Curious Self-Desctructive Furnace

[Lotare]

Anyone seen this? Call to residence, noise from indoor unit. Arrive, loud banging from duct work. Go to furnace in basement - 80% furnace, older Lennox. Not vibrating but man, it was a loud banging. Turn it off, pull the access doors, make the door switch. I notice nothing unusual, no hard vibration from the blower motor nor can I make out what is banging. I pull the motor. Some of the metal housing is on the loose side from the spot metal welds. I shrug it off and run some screws here and there. My goal is not to repair it, but, but tightening it up, I could perhaps accurately pinpoint. Everything seems tight. I turn it on before reinstalling, it doesn't vibrate. Put it in, start it up - same noise. I pull it back, lay down in the furnace and look at the heat exhanger. It is pressed plates, individual. The middle two seem loose. Too loose. I pull the burners out and check the screws mounting the heat exchanger Everything seems alright. There isn't anything until the flue gas collector box that really seems to hold it in place..... I tighten everything anyway, put the motor back in. Sound is gone....for two days. i go back, over size the screws to the exchanger... Goes away for 2 weeks.... I shall be an old man early I think.. Notes : Despite zoning, there is no air flow issue - however, 1 zone closed increases the noise and speed of the banging. Duct work should be sized close enough to handle it.....

[Lotare]

My post count is low. I'll work on that

[Texas-Tech]

Does it only do it when the heat is on or will the fan running cause the noise.

[Lotare]

Fan.

[JPJ13]

Sounds like a possible return air issue. When you have the blower door off the furnace is getting sufficient return air through the opening and it runs smoothly and quietly...then once the door is back on and the static pressure increases the chance of the noise increases. I wouldn't worry about the needle in the haystack that is a potential loose screw or broken mount somewhere, but instead would focus solely on ducting issues. I would take return and supply static pressure readings for each zone, and find where the duct restriction is.

[Lotare]

Here is the catch. If I manually wire both dampers open, and take the blower door off.....it is worse. And in case it is a question, yes the dampers were actually open. MORE airflow enhances the ability of the burners to bounce around. Finally chose to replace the furnace. It will stick around in my head tho because I never really accurately deduced the exact issue that caused it, other then the argument that age caused the burners to be too loose - an answer that I dislike because it doesnt truly address what problem arose. Never seen it before like that either. Strange....

[Samshawn]

it need to be inspected while it's running under air pressure.