idea of how inefficient my current furnace might be so I can estimate how much I might save each year and factor that into my replacement decision. I would also like to find out more about venting a HE unit.

The existing furnace is a gas fired forced air unit made by Carrier - Model 58-BV125-1B-C, serial 9052890. If the info I have found on Carrier serial numbers is correct, it would have been manufactured prior to 1969. All of the homes in our development were built by the same builder and some of them were completed in 1967, so the heater date could be prior to 1969. It says 125k BTU input and 100K BTU output which I would think makes it theoretically 80% efficient, but I have read that furnaces of that vintage are probably no more than 60% efficient. Would the 60% figure be a typical operating efficiency due to their age or were they never really 80%? Is it safe to assume that a new unit rated a 90% is actually 90% efficient when installed?

The other issues I am thinking about relate to venting a new furnace. Going 90%+ will orphan the hot water heater exhaust, so I will need to have the flue lined for that. Also, although the furnace is close to an outside wall where it can be vented, it is in a basement and there is no drain anywhere near it. The AC condensate is pumped out, but I am wondering if that will present an issue with freezing in the winter since it can't exit the basement more than about a foot above ground level. I also looked into a power-vent hot water heater since it is due to be replaced as well, but it seems like a lot of people are unhappy with the noise their exhaust pumps make. Is that an issue with 90+ furnaces as well?

I am in SE PA and my gas bills ranged from about $250-$325 a month last winter, although they would be a lot more if I didn't keep the thermostat down so low in 2 of the 3 zones. I expect that we can gain a lot of comfort with any new system and maybe even reduce our gas bill a little.


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