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Old 03-10-2009, 11:09 AM   #1
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Lennox Merit watchguard


Hi All,

I am having some trouble with a Lennox Merit series furnace. At times, it runs fine, and sometimes it won't successfully run. To fix it, I turn off the power to the furnace, wait five seconds and power it back on, and it then runs for a random amount of time before I have to do it again.

When it fails, it does the following:
  1. a small evacuator blower motor starts.
  2. The glow plug? lights up.
  3. The burners ignite,
  4. three seconds later, the gas shuts off.
  5. The evacuator blower still runs for about five seconds
  6. It returns to step 3 three more times, performing the same.
  7. The glow plug? turns off.
  8. after about 10 seconds the blower stops.
  9. The diagnostic lights alternate in a slow-flash pattern, which appears to be a watchguard fail. Fortunately, Lennox puts everything other than installation faults in one damn light sequence, so I can't tell if under-pressure, no flame, etc.
When the furnace works, step 1, 2, 3 all execute and the burners only shut off when the furnace reaches the set temperature.

I replaced a cheap programmable thermostat, just in case it was doing something - and still the problem remains.

The furnace is only 4 years old - is there anything I can do, or is it time for the repairman to pay a visit? (AKA, charge me for something expensive!)

Any help is greatly appreciated!

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Old 03-10-2009, 04:55 PM   #2
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Lennox Merit watchguard


Post the model and serial#. I am a Lennox tech. Sounds like the flame sensor is dirty. Sits in front of the left burner and can be cleaned with steel wool or emory paper and looks like this. Also take some emory paper and lightly clean the face of the burner it sits in front of:
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Old 03-11-2009, 06:14 AM   #3
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Lennox Merit watchguard


Agree, has the sensor ever been cleaned?
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Old 03-11-2009, 09:36 AM   #4
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Lennox Merit watchguard


While I don't have an answer yet (the furnace never has a problem immediately after switching the power off and then back on - even for a couple of days) the sensor was noticably dirty in and especially on the outside of the burn area.

The burner didn't have much on it from what I could tell.

I cleaned it and will report back. To my knowledge (we just purchased the home) the furnace hasn't had any maintenance done. I may schedule that soon.

Just for information's sake, I posted the 'before' image. It was really easy to remove, clean and put back in.

Thanks for your suggestions!
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Old 03-11-2009, 12:36 PM   #5
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Lennox Merit watchguard


sounds like sensor,as well those units are prone too filling up with water[ventor] due too blockage at the bottom of the flue trap.pull little black hose off and blow thru it may be tripping off on pressure switch.
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Old 02-04-2012, 01:01 PM   #6
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Lennox Merit watchguard


Quote:
Originally Posted by yuri View Post
Post the model and serial#. I am a Lennox tech. Sounds like the flame sensor is dirty. Sits in front of the left burner and can be cleaned with steel wool or emory paper and looks like this. Also take some emory paper and lightly clean the face of the burner it sits in front of:
I had the exact same symptoms, so I decided to try this solution and it worked! Thanks for saving me $120+ on a service call.
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Old 02-04-2012, 01:04 PM   #7
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Lennox Merit watchguard


send some $$ to your favorite charity. Pay it forward, works for me.

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