How to tell when a furnace is on its last leg?
October 20, 2009 10:19 AM   Subscribe

I have a 20-year-old furnace.

The HVAC person is coming today to do a minor repair (expected cost less than $400). When s/he sees how old it is, I fully expect him/her to push me to have a new furnace installed.

Should I? I know it's lived past its planned lifespan, but there aren't any moving parts. If nothing else is wrong with the furnace, should I insist on just the minor repair? If there are a few other minor items that need adjustment, what is the cutoff at which I should say fuck it, install a new furnace? $1,000? Or more or less than that?

It's a SnyderGeneral model GUG117A016N manufactured in 1989.
posted by Number Used Once to Home & Garden (15 answers total) 1 user marked this as a favorite
In my previous house I had a furnace older than me (installed mid-sixties I believe) before I replaced it. It's probably more than $1000 to get a new one - at least $2000 I would guess. If the current one is otherwise working well there's no need to replace it - things break down and need repairs. The major reason to replace a furnace is if the existing one is low efficiency. Some old furnaces are only 60% efficient, versus 95%+ for new high-efficiency furnaces. Ar the winters where you live very cold? Do you use a lot of gas or oil? If so a high-efficiency furnace might be a good investment. Additionally, if you house is well-insulated then some newer furnaces are able to be more sophisticated with keeping the house warm by constantly circulating slightly warned air as opposed to cycling very hot air on and off. This is also more efficient - these furnaces also typically have DC motors which consume less power (like 50-ish W vs several hundred W) which is another savings.

In general, you should install a new furnace a) when the old one dies completely or b) if the old one is low efficiency and the savings in gas/oil/electricity will pay for the cost of the new furnace is some reasonable period (10-20 years). Otherwise just fix it.
posted by GuyZero at 10:27 AM on October 20, 2009

being that old, your furnace is probably in the 60-70% efficient range(i think that's what mine was, I am not a furnace specialist, this is just my home owners finding after looking at replacing the heat exchanger on my own 20 year old furnace), newer high efficiency furnaces range in the 90's.

A simple way to look at it is to take one of your bills and multiply it by .65(this is just an estimate if you can find the exact efficiency of your furnace use that) and you will find out that you are paying %35 more to heat your house because the furnace does not convert gas to heat that efficiently.

I crunched the numbers and found out that i would start saving money on my heating in a little over 2 years with a new high efficiency furnace instead of paying alot to just keep the one i had going, so i replaced it. Granted i live in Minnesota and run the heat longer than most people will.
posted by Mesach at 10:28 AM on October 20, 2009

I've only replaced one furnace in the 6 houses I've owned over the years, and that was because it was and undersized oil furnace which was super costly to run and didn't heat the 3rd floor. Replaced with a high efficiency gas furnace that paid for itself in 3 years compared to the oil.

So, to me, the questions are: rust/extensive damage to the furnace or other condition that will impede it's doing an efficient job; right size for the house; as efficient as you can get; what improvement(s) are you seeking and what is the payback time. I'm trusting that you've had regular maintenance to clean out the gunk that collects around the burner, kept the chimney in good repair and have weather stripped, etc. the house. As well, you might find the new furnace smaller than the existing one, if that matters to you.
posted by x46 at 10:31 AM on October 20, 2009

The most important factor when deciding to replace the furnace is efficiency. Furnaces have become much more efficient over the past 20 years and you can save a significant amount in fuel costs by switching to a better model. You can also save significantly on electricity by buying a model with a more efficient blower. It is fairly difficult to say when this might be worthwhile for you without knowing the details (fuel type, average annual fuel consumption). There are also often incentives for the purchase of high efficiency furnaces, but that depends on your location.

Without knowing the details, I'll guess that it is probably worth it in the long run to replace the furnace with a high efficiency model.
posted by ssg at 10:31 AM on October 20, 2009

Oh yes - depending where you live you can usually get a few hundred dollars back from the government as an efficiency incentive.
posted by GuyZero at 10:33 AM on October 20, 2009

I got a similarly antiquated, but efficient, furnace repaired in 2007; got a new furnace when it totally crapped out in 2008. Ask yourself how upset you'd be if you ended up having to buy a new one fairly soon after getting the old fixed. And shop around; "Direct Energy" wanted just under $1k to make the same repair a little local co did for about $300.
posted by kmennie at 10:49 AM on October 20, 2009

Response by poster: Some great perspective! I'm in Virginia, and the winter's aren't so bad. I'll have to crack out the calculator if it comes to that and see. If it'll pay for itself in 2 or 3 years, that's one thing. If it's longer than that, I'm not so sure.

