My Lennox G20 furnace stopped working this afternoon during heavy snow... Symptom: pilot stays lit and the ignitor keeps sparking, but the main burner doesn't turn on. Fan starts after probably 30 seconds, but only blows cold air because the main burner is off. Since it keeps sparking forever, I thought it could be either the flame sensor or controller?

The ignition controller is Robertshaw SP735L
The gas valve is Robertshaw 7100 (DER S7C 71F-11A-017)

Here's what I observed so far: The gas valve P terminal got 26V AC. If I short P and M, the main burner will turn on as usual (while the ignitor still keeps sparking). So I believe the valve itself is fine. I checked the flame sensor current, it is 0 before ignition and 1.8~1.9 microAmp DC after the pilot is lit, a little bit low according to the service manual (should be in 1~5 microAmp range). Is this number normal for a flame sensor? should I open it up to clean up the sensor first? or focus on the controller? what else can I check? your help is highly appreciated!

PS: the sensor is installed behind the ingintor, kinda difficult to replace without disassembling a lot of other components.

 

Your reading is good but you probably need to clean your pilot orifice.





I don't see anything about cleaning the orifice in the installation manual, but that may be because it isn't a service manual...

http://www.lennox.com/pdfs/installat...ox_G20_IOM.pdf

 

Thanks Houston204! I was looking at a slightly different manual, which says the sensor current range is 1~5. www.completeheating.ca/manuals/LennoxG20.pdf

so you think the pilot gas flow is too strong, which prevents the controller from turning on the main valve?

 

Clean the orifice in the pilot burner and make sure it is properly aligned to light the main burner. A dirty orifice will cause a smaller-than-normal pilot that might not light the main burner.

Also, although I am not familiar with your furnace I suspect the pilot assembly is similar to the Carrier three-wire pilot that heats an internal bi-metal thermostat that both shuts off the spark AND opens the main gas valve when the proper pilot is established.

 

I have never read over 2.4uA DC on a Lennox furnace. 1.7 to 1.9 is average with Lennox. I check flame rectification on every furnace that I check on a maintenance call that has a separate wire for flame rectification. I don't see many spark ignition systems on maintenance calls.

I see closer to 5uA DC with Carrier and Trane.

I don't think that your pilot gas pressure is too high. You will probably need a new control board, but I would clean the pilot orifice and all ground connections before spending money on parts.

 

sorry I was not clear in the first post. while the pilot is lit and ignitor keeps sparking, I got 26V from the gas valve's P terminal, but 0 on the M terminal (MV on the controller). So the main valve is shut off. I can manually turn on the main burner by short P and M, while the pilot is lit. So this seems to me either sensor signal is too weak, or the controller for some reason decides not to turn on the main burner?

 

Thanks Houston204 for confirming the sensor reading with Lennox! I will definitely check the pilot orifice and ground connections.

This is a quite old furnace (30yrs?). I didn't expect amazon is selling the control board for $169...

 

I remember when those boards were 50 bucks, but I am getting old.

If you end up replacing the board, there is usually an issue with the 24V and TH terminals.
Read the instructions closely.

 

As others I suspect you have only a dirty pilot to contend with. So far anyway. Are you able to blow back thru the pilot hood, try this before disassembling.
This not a bimetallic system, pilot and flame rod.

 

quick update. I managed to locate a Honeywell S8610U from local craigslist listings on Sunday afternoon. Replaced the old Robertshaw SP735L and everything back to normal at the very first try! The new Honeywell control board is much advanced than the old one it replaces. There's even has a LED indicator to show error messages, including sensor current (it blinks 9 times meaning 9uA after the main burner is fully running).

Thank everyone for the suggestions. Happy holidays!