Loud low hum in furnace at startup

Bruce and Jenna · 12-19-2010, 08:14 AM

My neighbor's furnace makes a LOUD low humming noise at startup. So loud it wakes me up at night.

I want to talk with him about it. But would like to know what to suggest when I do. Dunno what kinda furnace it is.

Oh yeah. I also noticed it takes more than one ignition to get it going, i.e. it flames out and the ignition fires again.

Any thoughts? Thanks.

Zzzzzzz.

**"007"** · 12-19-2010, 12:35 PM

May have a dirty tube and is looking for air to keep burner lite, cob webs in tube for starters or the slide air adjuster is loose and not open enough.

davecampbell · 12-19-2010, 03:03 PM

the gas valve is a solenoid, it may be humming? Normally the only thing that hums is the control transformer (110V to 24V AC to run the thermostat and gas valve).

Transformers can hum if the windings or laminants are loose.

**RV Wizard** · 12-20-2010, 01:23 AM

There are many things that can cause noises and lack of maintenance is a sure way to shorten the life and invite improper operation of the furnace. What year is your neighbors coach? If it is an older unit, the noise is probably the fan motor as the bushings have worn. This can also cause the unit to not fire up properly if the air flow is insufficient to set the sail switch. You might just mention to your neighbor one cold morning that you noticed his furnace struggling to keep him warm the night before. It might motivate him to get it serviced, or if need be, replace the motor. and clean the unit.

Bruce and Jenna · 12-20-2010, 10:06 AM

Thanks to everyone for suggestions. I will talk with my neighbor in the next few days.

BTW - it's a Bighorn/Heartland fifth wheel. Looks fairly new.

wa8yxm · 12-20-2010, 11:34 AM

I doubt it is the gas valve or the blower motor or anything else that burns 12vdc.. What can humm is the ignition transformer.

The auto-spark igniter starts by converting DC to AC (inverting) then uses a transformer (most likely and auto-transformer not that that matters) to jack the voltage to the thousands range,, This is what jumps the spark gap and fires the burner.

Since it's the only place that AC exists.. It's the hummer.