York limit open failure help

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  #1  
Old 12-18-16, 06:05 PM
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Question York limit open failure help

Hello all, hope I can get a lead. Intermittent issue here...

12 year old York upflow furnace. Furnace turns on with call for heat. Works fine, hits temp at thermostat, burner turns off as normal, vent blower keeps running for minute then turns off as normal, main blower runs for a minute, then turns off as normal. But then instantly after that, the main blower turns back on and 4 red flashes start. Blower stays on until I turn furnace off. Did some reading online and got a few times... changed the filter 3 days ago for a new one (was dirty) and still happens intermittently. Some vents/registers were closed in unused area since it’s winter here now. I opened them all two days go, same result. Right after one of the failures, I turned off furnace and I put a meter on the L160F-30 switch and it was closed. This as I understand it means the switch is likely ok. Any time there is a failure, if I turn off the system, wait a minute and then back on, it works until next failure. Fails about 1-2 times in 24 hours currently.

What is next to try?
Thank you!
 
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Old 12-20-16, 06:53 PM
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When you measured across the limit, how did you do it? On some furnace boards you can adjust the "fan off delay". If so on your furnace, you can try a longer fan off delay.
 
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Old 12-20-16, 07:05 PM
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I just disconnected the wires to the switch and put a meter on it and it showed the switch was closed. My understanding is this meant the switch was fine? I'll see if I can find a way to adjust the duration. but what does that accomplish? Is maybe not blowing well enough and something too hot possibly and need more time to cool down?
 
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Old 12-20-16, 09:48 PM
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You need to be monitoring the limit line in operation. The limit only opens right after the burner goes off and the blower stops. By the time you disconnect the wires and check it.... its cooled off and back to normal.

Is maybe not blowing well enough and something too hot possibly and need more time to cool down?
That is absolutely correct. Most boards allow the blower to run for 90 seconds after the burner shuts off. How much time is yours allowing ?

Your furnace could be operating near the limit temperature and the temp rises quickly when the blower shuts off.
 
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