Strange problem with lennox furnace

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Old 02-03-19, 04:59 AM
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Strange problem with lennox furnace

Hello

I have a lennox gas furnace that is about 12 years old the other day I woke up to a very strange chemical smell almost like acetone varnish smell. House was a little cooler than normal and I could not locate the source of the strange smell. The next morning my thermostat trad low batteries so I put a fresh set in the display showed and then went blank. Thinking it is a bad thermostat I got a new one installed all wires are correct the display works off the unit however as soon as it makes connection with unit on wall (hard wired) it goes blank. Furnace then only operates blower and cool air blows. I tried taking display off again and seeing if maybe connection was not being made but display comes on as soon as it is taken off wall. I got the furnace to kick on by connecting it not all the way (thermostat face plate) but then furnace did not shut off when set temperature reached I am quite baffled as what is going on. Could it be the control board is fried but how is it still working ? Any suggestions??
 
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Old 02-03-19, 05:36 AM
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Hi, what type of thermostat are you installing, is it exactly the same as the old one?
Geo
 
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Old 02-03-19, 05:55 AM
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Yes both are honeywell thermostats
 
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Old 02-03-19, 06:00 AM
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Could it be the control board is fried but how is it still working ?
Yes. Many times when electronics get fried relays will get permanently closed (IE welded) which is why things will run in the furnace.

You can run a furnace without a stat by just jumping the wires. Red should be your 24 volts from the furnace. If you connect that and the white it will trigger heat on. Red to green will run the furnace fan. Red and yellow should be the A/C. Some cases you have to connect the green at the same time when calling for heat or cool so the fan will run.

The above assumes you have a standard gas forced air furnace and "standard" stat wiring. There really is no hard rule on wire colors so yours might be different. It is best to check how the stat is wired in the furnace.

I would start at the furnace and make sure the transformer is putting out 24 volts for the control wiring. That could be the funny smell you smelled, the transformer overheating.
 
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Old 02-03-19, 06:33 AM
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Thanks for the response!

I shut the furnace completely off last night as I have a switch for the furnace. I turned it back on and now its running as it should it is shutting off at set temp and blower is not continuously on. This is very strange and I will have someone look at it as it seems to be a intermittent issue and I do not want to be left without heat. Thermostat is reading normal on wall too😧🤯
 
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