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Just replaced all the goodies in the furnace aside from the limit switches, and the heater elements. I got the unit to come on without sparks, but I do hear a slight hum when I turn the breakers on. It does it whether the thermostat is turned to Cool, Heat, or even Off. Is it something I should worry about, or is it normal? I don't remember ever hearing it before, but I don't think I ever paid any attention, either.

Hi,

Most likely the transformer. It happens as they age and wear. Wouldn't necessarily mean it's going bad, it could go like that for years. However, they usually fail at worst time, so I always recommend replacement when they begin to hum.

Take care and best wishes,
Robert

That's good news. I just finally got all the parts replaced that were fried in the previous owner's attempt to thaw the A-coil. The transformer was the first on the list, because it had no output voltage.

But when I get the new one in, the sequencer, the fan relay, and the fan motor were all shorted really badly. One phase of the motor winding is burned to a crisp. So now I have a new blower motor, blower relay, sequencer, transformer, blower capacitor,

But now that is all fixed, and the only thing I notice is the hum. So as long as that isn't gonna burn me to a crisp in my sleep, I am happy.

Nope, it will just quit working. It could short out though and any short always has the potential to be a fire starter.

Before replacing though, I would want it inspected since it's new.

Also, any time you replace a transformer or on any unit, install a 3 amp ATC Automotive Type fuse and fuse holder in the hot/power wire of the 24v side of transformer.

Some may have a circuit board and have this protection, but most do not.

Take care and best wishes,
Robert