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First blizzard warning in Wisconsin for over 10 years and our furnace decided to stop working this morning. At least I was home because of the weather. Still had to drive a hr round trip in the snow and hope the after market sensor works. It is, for now....
bimetal flame sensors like that can degrade over time especially if they are in the vicinity of your washer/dryer. if you use dryer sheets or fabric softener the waxy component of them can build up on the sensor. add in lint and dust and the sensor life can be cut drastically.
couple of old HVAC techs told me that you can get by in a pinch by rubbing a newish dollar bill along the sensor to clean it off. it's just abrasive enough to clean it and lint free enough to not leave anything behind.
might buy you a couple days to a couple months depending on how badly damaged the sensor is.
the best solution is to properly vent your dryer or move it entirely away from your HVAC unit.
Which part is the flame sensor? Our furnace is getting old, last year I had to replace the exhaust fan pressure sensor. What were the symptoms?
It's attached to the white wire, top right. Looks like the piece in my hand. One scew holding it in. One electrical connection. And is basically a metal rod with a bit of porcelain insulator.
I got a universal part that I needed to bend to match the one in the furnace.
Symptoms were that it wouldn't stay lit. It would try, light up for a second and go out. Exhaust fan stay on the whole time.
We got up normal time for work but she had off and I had a snow day. She noticed that it was getting cold in the house so I went to check. The furnace is plugged into an outlet mounted to the side of it's self. The GFCI was tripped so I reset it. It came to life but just the fan. Then I heard it trying to light and fail a couple times. Looked up some videos about "my furnace won't stay lit" and they all said start there. Just happened to be the problem.
Ah I see it now! Thanks for the info.
Here is how I see how the flame sensor works in my mind:
Is it a flame? 🔥 Yep. Return signal.
I always thought it neat that the flame acts as a rectifier to create a dc circuit from AC.
That’s how I do it too.
MCP: Is there a flame?
Me: *checks, sees flame*
Me: Yup.
End of line.
Nice job! I had a fun one 2 winters ago, the glue on the pressure sensor switch just failed, I've never heard of this happening before. (Admittedly I'm not reading stuff about gas furnaces daily, but still...) The black piece and the white piece separated in the pic.
So the gas turns on, and the fan starts speeding up and there's a pressure switch I guess to make sure there's gas flowing, and the fan is blowing before igniting and starting the whole reaction, and since the switch wasn't working, it wouldn't ignite.
I tried to re glue the 2 plastic halves back together but the glue wasn't holding, so I just zip tied the hell out of the thing, and its been working great for 2 years so far...
Just as a clarifying point(I’m sure some HVAC person can correct me). The fan spins up to make a draft, the pressure sensor is detecting negative pressure(vacuum) and that successful check allows the gas valve to open prior to the igniter/flame sensor. The draft helps pull the gas into the combustion chamber and not just leak it out into the basement or wherever(especially if it fails to ignite). Furnaces kicking on are a series of do an action, verify that action happened, repeat until it gets through its todo list. High efficiency furnaces work similarly but also different…
Knowing the general boot up can help figure out what isn’t working. Igniter never goes off?…pressure sensor is right before it. Furnace ignites but shuts off a few seconds later…Flame sensor didn’t report success.
I keep a pressure sensor, igniter, and flame sensor sitting next to my furnace. It’s like ~$50 for the OEM parts and I can generally fix my furnace in 5-10 minutes.
Measure it's resistance if in a pinch. If it is in the kilohms range it is a thermistor. If it is super low resistance, it is a thermocouple. 10k ohms is the most common thermistor used in nearly everything consumer related. Nearly all common thermocouples are the same type too. It has been awhile, but IIRC they are k-type. The main difference in function is heat range. The thermistor is for lower temps and is less linear across the range in the cheap common ones. The thermocouple is two different types of wire welded at a junction at the tip. The heat causes a tiny voltage potential due to the different metals bonded together.
At scale of mass manufacturing, the thermistor is a fraction of a penny, while the thermocouple is a few cents. However the thermocouple requires an analog amplifier circuit to function, so this adds complexity in electrolytics. A thermistor is stupid simple and only requires a resistive voltage divider and any voltage threshold trigger circuit, so like a zener diode, capacitor, and single transistor.
The packaging of the sensor is the only thing you are paying for, and that is just for its mechanical mount and position, maybe some heat mass stability. A thermistor cannot handle direct flame temps, but a thermocouple technically can. In practice only the packaging of the sensor will contact a flame in some cases. Thermocouples are more rare but usually in any appliances that use natural gas. Thermistors are the third or third and fourth connection in most battery packs and found in almost anything with heat or temperature sensitive constraints.
In a pinch, all you need is the same sensing element. A coat hanger or anything similar may be a way to improvise holding it in place... should you ever need it.
The other type of common temperature regulation is a mechanical switch that uses a bimetal strip that deforms to close contacts. These will not work with the other two. Any device you hear a faint audible click from when heating, is using this type of temperature regulation. Typically old dumb coffee pots, clothes irons, etc. Thermistors are used in most small devices with some type of digital interface and a battery or heat.
flagged for not being dull
I had a flame sensor go out last year. The tech could get it going by just cleaning the sensor, but did come back and replace it when the part came in.
Sadly, I gave up my own troubleshooting when I saw the computer control was flashing an error. Would have saved me a couple hundred bucks.
I looked for some videos about why it wouldn't stay lit. They said the same, clean it off. But mine had a pretty obvious bad spot so I went and found one.
Why does the furnace never stop working in the middle of summer?
You mean it stops working when you are not using it? How often does your car fail while parked?
I don't drive a car.
Fascism