Furnace start-up hum

[MTPockets]

I've noticed that there is an occasional loud hum just as my furnace is beginning. The sound is similar to a transformer or electrical hum. It's not every time. More often, I hear a click, then the blower comes on, but occasionally, I get the 1 or 2 second hum. Never a problem as everything works well, just trying to understand the source of the sound. It's been like this pretty much for four years. Anyone know?

 

[danemayer]

Terry,

There thermostat fires a relay in a box located above the air conditioner return. That relay provides the 12V to start the furnace. If that's where the click and hum are coming from, I would assume the relay has some issue.

 

[TravelTiger]

I've noticed that there is an occasional loud hum just as my furnace is beginning. The sound is similar to a transformer or electrical hum. It's not every time. More often, I hear a click, then the blower comes on, but occasionally, I get the 1 or 2 second hum. Never a problem as everything works well, just trying to understand the source of the sound. It's been like this pretty much for four years. Anyone know?

Ours occasionally makes a loud low hum, almost a growl, like "grrrrrrrrrrrrr, grrrrrrrrrrrrrr" once blower is on, and just after the click, but the furnace has always worked fine.

 

[MTPockets]

Ours occasionally makes a loud low hum, almost a growl, like "grrrrrrrrrrrrr, grrrrrrrrrrrrrr" once blower is on, and just after the click, but the furnace has always worked fine.

Yep, that's it per-zactly. Since it's always been that way, think I should get a relay to have on hand just in case?

 

[danemayer]

Yep, that's it per-zactly. Since it's always been that way, think I should get a relay to have on hand just in case?

You'd have to check if Dometic sells the relay separately. If not, you could probably find a comparable relay, same form factor and rating. But if it's not pluggable, you might have to do some soldering. Depending on how it's designed, it's also possible the board could be the source of the problem; i.e. if the thermostat tells the board to apply voltage to the relay, there could be a component problem on the board.

 

[NHCelt]

Could it be coming from the furnace itself just after ignition. Depending upon weather conditions it could be a type of flame roar caused by an initial lack of draft. I've heard some furnaces that are periodically funny sounding.