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timp 11-26-2007 02:26 PM

Furnace, starved for air? Thunks at start/end of cycle
My forced-air gas furnace makes big thunking noises at the beginning and end of its cycle. The fan is actually sucking in the sides of the unit's case, so it thunks at the beginning when the sides are sucked in, and then at the end when the sheet metal relaxes. The noise echoes through the ducts all throughout the house.

I added a cold-air return, thinking it would help (there were only two for a 3000 sq. ft. house). It didn't. The unit didn't do this thunking while I had the ducts all open when I was adding the return. It also doesn't do it if I remove the furnace filter so that it can suck air in through the filter slot in addition to the returns.

One return goes through a homemade pan that I could crawl through it's so large; there's a 2' by 3' register grate feeding that pan. The other return goes through 12" duct. And the one I added through 7" duct. All this feeds into the rectangular inlet duct on the furnace inlet. Is there a rule of thumb for how much return space I need for the size of the house or number of heat runs?

I'm using a good 3M filter, though one of the 50-cent cheapies would probably let more air through (and dust). Are there high-flow filters? I didn't see anything like that at Lowes & Home Depot.

Overall, I'm worried that the unit is undersized for the house. The former owners cut corners wherever they could. I'm thinking maybe the installer jacked up the fan speed to force the heat further through the house or something. Could it be something like that?


#CARRIERMAN 11-26-2007 07:26 PM

Hi timp

You have in a nutshell just let everyone know what I am talking about. You are using the 3m with the tight pleates. Use an open pleated filter with the soft media. If your wife sews, have her give you a piece of inner facing that is used to stiffen a shirt collar. Try and suck air through this innerfacing, this is exactly what is used in the high end filters. The polyester filters are the best performing filters in a pleated type. However they are only a thirty day filter and must be changed regularly. Hope this helps with your request.

Good luck

timp 11-26-2007 08:41 PM

Thanks for the info! My wife does sew and laughed when I mentioned the inner facing...she agreed it would be very tough to suck air through it.

I was buying the better filters to cut down on dust, etc. My son has asthma and I wanted to provide as good a clean air flow as I can. I'll try some other filters to see if I can find a good balance between clean and quiet.

Thanks and best wishes,

timp 12-03-2007 08:13 AM

To close this thread out right, changing the furnace filter has solved the thunking problem. My wife picked up a Filtrete Basic filter at Target for $4. On their site, 3M lists this as a "high air flow" filter. I suppose it won't do as good a job filtering the air flow as the filters we had been using. But I'm just glad to not have to listen to the furnace clanking and banging at the beginning and end of each cycle!

Thanks again,

dcd22 12-03-2007 12:24 PM

have you had the furnace cleaned recentely? Mine did the same thiung regardless of filter. I had it cleaned and now all is well even with a very tight pleat filter.

timp 12-03-2007 02:43 PM

No, actually we planned to get it cleaned this year before the heating season began...well, we missed that goal! It's 20 degrees outside and snowing to beat the band. Thanks for the tip.


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