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.

Greg

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Comments (4)

baldloonie
16 years ago
You are confusing the output figure with AFUE or annual fuel utilization efficiency. That oldie, when fully warmed up and burning perfectly, can put out 80% of the gas being burned. BUT, AFUE will look at the losses while the beast starts, the losses as it cools down and pours heat out the flue and the stand by losses as it sits there with the flue open sucking air out of the house 24 hours a day. So AFUE on that beast is probably 55-60%.

Yes, you should have the chimney lined for the orphaned water heater, no big deal. Some 90s can put some noise out of the flue but since you are rarely sitting beside the vent in the winter, I wouldn't worry... unless you or your neighbor have a hot tub near it :)

Pumping condensate out in the winter is trouble, can you pump it somewhere in the house like a laundry tub?
delcogreg
Original Author
16 years ago
I've seen the AFUE term and guessed that it was something like that, but didn't really know what all figured into it. Thanks for the explanation.

The laundry sink is ~40' away on the other side of finished space. There is a 2" cast iron stack from the kitchen sink about 20' away in the same room. Is that an reasonable option?

I wasn't thinking about the water heater being an issue in the summer, but it makes sense that any noise from the heater wouldn't be such a problem in the winter. I read one guy complaining about a water heater vent across the street from him though.
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User
16 years ago
I don't see why you couldn't pump condensate all the way to the laundry sink. Our 87% boiler is a condensing type. The boiler has a condensate drain coming out of the bottom of the flue. This drain is routed to a condensate pump which hoists the water up and over my water heater, then gradually down to the washing machine drain. The boiler has been producing hot water of around 120Â°F lately (around 32Â°F outdoors). This low temperature means a lot of condensate from the flue, so the pump has been busy. I never hear the pump run, though.

As far as vent noises go, I would DEFINITELY have the air intake plumbed to the outdoors if I could do my install all over again. The air intake makes a considerable "whoosh" noise while it's running. (Of course, drawing air into the house.) The blower fan does have a slight vibration that I can hear in my kitchen, which is just on the opposite side of the wall from the boiler.

In the case of a boiler, I can connect an indirect-fired domestic water heater that the boiler services. This means one unit does two jobs by effectively making an additional zone. I don't know if this can be done economically with forced air heat.

I can't hear any noise from my boiler when I'm outside. Then again, I don't have it vented through the wall.
bobcroker_yahoo_com
13 years ago
I just went through this same scenario. My choice was to get the two stage 80% efficient furnace (saves $500 including losing out on the tax credit), not line the furnace (saves $1500) and put the savings toward duct sealing, and sealing drafts with weatherstripping and spray foam. In an older house like mine, replacing old can lights that vent into the attic should get me a better return on investment and more savings. High efficiency (notwithstanding the emotional aspect of feeling green)is kind of lipstick on a pig.

My two cents. We'll see how it turns out.

How inefficient is my 40 year old gas furnace?

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baldloonie

delcogreg
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How inefficient is my 40 year old gas furnace?

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baldloonie

delcogregOriginal Author

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delcogreg
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