Fall has arrived and the temperatures at night are starting to get uncomfortably chilly. If you haven’t already, now is the perfect time to do a checkup on your furnace to make sure it is ready for another cold winter.
One of the side effects of being a landlord, is that I now have 5 furnaces to maintain. Three of those furnaces are older than me, so breakdowns are a fact of life.
A couple of days ago I went to turn on the furnace at one of our vacant apartments. I was expecting some trouble because this same furnace acted up last winter and made a fool of me. Last year, I had to call a HVAC company to come in because I couldn’t figure out how to get the pilot light lit. In my defense, if I had the benefit of more time I could have figured it out. Alas it was mid December, the temperature in the apartment had already fallen to 50°, and I didn’t want to leave a tenant overnight without heat so I broke out the wallet and bought a life lesson to the tune of $110.
Ouch. Lesson learned. Always bring an extra long lighter that can reach into those nooks and crannies to light the pilot.
Fast forward to the present and I swaggered into the apartment with my extra long lighter. BAM, pilot light lit. I felt suave.
Then nothing else happened. The main burners didn’t fire up, the furnace did not make any heat, and my overconfidence was cut down where it stood. This was going to be more complicated than I thought.
It was time to run through the standard checklist.
- Has the gas bill been paid? (Yes)
- Do any other gas appliances work? (Yes, the hot water heater next to the furnace was working)
- Is the electrical breaker on? (Yes)
- Is the electrical shutoff switch set to On? (Yes, these look like light switches and are mounted near the furnace itself)
- Is the thermostat set to Heat? (Yes)
- Is the thermostat temperature set to higher than the room temperature? (Yes)
- Is the pilot light on? (Yes)
Well fudge buckets. That exhausted my standard troubleshooting list. It was time to learn more about how gas furnaces work!
Here is what a 40+ year old furnace looks like with the front panel taken off.
They are pretty simple which is good for us DIYers. The black pipe that comes into the furnace cabinet from the right side is the gas pipe. You can follow it around until it runs into a big silver metal box with the red knob on it. That is the gas valve. The gas valve has two electrical connections to it, a small silver pilot gas line, a large black gas line that goes to the two main burners at the bottom, and a small copper tube that runs to a sensor. In the above picture, I have pulled that sensor out of its normal position.
Let’s talk more about that sensor.
It is called a thermocouple.
I bought a new one for $8. A thermocouple is an analog sensor that detects whether or not there is a lit pilot light. If the pilot light is off, then it tells the gas valve to close so your house doesn’t get flooded with gas and blow up. It works by generating a tiny electrical current when it is heated up by the pilot light. A working thermocouple generates between 25 and 35 millivolts. For reference, a AA battery generates 1500 millivolts. You can see that thermocouples produce a tiny amount of voltage. That voltage has to be strong enough to activate a solenoid (or in simple terms a door) within the gas valve. If the thermocouple is worn out or not positioned properly and getting bathed by the pilot light, then the gas valve will shut off the flow of gas to the pilot light and the main burners.
With the thermocouple replaced, I measured 27 mV of DC with my multimeter. The new thermocouple was operating within specifications, but the main burners STILL weren’t coming on.
The next thing to test is that the gas valve is getting the call to heat from the thermostat. On older gas valves, this is a piece of cake. Set your multimeter to VAC, volts AC, and put the test leads on the two electrical connections on the top of the gas valve like so.
We should expect to see 24 V on the multimeter when the thermostat calls for heat. It was showing 0.
Let’s take a quick look at the thermostat on the wall. After removing the faceplate it looks something like this.
Thermostats are pretty darn simple. They take 24 volts AC from the red wire and send it to the yellow, white, or green wires.
- Yellow = Cooling
- White = Heating
- Green = Fan
With the furnace power off, I simple connected the red to the white wires. Basically, I took the thermostat out of the equation. This is a constant signal to the furnace for more heat.
And would you believe it? I got 24 volts back at the gas valve!
The main burners came on now that the gas valve was energized. The problem was with the thermostat. Rather than try to troubleshoot the thermostat electronics, I simply replaced it with a spare one that I had laying around.
A multimeter is an extremely useful tool for troubleshooting furnaces.
There are many points of failure for a furnace, but there are also many points that you can quantifiably test. If there was still no volt reading after I jumped the red and white wires at the thermostat, I could have jumped red and white directly on the control board in the furnace cabinet. That would have ruled out any faulty cabling from the furnace to the thermostat. Sometimes, an overzealous picture hanger can put a nail right through the thermostat cable. 🙂
I’m glad that I was able to figure this one out. I was getting close to the end of DIYer troubleshooting and into paying big bucks for a professional to come out. The most important thing is to keep your confidence level up. You can do this! Think it through and use that magnificent brain of yours to solve the problem.
Such a helpful guide for troubleshooting gas furnaces! The tips on checking the pilot light and thermostat were exactly what I needed. I feel more confident tackling minor issues myself now. Thanks for the great breakdown!