Before I call the repairman, I thought I would throw this out there.
My furnace is about 15 years old. I don’t know much about it except that it is a high efficiency furnace with a variable speed motor which is a bit unusual. I had the motor assembly replaced last year. Very expensive for that type of motor.
So my furnace works fine but there is this annoying clunking sound that repeats in a very regular pattern as the furnace goes through its cycle.
What happens is the blower motor turns on and this stays this way for about 10 minutes. Then, as the blower appears to change speeds there is this odd clunking sound like someone is hitting the duct work with a wrench.
It is somehow connected with the blower motor because it clunks whenever the motor changes speed.
The clunking is worse at the end of the cycle when there are several louder clunks a few seconds apart before the cycle ends.
I have listened carefully in the furnace area and cannot pinpoint the source of the sound except that is in NOT coming from the burner area or the blower but somewhere in the duct work above the furnace.
It is also definitely NOT a noise that comes from expansion. It is a very regular, predictable pattern.
I wonder if there is a solenoid up there. It almost sounds like a switch banging against the duct.
I would love to hear any suggestions.
Thanks
It’s just the variable-speed motor. Mine makes a somewhat similar noise, and has for the last 10 years.
This is a fairly new sound. I heard it last year but don’t remember it before them.
Might be worth recording and playing for your repair guy, but I’d bet it’s just a motor resonance interacting with the duct work.
I’d give this a low probability but I had a friend who complained about a clunking noise whenever his furnace came on.
Turns out that he needed to replace the air filter. It had gotten clogged to the point that the airflow would make it smack against the duct.
You seem sure it’s not the duct work flexing because you hear the noise on a regular, predictable basis. How often are you hearing it? Every few seconds, few minutes, once per cycle? Does it happen the entire time?
Do you have a zoned system that can be controlled automatically. That’s the only reason to have anything moving around up there. Not so much solenoids as small motors that move dampers.
Speaking of dampers, there are probably some dampers in ducts. Look around for small wingnuts or screws in odd places. If you can find one, try turning it a little to see if there’s a damper hitting the duct.
One thing you should do is turn the heat off and the fan on at the t-stat. If you can replicate the noise with no heat, you can mostly rule out anything on the heating side of the furnace as well as hot air making things expand.
I’d suspect the motor mount is broken or loose.
If the motor has a clutch then it may have locked up. I have an older furnace with a belt driven blower. It’s old school with a big/heavy squirrel cage. The motor has a clutch that allows it to ease into startup. When the clutch fails there is a noticeable bang on startup.
You might get an idea of what is going on by pulling the front panel off and watching it start up.
I would need a stopwatch to time it all but I think the pattern is pretty predictable.
Roughly 15 minutes before the the first clunk, then 15 minutes before a series of clunks and then the cycle ends.
After more observation, I wonder if the humidifier is involved. There is a flex tube from the humidifier to the furnace that shakes when these clunks occur. There is an electrical connection between the furnace and the humidifier.
I should say the clunking is loud enough to keep me awake at night and I will say again that it definitely does NOT originate from the burner or the blower motor but somewhere above or near the furnace and I now suspect the humidifier. I have recorded the sounds and it is a half hour recording.
I hear the clunking as I type
Have you had any pets gone missing? When i was growing up, we had hamsters escape and get into the ducts. Eventually thay made their way to the squirrel cage blower and ran on it like it was their wheel. Then when the blower kicked on, the poor critters got flattened by the centrifugal forces. This resulted in making the blower out of balance and shaking itself apart, causing banging.
I’m still guessing it’s a portion of the duct work flexing. It might just happen to be the plenum where the humidifier is mounted, but I wouldn’t think the humidifier, in and of itself, would be doing it. You could also try putting your hand on it when you hear it, see if you can feel or see it moving.
Something else you can do is to go look at your ductwork. Somewhere on the duct, or the plenum, you’ll see a large X bent into the sheet metal. Push on it so the duct flexes in and out…is that the noise you’re hearing? If it is, then we can be pretty certain that’s what you’re looking for. Unfortunately, they can be hard to find.
There’s a solenoid with your humidifier. At least, that’s how my humidifier works. The solenoid is on the outside of the humidifier and very apparent. It does make quite a noise when it does whatever a solenoid does.
You are absolutely right!!
That was the problem.
$50 repair.
Thanks everyone.
Since it’s something new, I doubt it’s the same cause we found when our furnace (in our previous house) had a loud banging noise.
The place was less than 2 years old when we moved in - in March, so we didn’t use the furnace until the autumn. When we turned it on that fall, there was a banging sound - my husband described it as “sounds like someone’s inside, kicking the side to be let out of it”.
I called a repair place and scheduled a visit. I asked the person on the phone “Does this sound like it might be something dangerous, as in we should turn it off until you get here?”.
She put me on hold, checked with some technicians, and came back laughing: “The technician said that sometimes construction workers will leave a soda can in the furnace”.
The repair guy got there the next day, I told him about the conversation, and he looked skeptical.
Turns out, it was NOT a soda can at all… it was a 1-quart plastic drink jug.
I don’t know why the previous owner didn’t notice the issue and have it looked into - maybe she assumed it was normal.
And my guess is that the workers do this deliberately - really, there’s no way for something to be that common without it being a known prank.