Lennox Propane 90% Furnace Will Not Stay Lit

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Old 12-20-10, 08:03 AM
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Lennox Propane 90% Furnace Will Not Stay Lit

Any help would be appreciated. The error code is alternating slow flashes stating the furnace fails 5 times to ignite. The troubleshooting guides says that it could be the flame sensor or condensate back-up. I changed the sensor and that did not fix the problem. Then I cleaned all the condensate lines. There was some black fine particles in these lines, althought they were still draining, I thought the condensate might be backing up. I've checked/tested the pressure switch and it is shutting the gas off at 0.75" WC (as normal) and the pressure drop is 1.4" WC when the furnace runs for 15 minutes and completely warms up. The unit shuts down more often when really cold <25 F. I have a heat pump which runs when outside temp is >40 F.

I suspect the black soot may have fouled the heat exchanger and the condensate may still be backing up inside the furnace. Any ideas? My next thought is to call a Lennox repairman, but I'm worried that he won't be able to solve the problem since it is intermittent. My model number is GHR26Q4/5-100-6. The flame is blue and looks to run well through all burners. I think something is making the gas valve shut off (not the flame sputtering causing the flame sensor to kick it off).

The furnace usually turns off in the middle of the night, then tries to reignite 5 times and then gives up. The air handle stays running.

Any thoughts? Thanks in advance!
 
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Old 12-20-10, 09:31 AM
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Good job on checking the pressure --- but is it the pressure switch opening that shuts off the furnace?

Please quote EXACTLY what the diagnostic code says.

It would be unusual for the same diagnostic code to detect a pressure switch that's opening (plugged condensate drain) and a bad flame sensor.


Since the flame sensor is OK, you might have a bad control module that's failing to detect that the burners are lit. To check that, start by checking to measure what AC voltage you have applied to the flame sensor when the burners are lit. If the 24 or 120 VAC voltage remains steady when the furnace burners shut off, report that.
 
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Old 12-20-10, 11:50 AM
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Seattlepioneer, thanks for your reply. I am not sure if the pressure switch is cutting the furnace off. I borrowed the diff pressure gauge from my work and only measured it after I cleaned the condensate lines. After cleaning, every thing was working for several days but now it is cutting off again. It is hard to be there exactly when it cuts off to see if the pressure switch is the cause by watching the guage drop below 0.75"WC (that is the switch set point). I did test that by squeezing the tube and when the guage drops to 0.75", the gas valve closes. So the switch is working properly.

I tried testing the flame sensor voltage, but it is measured in microvolts and my multi-meter doesn't go that low. Lennox says you have to buy a converter to boost the signal for a multi-meter to read it.

If condensate backs up in the furnace, what would causes the furnace to shut down? Is it a drop in differential pressure or lack of air flow to the burner and then the flame sensor drops it out? This is just a general question and may not be applicable to what is going on in my unit.

The error code is alternating slow flashes on the two LEDs. The manual says there are 4 causes "Sensor or wire not installed properly, sensor or sensor wire broken, sensor or sensor wire grounded to the unit, control does not sense flame"

The above was the last error code I got - at 5 AM this morning. But previously I've got LED #1 off and LED #2 slow flash. The manual says on this failure " Condensate drain line is not draining properly or low pressure differential at the pressure switch.

I've also checked the inlet and exhaust pipes. They are not blocked. This unit ran well for 9 years and now it is having trouble staying lit on really code mornings. The burners look very clean and there isn't any soot near them. But I do have black fine particles mixed with the condensate. They formed a sludge that I cleaned out of the drain lines and trap unit last week. After that was cleaned it ran well, but just wondering if the sludge is building up again and causing poor drainage.

I'm open to switching the flame sensor wire and/or the board, but my wife is losing confidence and ready to call the repairman. I'm just afraid that he won't be able to quickly diagnose the problem beause it is intermittent and run up a big bill. I'm okay calling one, but I just wanted to get some good trouble-shooting done first.


Thanks again.
 
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Old 12-20-10, 04:38 PM
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Well, Lennox is wrong.


The flame sensor works by a process of flame rectification:

An AC voltage is applied to the flame sensor via the wire. When flame engulfs the flame sensor it rectifies a small amount of the AC voltage which causes a small DC current to flow through the wire, through the flame and to the chassis ground.

So you typically find 24 VAC or 120 VAC on the flame sensor wire when the flame sensor is engulfed by flame. That's the first thing to test.

Then you can measure the DC current that should be flowing. Typically 1 microamp of DC current will keep the burners on, and 5 microamps is a typical good reading with a clean flame sensor.

Keep in mind that 1000 microamps = 1 milliamp.

Are you sure you weren't confusing millivolts with microamps? It takes a good digital multimeter to measure down to one microamp.
 
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Old 12-20-10, 07:46 PM
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Thanks for your valuable explanation on the flame sensor. I understand now about how it works. You are right, I was unable to reach the low amp current reading with my standard multimeter.

Can you also help me understand how the condensate backing up in the the furnace would shut the unit down? Is it the differential pressure switch or lack of air (oxygen) to the flame and the flame sensor shuts it down? Just want to know your thoughts on the most likely situation. I'm not even sure condensate is backing up. It seems to flow quite freely from the trap.

I also need advice on how to trouble-shoot my situation. Thanks!
 
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