Trane furnace question.

[lsrx101]

It's not in my garage...yet. I need to get through this season before I can update the heating/cooling plant in my house. I'm hoping to do geothermal next summer, if finances allow. This furnace is slated for the shop if all goes well.

I've got a 1992 vintage Trane XL80 furnace running on propane. I can get the exact model # if necessary. It uses a hot surface ignition and a flame sensor rod.

When I bought the house 2 years ago (almost to the day), the furnace would light, go out, light, go out...and eventually default to running the blower. I found that the flame sensor was bad and replaced it. This is a very common issue and it worked perfectly after that.
Fast forward to 2 weeks ago. I noticed that, when the thermostat called for heat, the burner would light for a few seconds, go out, then light normally. Not every time heat was called for, but about every second or third cycle.

Thinking that the flame sensor was failing again, I replaced it. The old one had 2 "divots" burned into it so I thought it was quite obviously the problem. I guess I was was wrong.
It is still doing the same thing. Burner fires for ~4seconds, shuts down, burner fires normally, blower comes on.... About every 2-3 cycles.There are no codes flashing on the LED telltale.

Any suggestions before I call a Tech?

 

[mshell56118]

Mine did same thing different brand furnace but they all kinda work the same mine ended up being a faulty pressure switch


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[lsrx101]

Mine did same thing different brand furnace but they all kinda work the same mine ended up being a faulty pressure switch


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Hmmm, that's interesting. I was pondering about that pressure switch, trying to think of a way that it might be the culprit. I'll check the voltage drop across it and see what happens when it's supposed to close for a couple of cycles. Could be debris or a spider nest in the tubes, or a flaky switch.
Thanks for the mental jump start.

 

[mshell56118]

Mine had 2 one was safety control the gas valve and one was signal into Ctrl


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[zmaxmotorsports]

It's not in my garage...yet. I need to get through this season before I can update the heating/cooling plant in my house. I'm hoping to do geothermal next summer, if finances allow. This furnace is slated for the shop if all goes well.

I've got a 1992 vintage Trane XL80 furnace running on propane. I can get the exact model # if necessary. It uses a hot surface ignition and a flame sensor rod.

When I bought the house 2 years ago (almost to the day), the furnace would light, go out, light, go out...and eventually default to running the blower. I found that the flame sensor was bad and replaced it. This is a very common issue and it worked perfectly after that.
Fast forward to 2 weeks ago. I noticed that, when the thermostat called for heat, the burner would light for a few seconds, go out, then light normally. Not every time heat was called for, but about every second or third cycle.

Thinking that the flame sensor was failing again, I replaced it. The old one had 2 "divots" burned into it so I thought it was quite obviously the problem. I guess I was was wrong.
It is still doing the same thing. Burner fires for ~4seconds, shuts down, burner fires normally, blower comes on.... About every 2-3 cycles.There are no codes flashing on the LED telltale.

Any suggestions before I call a Tech?

Usually you can clean the flame sensor with a green scuff pad and just reinstall it.

 

[sms1974]

Model # should start TUD unit has green doors? It could be flame sensor but if you have cleaned it and it still lights, goes out, relights ect. It may very well be the ignition control... Post your model # and I'll be able to get you more info

Have you pulled out the burners and cleaned them? Especially the burner in front of the flame sensor. These units use flame rectification to sense there is a flame present this requires both a good flame sensor and a clean burner.

Potential culprits: flame sensor, dirty burners, bad or corroded ground, bad integrated control...
Yes a pressure switch could cause the drop out too but it's a rarity, if that is the culprit I'd bet you have a bad heat exchanger. If the secondary heat exchanger is restricted when the burners fire off it will cause a momentary rise in pressure inside the heat exchanger. Remember the pressure switch reads negative pressure, a rise in pressure would cause the switch to open and drop the burners. You should be able to see that as a voltage drop across the switch terminals.

Theses were 2 stage units if I'm thinking correctly, first stage is low flame and the units are sensitive about flame proving. Being that the unit will run after a few tries I'm betting you have a dirty burner and or a bad flame sensor.

Oh and 22 years is a long life for these they were only given a 20year service life expectancy...