Bad flame sensor?

[Owen Lucas]

My Nordyne furnace was turning on then off before the fan would kick in. I can see the flames going into the heat exchanger but then they would shut off after 5 seconds or so. It would do this a few times until the fan turns on and starts to heat the house. I figured its a dirty flame sensor so I cleaned it up with sand paper as there was soot on both ends. Operation was better and the system would turn on the first try but now it is starting to do the same thing, not as bad as before but I want to avoid it from getting worse.

What would you do?

A. Change flame sensor. ($43)
B. Maybe the limit switch is bad? ($11)
C. Something else is going on.
D. A & B

 

[bullwinkle]

What happens if you set the fan to “on” and then call for heat? Some furnaces, like my Lennox Pulse, use the same switch for fan control (in auto) and as a high limit. My guess would be flame sensor, personally, since you cleaned it & it got better.

 

[Owen Lucas]

What happens if you set the fan to “on” and then call for heat? Some furnaces, like my Lennox Pulse, use the same switch for fan control (in auto) and as a high limit. My guess would be flame sensor, personally, since you cleaned it & it got better.

Just just this, it fired right up.

 

[LazyDog]

Flame sensor do need to be clean some times but I normally keep a spare and also the igniter.

 

[LazyDog]

You can learn a lot about your furnace by watching or paying attention to the starting sequence.

After this, you can almost always tell what part you need because it fails to start at a specific point

 

[oldcodger]

I would change the flame sensor. That is almost always the first item to fail on a gas device.

 

[brianl703]

Most furnaces new enough to have a flame sensor are also new enough to have a light that blinks an error code, along with a list of codes, which will tell you why it shut down. Normally there's a window in the cover to the blower compartment so you can see this blinking light.

 

[Dinoburner]

The flame rod and lead to the control should be changed. Flame impingement on the rod should be constant as when the signal is lost for more than the design limit the control will lockout. Since this is part of the safety system a pro should be called.

 

[Hall]

No circuit board with diagnostic light(s) ?

What's your filter look like ? How long has it been installed ?

 

[Owen Lucas]

No circuit board with diagnostic light(s) ?

What's your filter look like ? How long has it been installed ?

I haven't paid attention to the diagnostic lights, I'll have to run the furnace with the door open. Problem is that this is an intermittent issue. System is running fine now but maybe an hour later I'll hear the system restarting. Filter has been recently changed.

 

[Zahndkile]

It's extremely rare to have to replace a flame sensor they can almost always be cleaned. polish that thing up with a Scotch Brite green scrubber until it shines. I Cary a green scrubber in my tool bag for that very purpose and I clean the sensor on almost every furnace I touch.
Also clean the burner face as it can have the same effect.

 

[CDX825]

You cleaned the flame sensor so my next suggestion would be to check the pressure switch or switches if there are multiple.

 

[Cujet]

Prob the flame sensor. The tension on the internal sheetmetal (it's a spring really) is probably a factor. I've had luck bending them for better performance.

 

[brianl703]

I haven't paid attention to the diagnostic lights, I'll have to run the furnace with the door open. Problem is that this is an intermittent issue. System is running fine now but maybe an hour later I'll hear the system restarting. Filter has been recently changed.

Opening the door to the blower compartment usually kills power to everything.

 

[Hall]

Opening the door to the blower compartment usually kills power to everything.

Oddly enough, I can remove the panel to the blower and burner section and it keeps running but the bottom panel, where only the circuit board is, has a safety switch built-in. I think this is standard on Trane and Lennox (and certainly others too).

 

[Owen Lucas]

Oddly enough, I can remove the panel to the blower and burner section and it keeps running but the bottom panel, where only the circuit board is, has a safety switch built-in. I think this is standard on Trane and Lennox (and certainly others too).

I just tape the switch closed, this is a weird Nordyne unit I've never heard of before. It used one of these massive accordion filters that you have to manually insert into a metal cage, pretty annoying. I've owned Trane, Carrier, Lennox, and Goodman in my previous residences.

I went ahead and ordered the parts, so cheap at this point it's just worth swapping them out.

 

[brianl703]

Oddly enough, I can remove the panel to the blower and burner section and it keeps running but the bottom panel, where only the circuit board is, has a safety switch built-in. I think this is standard on Trane and Lennox (and certainly others too).

Is the blower on the top and pulls through the heat exchanger? Not a common configuration, usually the blower is in the bottom.