Really, really need advice on old gas furnace

paigegirl

Hi-I inherited my Grandmom's house a couple of years ago, & I've been living in it since. Love the house but everything in it is old. It has an old natural gas furnace in the basement (not sure how old) that makes noise when it runs-not so much when it's running, but when it cuts off, I hear a slamming sound and later, some other noises. I had the gas company look at it & they told me it is @ 80% efficient ( I think) & might go out any time. So now I'm scared-we have some pretty cold winters here, but I can't afford to replace it now (@$3,000 they said). So-how worried should I be? If it gets through this winter, I will be in better shape financially next year. The company did what they could-not much apparently-& replaced the filter. If I remember the brand and the age, I'll post that here, but any advice on what more I can do at this point? I'm a liberal arts major=not technically oriented. Thanks for any advice!

Jamie Hall

I'm a liberal arts major, too. Doesn't mean that you can't learn about your heating system! Of course that was a long time ago...

People who replace furnaces like to say that old ones may go out at any time. Sometimes they are right. Sometimes not so much. Can you post any information you can find on it regarding make and model number and that sort of thing? And maybe a couple of photos of it?

The bang may be a duct changing shape when the blower stops. That is certainly not unheard of. Can you time it quite precisely -- like maybe watching the furnace when it turns off -- to say whether it happens when the flame cuts out or when the blower stops?

Can you describe the "other noises"?

And -- besides that -- can you find a good independent heating contractor in your area (you might try the "find a contractor" section here at Heating Help)(where are you located? We might know of someone). If so, it would be worth having him or her come out and check the unit over and make sure it is adjusted properly. Not that the gas company isn't probably reliable, but... sometimes...

paigegirl

Jamie-Thanks! Yes I will try to post some more info-the unit puts out good heat, but the age & the noises are a little scary. Besides the "slamming" sound, which happens a few seconds after the blower shuts off, there are some sort of popping noises in the next few seconds. It has worked OK for a couple of years, but with winter approaching-I'm getting concerned. Thanks for your suggestions-anything else? I'll try to post the age and type tomorrow.

Tim McElwain

Need make, model of furnace along with make and model of controls such as gas valve, fan control etc.

paigegirl

Gang-Sorry I took so long-the furnace is a Magic Chef Forced Air Furnace, model G5A100C36-7. Also there is a little patch that reads: ANSI Z21-47 1983. So, does that mean the furnace was actually made in 1983, or is that the date of the model? If it's that old (1983), then it's old, obviously. I took some photos but can't find the cable to down-load them-try to do that tomorrow. If anybody has any thoughts on how to make this thing last, I will be grateful! Thanks-Paige.

Tim McElwain

The ANSI is the standard for furnaces from American National Standards Institute and the standard was last updated for that time in 1983. It does not necessarily mean the age of the furnace.

You think that is old, my furnace in my house was installed in 1955 and is still running fine with the original blower motor.

What are the number and make of controls?

Try to pinpoint the exact location of the sound is it duct work, the motor etc.

The folks you have had look at it should have been able to pinpoint any problems and give you some solutions.

I would look for some professional HVAC company that can give you definite answers.

ChrisJ

The house I grew up in had a 1958 forced hot air furnace that was still going fine last I knew in 2006 when the house was sold.

For a few years it had become quite loud and we assumed the blower was shot, but it turned out the blower kept stopping in the same position and accumulating dust in the same spot until it became very imbalanced.

A good vacuuming and the thing purred like a kitten. Far quieter than any modern furnace I've ever heard.

paigegirl

Gang-Thanks for this information-very useful & reassuring (glad to know other folks have an oldie!) I will try to post the control type and the photos. I think the slamming noise comes from one of the main ducts, leading to the front of the house. The heater has a big duct covering the whole top of the furnace, with ducts leading off from that. Is there anything in a duct to make that noise? The "popping" noises seem to come from the burner -whatever the tech. word is-after it stops. could it just be cooling off noises? Still a little scary. Thanks again-I'm learning a lot.

ChrisJ

Our furnace used to cause the ducts to "oil can" so to speak when it would turn on and off. It was the sheet metal springing outward from the pressure and then collapsing when it stopped.

It's also normal for things to make a quiet knocking/clunking/clinking sound as they cool. Not loud though, you'd need to be within a few feet to hear it.

Can you record a video of the sound and post it on youtube?

Tim McElwain

The popping noise when the burners shut down is called "extinction pop", it can be corrected by slightly closing the primary air shutters on the burners. This should be done by a professional and also a combustion analysis should be done to insure safe combustion and protection against high levels of carbon monoxide. Make sure the input to the furnace is correct by having someone clock the burner on the gas meter. The so called pros' you already had should have done all of this. I hope you did not pay them.

As for the ducts have them checked by someone who does duct work a sheet metal license if they have such a thing in your area is preferred.

Jamie Hall

As has been said, the banging noise when the fan stops is almost certainly "oil-canning" from one of the bigger ducts -- but it could either be a supply or a return, to make it more confusing! Any large flat surface on one of the ducts can cause it. If you can find out which one is doing it, anything done to stiffen it will cure it -- such as simply using Liquid Nails or something of the sort to glue a board across it.

Bob Bona_4

We zip screw S slip diagonally across flimsy duct to stiffen things. Also could be short on return ducting, very common in old homes.

paigegirl

Gang-Thank you again-I found my grandmother's Owner's information stuff from Magic Chef, unfortunately the installation date isn't on it. It is described as a gas fired sectional basement furnace, part # 30824A084. There is a date on the front: 1-26-84. Do you suppose that means the unit was installed then, or maybe a few years later? How often do companies change models ? I'm hoping the unit was actually installed after that date, but don't have a clue. I posted some photos. Anybody care to guess the age? It's running O.K. just now-just concerned about the age & the noises: by the way-the slamming sound doesn't always occur when the blower stops. Thanks for any input! Paige.
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John Mills_5

In the real world, that's 60% annual efficiency tops.

paigegirl

Gang-thanks for all the input-very useful! To return to the subject-so far (early Jan.) the furnace is still chugging along- making some noise but still putting out good heat. I note that some of you posted about the typical "life-span" of units like this-I'm guessing it's @ 30 years old. So, we have @ 2 1/2 months of winter left-is it reasonable to expect it to make it through? Over the summer I will have time to consider my options. If I thought it would last the rest of the winter, I will just hold on & wait for Spring. Are there a lot of things that can go wrong with these types of furnaces? Thanks again.

Jamie Hall

You'll probably make it to summer. There isn't that much that can go wrong -- burner problems, but they're usually fixable; fan problems, ditto. The only baddy which isn't fixable is a crack in the heat exchanger, but that's not all that likely.

I would be inclined to have at least one CO monitor, though -- but that applies even to a brand new furnace!

paigegirl

Jamie-Thanks. I got one of the Lily 1800 heaters, and now I'm planning to use it to extend the life of the furnace. It's a real relief to know these old furnaces last longer than I thought. I signed on with a yearly "check-up" program. If you change the filter often & get the burners (whatever you call them) checked every year by a reputable company, any reason it can't keep going? Thanks again-I am printing out all the info here to put in a binder.

Jamie Hall

No -- it will keep going a long time. The yearly checks are really important; they can make all the difference.

The only thing is that newer furnaces are sometimes more efficient, but are they enough more to justify the cost of changing? Often not...