The sound seems to increase for several minutes and then will go to a normal operating sound. Our plumbers, who installed the unit, have looked at it and check the fans but can't determine what the is causing the very loud noise. They thought it had something to do with air flow but weren't able to resolve the issue. Any suggestions?
Mistake #1, calling the plumber/installer to diagnose a heating problem; unless of course he was dually licensed as a plumber and a HVACR technician. (Heating, Ventilation, Air-Conditioner, Refrigeration)
I would contact the plumber/installer in case of parts / unit warranty or installation issues since you purchased it and had it installed by him.
Your unit has a combustion motor with a "squirrel cage" type fan attached to it.
The purpose of the combustion motor-fan unit is threefold; it acts to purge (exhaust) out any gas build-up before ignition occurs; it pulls outside air (oxygen) in through the intake pipe which is needed for flame ignition and to maintain it; while at the same time - vents spent/toxic gases out the flue pipe.
There is also one convection fan motor controlling two convection "squirrel cage" type fans. It's purpose is to circulate the heated air created by the heat exchanger, into the room.
On initial start-up, it is the combustion motor/fan unit that kicks in, so if you are hearing the noise at this start up phase, this would be the source of your problem.
The convection fan motor doesn't kick in until there is a heat rise sensed by the low-limit switch; meaning, If the blower came on at the same time as the burner, it would pump cold air through the house at first. The limit switch includes a sensor in the heat exchanger to determine when the air is warm enough to circulate. Once the air reaches a preset temperature, the limit switch turns on the blower.
Therefore if you hear the squealing sound when the convection blower turns on, that would be the source of the problem.
Why would the noise occur, most likely the sealed bearings in the motors are going bad; or perhaps the fan blades are loose and wobbly; or something got inside the fan housing and is chaffing.
A major cause of early, premature combustion fan motor wear is improper installation of the flue pipe; condensation flow back to the motor will damage it
Another reason, manufacturer defect in the motor; it happens.
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