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I have a Lennox HE propane furnace in the cottage that is set to 41 degrees when we are not there. We head up most weekends and when we get their I boost the temp to 65 deg and get a fire going in the wood stove.

The problem is the furnace will run for 40 to 45 minutes then shut down, although the thermostat may only indicate 45 degrees. The code points to burners failed to ignite. I reset the trip and again we'll run another 30-40 minutes and shut down. So it's takes for ever for the cottage to heat up. I have cleaned the ac condenser, replaced filter, cleaned flame sensor and igniter but problem still exists. Any ideas before I call a service guy which is over an hour away?

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    Does the furnace light and put out heat while running?
    – d.george
    Nov 5, 2017 at 14:13

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I will assume it is indeed a failure to ignite problem. The way it works is your circuit board puts out about 5 milliamperes through to your flame rod. When there is flame the electrical signal jumps the flame and the ground completes the circuit back to the board. If it doesn’t read approximately 5 milliamperes back to the board it assumes there is no flame and shuts off. It sounds like there is something that isn’t quite making and finally gives up. Common problems are a dirty sensor, faulty board and bad ground. Unless the flame rod is physically damaged it almost never needs replacing. A resistance test should show 0 ohms. Usually the circuit board is bad but I have had issues with the house ground where more than 2 volts were leaking into the ground. This takes a good repair electrician to track this down. If the ground checks out good then by process of elimination I would assume it is the circuit board. The problem with your scenario is that it takes a long time to happen so it may take a while to find.

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  • The furnace does provide good heat while running and we'll maintain the heat once it gets to determined temp. It's just getting there that is the problem without issue. I'll try to give the flame rod a better clean up. Their wasn't any carbon build up and just gave a wipe with a kitchen scrubby. Maybe some very light emery cloth next trip up before calling a service tech.
    – Mjack7
    Nov 5, 2017 at 18:12
  • If it looks clean it is clean. Avoid putting scratches in the rod as this will invite future fouling. I just use a piece of plain white paper to clean but in my area they almost never get dirty as we have good quality gas.
    – user76730
    Nov 5, 2017 at 19:06

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