Lennox Condensing Furnace will not stay lit more than 1 hour


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Old 12-21-10, 04:44 PM
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Lennox Condensing Furnace will not stay lit more than 1 hour

I'm reposting this thread to see if there are any other input. The initial problem is shown just below.
"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 handler stays running.

Any thoughts? Thanks in advance!"
This is the rest of the thread.

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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Yesterday 06:38 PM #4 SeattlePioneer
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Posts:2,944Well, 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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Yesterday 01:50 PM #3 kajayda
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Posts:3Seattlepioneer, 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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Yesterday 11:31 AM #2 SeattlePioneer
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Posts:2,944Good 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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Yesterday 10:03 AM #1 kajayda
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Posts:3Lennox 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 12-22-10, 06:02 PM
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i think that you answered your own question,the secondary heat exchanger is probably dirty with black soot and gunk. you would have to take the furnace apart,shut down power first,and clean the deat exchanger.you would need the parts manula to show how to access and clean. this is probalbly best ;eft to a company,unless you feel up to the task/
 
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Old 12-22-10, 06:38 PM
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It's not unusual to have some black mold growing in the drainage system of a condensing furnace.

The diagnostic code you are getting implies that the flame sensing system of the ignition control and flame sensor are not detecting that the main burner is lighting, and the main burner gas is shut off because of that failure to light.

If the condensate drainage system or a plugged heat exchanger was the problem you would be getting a diagnostic code for the pressure switch opening.

You've replaced the flame sensor. That leaves three possible problems 1) the burners are dirty and can't light off the hot surface ignitor 2) the hot surface ignitor isn't getting hot enough to light the gas or 3) the ignition control is failing to detect that the burners are lit and needs to be replaced.

Is the HSI glowing white hot? is the voltage being applied to the HSI 120 VAC or thereabouts? If so, the HSI is probably not the problem

Measure the AC voltage being applied to the wire to the flame sensor when the HSI or burners are lit and report that here.

Report on the condition of the burners. Is there significant amounts of crud and junk that has accumulated on them?
 
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Old 12-23-10, 05:19 AM
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Thats to Seattle Pioneer and Seabee570. I had a technician look at the problem and we were able to catch the flame shutting down. The pressure switch was shutting the gas valve. After a lot of trouble-shooting we suspect a fouled secondary heat exchanger. The burners are very clean and this furnace is 9 years old.

We did find the the gas valve was regulating to 4.5 "WC, so the technician adjusted to 10 " WC as was indicated by the manual for propane.

Do you think the low gas pressure could have caused the soot problem or is this a non-issue?

The differential pressure would also vary quite a bit between 1.25" to less than 0.75" (cut-off pressure). It was never consistent, except that eventually it would always drop after 1 - 20 minutes of running and shut the unit down.

I'm considering replacing the unit with heating strips for aux heat with the heat pump or trying to clean the unit myself. The propane costs are killing me!

Thanks for your final thoughts if you have the time.
 
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Old 12-23-10, 08:35 AM
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I would have used a calibrated carbon monoxide tester to measure the actual amount of CO in the combustion gasses, which would be a guide to how the combustion process is working in the furnace.

Adjust the gas pressure and check again to see on the effect your change has had.

It wouldn't surprise me that a markedly low gas pressure might cause combustion problems and high levels of carbon monoxide. Di you ever notice off odors coming from the furnace vent when the furnace burners were operating?

I would recommed against a DIYer tearing into the secondary heat exchanger to clean it. In many cases you are going to have to tear much of the furnace apart ----- and then put it back together again correctly. That's not something I'd recommed for the DIYer in general.

Upon completion the CO level should be checked again to verify that it's acceptable and normal.


Sorry to hear you are having a difficult problem like this. A fouled secondary heat exchanger is one of the risks of having a high efficiency (90%+) furnace. Having them get plugged is a rare event at least.
 
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Old 01-10-11, 05:22 AM
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Wilkie

quick reply from my iPhone. Just had exactly this problem and was not fixed by repair of flame sensor. Had new high efficirncy ac coil this summer. When air filter became somewhat clogged trouble started. First was fixed with new filter but as days got colder it started again. The noticed lower air flow than before new coil and raised fan speed. Problem now fixed. Hope this helps. I guess low air flow can trigger this and the slow alternating LEDs.
 
 

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