Is there a site that details what tax/government incentives there are to upgrading to a more efficient furnace? I'm surprised furnaces have gotten much more efficient, since in my mind anyway, fire is fire, and that is that.
posted by Number Used Once at 10:55 AM on October 20, 2009

Response by poster: Also: any particular brand or model or certifications that I should insist on if I do get a new furnace? What is the minimum outlay for durable efficiency?
posted by Number Used Once at 10:57 AM on October 20, 2009

The efficiency come from better heat exchangers - you don't pump the exhaust gas into your house because at best it's a lot of CO2 and at worst it's a lot of CO which is lethal. Your gas utility will be able to tell you what all the available rebates are.
posted by GuyZero at 11:00 AM on October 20, 2009

I just recently (as in, yesterday) heard that American Standard is offering rebates on their furnaces (varies depending on efficiency level). I think the $1500 energy tax credit also applies if you get a more efficient furnace. (What that means is you can get up to $1500 back, but the tax credit is, I think, for 33% of the actual cost.)

I've had to replace 3 furnaces at a couple thousand dollars a pop (including labor, etc.) & every time, it's been due to a cracked heat exchangers, which means (at least where I live) that the furnace is considered a "hazardous appliance" & may not be operated. I am really big on energy efficiency, but given how expensive it can be to replace a furnace, it's definitely a good idea to run the numbers before deciing.
posted by oh really at 11:30 AM on October 20, 2009

Assuming yours is a gas furnace, there's a 30% tax credit, up to $1500, for new furnaces rated at 95% efficiency or better.

Besides efficiency, the other thing to look at is the condition of the heat exchanger. This is the part that separates combustion gasses from the air that circulates through your house - when it fails, you have a carbon monoxide problem.

FWIW, I have a first-generation 90% efficiency furnace that's a bit older than yours, so your furnace is not necessarily an energy hog.
posted by jon1270 at 11:30 AM on October 20, 2009

A failure requiring replacement in mid-winter would be a big deal; a huge deal if you weren't home when it happened and a freeze-up resulted. Even if efficiency concerns don't motivate you at the moment, consider replacing it when you feel that putting up the replacement cost now is a less big deal than potentially bearing the consequences at a time not of your choosing.
posted by TruncatedTiller at 12:01 PM on October 20, 2009

We just replaced our boiler this year (different than your furnace, but serves essentially the same purpose).

Regardless, if you go high-efficiency, you likely qualify for a tax credit: you can check restrictions and such here, at
posted by pkphy39 at 12:43 PM on October 20, 2009

We just bought a new Bryant 96% efficiency to replace our 17 y.o. gas furnace due to a cracked heat exchanger, as oh really mentions above. Our HVAC guy by law had to shut off the gas. Putting a new heat exchanger into an elderly furnace just didn't make sense.

Price started out around $5k. $150 rebate from Bryant, $300 rebate from our local gas company, $1500 estimated tax credit brings us in the low $3ks, and Bryant offered 6 months SAC financing.

We weren't expecting the expense at this time, but knew it would happen eventually. The new furnace is quiet, very efficient and I love the programmable thermostat. Toasty is good.
posted by Sweetie Darling at 12:44 PM on October 20, 2009

You can check prices on furnaces also at

Many folks refer to this brand as "builder grade" - in usually a snobby tone.

I have had very good luck with their units in high end remodels (in DC)... they sell with a decent warranty, very high efficiency units, a lot of flexibility in how the unit is assembled (orientation of "guts" - technical terms)... sell all versions, air-air, air-water exchangers, variable speed blowers, very quiet! Anyway... even if you don't use them to purchase, it's a good place to price shop.

Also... if you have an old furnace, the concern is with the exchanger having holes/cracks... the exchanger in an older unit is likely to be leaking CO2 into the air that is blown throughout the house...

The company ref'd above sells direct to installers or to homeowners...
here's their site:

Goodman Manufacturing
posted by Jiff_and_theChoosyMuthers at 1:52 PM on October 20, 2009

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