Our 1986 Lennox Pulse furnace started making a low-pitched humming noise at times. There's also a bit of a burnt electrical smell in the area. The noise seems to be coming from the metal duct area beside the furnace, but it's hard to find any specific source other than 'the furnace'. I was able to do some testing today.
We had a service guy look at it this afternoon and so far can't find anything wrong. Of course, while he was here it didn't make the humming noise and operated perfectly. My wife commented she thought the smell was like when you start the furnace the first time in the fall after not running it for several months. It was strong enough this morning for her to notice and want to investigate.
The service guy checked the load on the motor and it wasn't drawing a large amount of current, wasn't hot, and the dust on it didn't seem burnt. When the system was cycled a couple times, all seemed normal. He then took out the tray with the relay and transformer, saying when they started to fail they get hot and sometimes make a buzzing noise that can resonate through the furnace. Both the relay and transformer weren't getting hot and tested fine, though. He did find a wire for the lower arm of the blower where some electrical tape had fallen off. When he pulled the tray out, it connected to the tray and started the blower when it shouldn't. He put a twist-on wire nut on that so that wouldn't happen again, but said that doesn't seem to be the real issue. After placing the tray back, he looked at the large black 24V ignition transformer, but that also seemed to be fine and he commented that they generally stop working completely when they fail. So, there's something wrong somewhere, but we're not seeing it.
After reading about issues with this furnace, I couldn't resist a bit more testing and that led to lots of fun tonight.
First, I turned the furnace circuit breaker on and kept heat turned entirely off at the thermostat. The humming noise started after ten minutes. It seemed odd that the duct just above was furnace was warm. I called the service guy and he had me feel the mount side of the motor. It was almost too hot to touch, so he thinks the motor might actually be going bad and is going to come back tomorrow for more testing and with a spare motor.
The fun started when I checked the First Alert carbon monoxide sensor in the bedroom and the reading was 400ppm. It wasn't beeping, but the manual said that was a dangerously high level. I reset it and it was soon beeping to indicate the levels were too high. A few calls later and the entire family was sitting in the car waiting for the power company's gas leak person to come and see if the house was safe. Thirty minutes later, he came and checked out the whole house for carbon monoxide and gas leaks. The result? Zero carbon monoxide or gas leaks. It looks like the First Alert meter wasn't working correctly. All that is a relief, as we were looking at airing out the house for a day and finding a hotel at 11pm.
Does all this sound like it could be caused by a motor that's starting to go bad? With a furnace almost 27 years old, I'm starting to wonder if it might make more sense to replace it before other parts fail.