# boom noise after heat goes off



## kenmac (Feb 26, 2009)

Sounds like the return duct is making the noise.. Is the furnace cycling on & off on limit ?? What has the temp in the house droped to when furnace cycles on & off ??


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## fabian (Dec 29, 2008)

kenmac said:


> Sounds like the return duct is making the noise.. Is the furnace cycling on & off on limit ?? What has the temp in the house droped to when furnace cycles on & off ??


 the temp must just drop a degree or 2 maybe 3 and it kicks on again. limit? i dont know, i just purchased the house and this is my first house so i dont know what im looking at. what would be making noise in the return duct? something i can strap down or screw down?


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## kenmac (Feb 26, 2009)

suction on the blower.. Lack of air flow .. You may want to have the evap. coil check /cleaned.. Previous owner may not have changed filters or had a filter at all.. About cycling on it the temp in the house is dropping . then, the furnace will cycle on to bring house up to temp setting.. Some t.stats are off by 2-4 deg + or - depending on type of t.stat It may not be level


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## fabian (Dec 29, 2008)

kenmac said:


> suction on the blower.. Lack of air flow .. You may want to have the evap. coil check /cleaned.. Previous owner may not have changed filters or had a filter at all.. About cycling on it the temp in the house is dropping . then, the furnace will cycle on to bring house up to temp setting.. Some t.stats are off by 2-4 deg + or - depending on type of t.stat It may not be level


 
? what?


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## kenmac (Feb 26, 2009)

what do you mean ?? what. I was telling you somethings that would make the return pop whan blower turns off air restriction , blocked coil ,etc, Or it just be the way the return is constructed.. Ie if metel not cross broke,


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## newtech (Mar 21, 2009)

Can you adjust the heat anticipator setting? If so that might help. Also turn the heat up and let it run for a while, then turn the stat down and go to where your duct work is an see if you can locate where the noise is comming from. If you have a restricted or partially restricted coil the supply duct would more than likely pop due to the metal getting hot(expanding and contracting) If the coil was plugged you are not moving enough air so I doubt it would be the return popping because the fan would not be drawing in any air. For example if you have a box fan and block the outlet will anything be sucked in(no)


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## kenmac (Feb 26, 2009)

May also try removing your filter & see if the noise stops, Return mabe too small an uninsulated return will sometimes do this


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## whatsthis (Apr 1, 2009)

Had to jump in on this one! I have had a similar problem,(electric Furnace) but mine seems to be cured now. I have a cat that on occasion would take a stroll through the void where the duct work is, and shortly thereafter I got the boom on startup and shutdown. I put it down to his weight having dented a piece of ductwork, and when it got really annoying I went searching for it, found the area that was visibly vibrating and cushioned it with a piece of that round pipe insulation. Job done! All is well with me now, except that once in a while, we seem to hear a slight "swooshing" sound from the basement area where the furnace is (Furnace not on) don't know if that has anything to do with the heating system, or is just totally unconnected!:whistling2:
But I do have a couple of questions though,

If your thermostat (digital) is set fot lets say 68 degrees, how far below 68 will the temp have to drop before the furnace kicks in, and is there any adjustments you can make to it, or is that built into the thermostat?

Also, I find that I will hear the thermostat click at say 68 and by the time the system cools / shuts down, the reading will be 69, so again, is there an adjustment on the furnace that shortens the "hang time" (if you know what I mean) between the thermostat telling the furnace to shut down, and it actually shutting down? At the moment I am setting the temp setting a degree cooler than I want it to allow for the difference.
Sorry about this LONG post!:icon_cheesygrin:
Thanks!


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## yuri (Nov 29, 2008)

Most of the better quality electronic thermostats are accurate to 1 degF differential. The leftover heat in the heat exchanger causes it to rise 1 deg. The old mercury type were 2-3 degF. Electric furnaces should be set for elec. on the switch at the back of the thermostat subbase or in the programming.


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## j.crugerhansen (Apr 3, 2009)

Fabian

I had the same problem, boom sound a few seconds after the forced air gas furnace shut down. I found it was the filter that was clogged, I replaced the filter and all was fine. Now I can tell when I need to replace the filter by the boom sound. Replacing the filter may also fix your repeated start/stop. The heat exchanger gets to hot because of low airflow and automatically shuts the furnace down before the thermostat has reached the correct temperature.


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## fabian (Dec 29, 2008)

j.crugerhansen said:


> Fabian
> 
> I had the same problem, boom sound a few seconds after the forced air gas furnace shut down. I found it was the filter that was clogged, I replaced the filter and all was fine. Now I can tell when I need to replace the filter by the boom sound. Replacing the filter may also fix your repeated start/stop. The heat exchanger gets to hot because of low airflow and automatically shuts the furnace down before the thermostat has reached the correct temperature.


i wish that was my situation but i changed my filter about 3 weeks ago.


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## beenthere (Oct 11, 2008)

Sounds like you have undersized duct work.

Which means it could be the duct is contracting from thermal expansion, or from the relief of too much pressure, or a combination of both.

A oversized furnace tends to have short on and off cycles.


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