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Old 01-18-2014, 09:19 AM   #1
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What is the hammer sound in the furnace?


Hi all,

I am writing today to ask about a sound that happens only in the winter in my HVAC system. This sound occurs only a couple times a day inside the actual "furnace" unit (right word?) and only after the heat has shut off during its normal operations. Not every time the heat shuts off, but the few times it makes the sound it is after the heat has shut off until the next cycle runs. It sounds like a rapid hammering sound and its loud enough to be heard one floor up throughout the main level of the home. It doesnt sound like its coming from the water heater next to it, but from the air unit itself.

FYI we run a heat pump via the AC unit outside. Never had any problems with the unit (other than one time when the line running to the outside AC froze over inside the house, which is when we removed all the extraneous air filers the builder had installed at every intake vent - since then no issues. But, this hammering noise is just weird. I used to think it was after running the hot water but the noise definitely sounds like its from the hvac unit not the water heater.

3 photos attached. Water heater (electric) is on the left, the ductwork comes down in the middle (and now has a humidifier installed not seen in this photo from prior to that), and the main unit is on the right side against the wall (excuse the spray foam and mouse traps...we had an issue once.)

All units are electric.

Thanks!
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Old 01-18-2014, 09:52 AM   #2
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What is the hammer sound in the furnace?


sounds like it may be related to the heat pump coil inside the air handler. when it shuts off the Freon pressures equalize and possibly that may have something to do with it. try stand next to it and hold the small copper line with your hand and run the unit for 5 mins and then get the wife to shut it off. if it pulsates then that is the problem. I don't do heat pumps but that is a possibility and maybe some of the other techs have an idea. may have to do it several times to catch it.

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Old 01-18-2014, 09:57 AM   #3
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What is the hammer sound in the furnace?


May be that the check valve inside the air handler at the coil is hammering.

If you can record it and post a link to the recording, that would help.
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Old 01-18-2014, 10:24 AM   #4
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What is the hammer sound in the furnace?


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Originally Posted by beenthere View Post
May be that the check valve inside the air handler at the coil is hammering.

If you can record it and post a link to the recording, that would help.
I will certainly try. Its very random. For example, it just happened a moment ago (11:22am) and it happened earlier around 8:30am. The heat has gone on and off multiple times since then since its so cold out, but I only noticed it twice. Not predictable... But if I can catch it, I will post here. All I can say is its very rapid and sounds like a hammer.
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Old 01-18-2014, 05:23 PM   #5
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What is the hammer sound in the furnace?


I should have noted the hammering sound (very rapid) can last between 5 and 10 seconds.
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Old 01-18-2014, 06:00 PM   #6
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What is the hammer sound in the furnace?


Reversing valve hanging up?
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Old 01-18-2014, 06:16 PM   #7
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What is the hammer sound in the furnace?


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Originally Posted by bobelectric View Post
Reversing valve hanging up?
RV hanging up wouldn't cause a hammering sound at the indoor unit.

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