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I have a '87 mobile home with a 3400A816 electric furnace. Some times ,but not all the time it "bangs" as the blower is shutting down. I checked for something loose, nothing, I even put a screw into a stud, noise still happens. As the blower is getting ready to stop it bangs , runs for a few seconds and then stops. I checked the motor by using the on/auto switch and things seem fine. Any ideas? Thanks JAD :?

Have you stood directly in front of furnace or sat there to closely try to pinpoint where it is coming from ?


One similar thing I found was the bottom door was expanding and contracting as furnace chamber heated up and cooled down.


As it contracted back to norm, when it got close, it would ~pop~ back to final form and make a bang sound as it did this.


It also did not do it always and it was just before blower shut down.


Check for that and if not it, try the close encounter to try to define where it is coming from exactly.



Thanks,
Robert

No, I have not. Yes,I will. I am the first one, usually. Thanks Jerry [jad]

Omigoodness, mine does the same thing. I call it a loud thunk.I assumed it was the door.
At first i thought the blower was creating a vacuum, and releasing it when it turned off, but there is plenty of air getting in there, with the louvred top part, and around the door itself.

I never think of checking it when I hear the click of the thermostat.
Hmmmm, something to check now.

Okay, I was correct about the vacuum. It is almost impossible to get the door off when the furnace is running. It is stuck tight until it goes off, and then it lets go.
I still couldn't tell tho, if the thunking noise was the suction letting go of the door.

Hi Barb,

I have found them both ways and a combo of the two.


Just depends on furnace and situation.


The doors are thin in middle area and the vacuum and/or expansion/contraction from heat will make it bow in and release out with that ~thunk~ sound.


I have removed the solid bottom door, placed thick sheet of plywood over the opening and held in place to run a cycle.


The thick wood won't give and if noise is not there, but returns with replacement of door, it is the door.

Thanks,
Robert

Robert, thank you so much for the idea of the wood door.

I am replacing my twenty year old Coleman 3400. Should be in next week
If the new one does the same thing, I will definately try the wood.
The furnace is in the hallway alcove, only 8 feet from my bedroom.
It wakes me every time the furnace goes on. *thunk* on....*thunk* off....all night long.

I can't keep it too cold, because I have birds, as well as cats and a little dog to keep comfortable.

Hi Barb,


Just for clarification, the wood door is only for a temporary test to know if that is what is making the noise.


If so, you then would need to have a piece of metal or something non-flammable attached to backside of door to keep it from bowing inward.



Do not leave the wood door there more than a cycle to test with.



Thanks,
Robert

Robert, I kind of figured that, when I remembered the inside of the metal door is partially lined with insulation. It's there for a reason.
I take no chances with safety.
Thank you.

Hi Gang, Well it does seem to be the door. Both expansion from heat and vaccum from the blower. The blower suction is so strong that I had to duct tape the tops of my socks to my ankles to keep from loosing them while I was kneeling by the heater. Anybody got a good cure? Thanks Jerry [jad]

Hi jerry,

Is your door backside insulated or bare metal ?



Thanks,
Robert

Hi Robert, The door is insulated with a 3/8" blanket. Just now it banged 3 times. However for a couple of days it didn't make any noise. Beats me. thanks Jerry

Hi Jerry,

They do it moreso without the insulation is why I asked.


The only cure is to place something across backside of door panel and attach it to prevent the bowing.

It must be non-flammable.

Maybe 2-3 small pieces of metal attached under the edge lips directly to door panel cross ways.

Attach with small bolts, washers and nuts.


Anything non-flammable to prevent panel from bowing inward.



Thanks,
Robert
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