Why does my furnace lose power randomly?
February 4, 2018 12:30 PM Subscribe
Occasionally the power to my furnace completely cuts out, and my thermostat goes blank. I have to turn the furnace power switch off and on a few times and it comes back, but it's very annoying. I've never caught it in the act of cutting out, but to the best of my knowledge it happens when it's heating up, and happens more frequently the colder it gets outside. What might be causing this?
Additional details: I recently installed a C-wire thermostat in the fall. (Honeywell Lyric T5) This never happened before when I had a battery operated thermostat but I'm not sure that it's related to the new thermostat. I originally suspected the thermostat but I contacted their support and ran through troubleshooting with them and they think it's something with the furnace. When the furnace has power, all the voltages on the wires are exactly within spec, but when it's out it has no voltage.
Additional details: I recently installed a C-wire thermostat in the fall. (Honeywell Lyric T5) This never happened before when I had a battery operated thermostat but I'm not sure that it's related to the new thermostat. I originally suspected the thermostat but I contacted their support and ran through troubleshooting with them and they think it's something with the furnace. When the furnace has power, all the voltages on the wires are exactly within spec, but when it's out it has no voltage.
Best answer: I had a similar problem recently. Turned out to be a faulty flame sensor.
The flame sensor detects if there's a flame. If it does not detect a flame, it shuts off the gas.
If it shuts off 3 or 4 times in a short span, the furnace will then shutdown and enter a lockout mode for a period of time.
There is a manual procedure to override the lockout enabling you to restart earlier
It was written somewhere in the manual
The symptoms I had were very similar to yours.
I thought the thermostat was toast. No display etc.
The furnace would run ok until it hit the preset temperature.
Then shut off. When the temp then dropped it tried to restart, but it wouldn't succeed half the time then entering lockout
The flame sensors get dirty, coated with carbon over time causing them to not function properly
posted by yyz at 1:56 PM on February 4, 2018 [1 favorite]
The flame sensor detects if there's a flame. If it does not detect a flame, it shuts off the gas.
If it shuts off 3 or 4 times in a short span, the furnace will then shutdown and enter a lockout mode for a period of time.
There is a manual procedure to override the lockout enabling you to restart earlier
It was written somewhere in the manual
The symptoms I had were very similar to yours.
I thought the thermostat was toast. No display etc.
The furnace would run ok until it hit the preset temperature.
Then shut off. When the temp then dropped it tried to restart, but it wouldn't succeed half the time then entering lockout
The flame sensors get dirty, coated with carbon over time causing them to not function properly
posted by yyz at 1:56 PM on February 4, 2018 [1 favorite]
Forgot to add: one of the procedures for disabling lockout was to flip the power switch a few times.
posted by yyz at 2:02 PM on February 4, 2018
posted by yyz at 2:02 PM on February 4, 2018
Best answer: Sounds like the flame sensor to me too. You can remove, clean, and replace the sensor fairly easily. I’ve done it myself, and I’m not particularly handy. Keyword search YouTube for instructional videos. Best of luck!
posted by Don.Kinsayder at 2:29 PM on February 4, 2018 [1 favorite]
posted by Don.Kinsayder at 2:29 PM on February 4, 2018 [1 favorite]
Best answer: This exact thing has happened with my furnace. I called a local furnace repair place and they talked me through how to clean the flame sensor over the phone—saving me a costly weekend repair visit. As said above, check YouTube and you should be good to go.
posted by bookmammal at 8:17 PM on February 4, 2018 [1 favorite]
posted by bookmammal at 8:17 PM on February 4, 2018 [1 favorite]
Response by poster: Thanks all, definitely sounds like flame sensor based on the symptoms. I found mine and it was a bit dirty so I cleaned it up, and if that doesn't permanently fix it I'll replace it.
posted by sputgop at 12:17 PM on February 5, 2018
posted by sputgop at 12:17 PM on February 5, 2018
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- Thermostat
- Control transformer
- Control circuitry internal to the furnace
- Bad power switch
- Something else
If you still have the old thermostat (assuming it works) I'd swap it for the current thermostat as an easy check.Also you may want to check the control board in the furnace to see if it has any lit diagnostic lights.
posted by Mitheral at 12:56 PM on February 4, 2018