Back in October I turned on the furnace to make sure all systems were go with my forced air propane unit for the winter. What usually happens:
Hum
Burner lights
Fan turns on
Humming stops
Heat
Fan turns off
What happened in Oct/Nov:
Hum
Hum
Hum
Hum
I called the outfit who installed it back in '98 and Arlyn and Buddy (no kidding) came to check it out and, of course, it worked fine. They made a note of the model and stuff in case I need a new motor.
Today:
Hum
Fan turns on. I don’t hear the burner light so maybe it’s staying on?
Humming continues
Fan turns off
Humming continues indefinitely unless I turn down the thermostat
Does anybody who knows something about furnaces have an idea as to what’s going on? I want to know if I should have the motor replaced or go for a whole new furnace while I have the money to do it.
Sounds like an intermittent/funky thermocouple. Strange that it would do nothing in October but worked when the boys showed up and now does a third thing. The hum is actually a small fan making sure the burner area isn’t full of gas. Then the pilot (which might be always on or - more likely in '98 - lights with electric spark) heats up the thermocouple to OK the main gas valve to open and give you heat. The fan goes on (theoretically) when the main burner does.
I had the “hum only” once and found a bird had dropped a stick down the vent and jammed the little fan, so the cycle could never start.
If it were mine, I’d get at the back of the valve body and tap it and the thermocouple (copper wire tube thingy) and try a few more times. The parts are cheap and readily available. You may not want to change a valve body yourself, but if you can change a sparkplug, you can do the thermocouple for a few bucks. If no joy, call the lads back and ask them to bring a thermocouple and/or a valve.
In my furnace (natural gas forced air with an electronic igniter), it does your October process when the flame-sensing probe needs to be cleaned.
Here’s how the system works (as I understand it). The thermostat signals the furnace to run, gas flow starts, the igniter sparks and the burner lights. The probe is looking for the heat from the gas flame; when it is dirty, it doesn’t sense the heat, thinks there is no flame and shuts of the gas flow to keep from flooding the house with gas.
When this happens, I clean the probe with some sandpaper or a file, and then it works normally again. I would think that Arlyn and Buddy would have done this during their visit, but it might be worth a try.
A couple more details on newer furnaces. Yes the initial hum is a small blower that supplies combustion air. It runs the whole cycle, you just can’t hear it any more after other stuff get going. Look for a knob that spins when the hum starts. Nothing happens until a sensor reports the last operation worked. Once you have air flow, the ignitor heats up, the gas comes on. It will go off again very quickly unless a conductive flame shorts out the probe to the frame of the furnace. Cleaning the probe may restore conductivity and keep the gas on.
My furnace is a Bryant 80+ installed in 1997. That is how it works.
on my furnace I have adjustable temperature switches that determine how hot the burner gets before the blower kicks on and also one that determines the temperature at which the blower shuts off.
From the OP’s description he needs to recall the repair service. He may need to call another repair service. Just because a service installed a system doesn’t mean they are the only ones that can work on it. If they are reputable and all then yes keep their number on the fridge.
On any forced air unit look to the air filter before anything else is done. Of course that isn’t going to fix everything but a clogged filter causes many other problems.
Vapor fueled heating systems are dangerous and I believe the OP needs to keep hands off until becoming schooled on subject.
As you should of noted, Rain Dog is the Man here and he is asking questions before any WAG’s
Thanks for all the responses. I’m not looking to make a repair myself. I just wanted to understand in general terms what might be going on. Of course, after I posted that I turned it up a tad and it hasn’t done it again since. So I forsee another visit from Arlyn & Buddy in my future. I don’t distrust them but if I’m paying $$ for another service call I want to know something about how furnaces work. The air filter gets changed roughly the first of every month during heating/cooling season.
