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Natural gas furnace not working properly
For the past month or so my forced air natural gas furnace hasn't been working properly. It will turn n on with a low thermostat reading then run for about 5 minutes or so then go into shut down cycle. As soon as the blower fan quits running the draft fan kicks back on and a new heat cycle starts. It will do this many times and still never get the thermostat up to temperature to shut down. Thermostat is set for 72 and best it will get is 71. If it does hit 72 it doesn't over shoot enough to trip the relay in the thermostat to shut down. It's been cold here in Illinois for awhile now and there's night's it will do this all night long. I've woke up to the house at 69 some mornings.
This is the first year in 9 living here that it has done this. I've cleaned the thermocouple thinking that might help but it did nothing. I've been told by a friend, ex HVAC, that it may be over heating and going into shut down mode. He said maybe adjust the gas flow alittle less and see if that helps.
Does anyone here have any ideas what's going on?
Furnace is an old Ruud 75,000 btu unit. Can't remember the date but thinking close to 15 or better years old.