HVAC Banging after being off for hours?

snikcillaJuly 6, 2009


What could be causing my system to be banging when it is "off" at the thermostat and has been off for hours?

How could it be the delayed ignition or oilcanning, etc when it is off? (And the fan is off too). Or is it something else?

A new e-coil installed last week, could this have anything to do with it?

This is pretty loud banging every few seconds, 40 or more times.

Any suggestions?

Thank you.

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Are you talking about heating or cooling? Where is the noise coming from (inside/outside?) What is an "e-coil"?

    Bookmark   July 6, 2009 at 8:52AM
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- Well it's a standard combined blower for Heating and AC system, but it's "off," so right now it is neither heat nor AC.

- Banging heard inside, throughout house. Seems most prevalent at night. (It's the same sound that you hear before the heat goes on in the winter. But usually that is just a few bangs until the furnace starts. That is why I am confused. The thermostat is set to off. Has been off for many hours, and is not about to turn on. )

- Evaporator coil for AC


    Bookmark   July 6, 2009 at 9:44AM
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Is the service truck for the guy who installed the new coil still in your driveway? You should check to see if he is perhaps stuck in the air handler and banging to get your attention.

Really, I don't normally hear banging when the furnace starts... so I don't know what you are talking about. The only think I can think of is the flat side of ductwork that becomes concave (or convex) when the blower kicks on because it's now under pressure (or reduced pressure, on return side). I would guess that's the banging that you are hearing.

In this case, I bet the ductwork is returning to its relaxed state after it has returned to normal temperature, thus the delay.

Look for large, flat runs of square duct work. If you can find the culprit, the solution is to support it, reinforce it, etc.

Also look for the service van in your driveway and legs sticking out of the furnace someplace.

    Bookmark   July 6, 2009 at 10:28AM
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Your original post seems to indicate that you have an oil fired hot air furnace, correct? It would concern me that you have had the noise before and more than once: . "(It's the same sound that you hear before the heat goes on in the winter. But usually that is just a few bangs until the furnace starts". When you say "starts" do you mean when the burner IGNITES or when the blower comes on? Thermal expansion can cause noise, but it seems odd that it is happening after hours of being idle. I suppose there could be a thermostat and/or short issue that is causing the burner to ignite on it's own and maybe going off on high limit but that seems really unlikely. Any chance there is a "critter" walking on the duct work?

    Bookmark   July 6, 2009 at 1:33PM
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Thank you for your thoughts--

It's a natural gas furnace.

I don't know for sure, but believe it's a banging before the blower comes on in the winter. (But really haven't heard it too many times.) Actually I remember the familiar banging moreso from another house, but that is a different story! I do realize any banging couls be cause for concern.

- No chance of critter. Very methodical banging. Consistent, even, bangs until it stops. Heard loudest in bedrooms at night via the return air duct in the hall.

Thank you.

    Bookmark   July 6, 2009 at 5:10PM
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I had a Carrier gas pack that did something similar. It turned out to be a bad control board. Every now and then it would momentarily pulse power to the blower motor. That caused the duct work to make noise. At times, it did it every few seconds. It did it even with the blower switched off at the thermostat.

I have flex duct in the crawlspace, so I didn't get any banging out of it, just a soft duct flex sound. If I had a sheet metal duct running to the unit, I would have expected it to bang somewhat from oil canning.

BTW, if you are not familiar with the term, a gas pack is a package air conditioner (everything in one unit) and natural gas/propane furnace all in the same box. It sits entirely outside the house hard against the side. Both supply and return ducts connect directly to the unit.

To check and see if you are having the same problem, put your hand by a supply vent when it happens. If you feel a slight puff of air, you may be experiencing the same thing.

My then AC tech claimed it was mice chewing on the thermostat wire and refused to go into the craw space and look. I didn't believe him and troubleshot it myself. I bought a new control board on eBay and repaired the thing myself. I never had another lick of trouble with it.

Good luck with your system.

    Bookmark   July 6, 2009 at 9:42PM
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Where is the furnace located? Is it inside of the home or in the attic? I have had people complain to me that in the summer time, their blowers will randomly turn on and off at times. Most of them are mobile home owners or people who that attic mounted gas furnaces and they leave their systems off during the day... heat builds up in the home and the fan relay closes and the blower kicks in.

I would call a tech to check it out.

Hope this helps

    Bookmark   July 7, 2009 at 9:12PM
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