Old age memory may be getting the best of me but I recall a couple of years ago there was an issue with the exterior furnace access panel having the louvers cut wrong allowing rain to get inside and causing issues.
Could that be a contributor to the problem?
Do a forum search on Furnace Louvers and yo will find a bunch of threads on the topic.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054
No mention of year of coach, or furnace manufacturer which can help. Did this problem start after or during a heavy rain? Were any changes/repairs being made in the coach recently around the thermostat or furnace?
I assume that you mean the furnace blower fan never stops running......or is the furnace continuing to heat also? Make sure the thermostat is not calling for heat and that the thermostat does not have a fan over ride control set to run/on. You could also try setting the thermostat to off or to cooling to see if that makes a difference.
Usually the furnace provides 12vdc to the thermostat and some thermostats also have a fuse inside which can be pulled to cut the 12vdc through the thermostat. If this does not stop the blower (there might be a time delay before it stops) then there may be a short cct somewhere or a bad control board as mentioned by previous posters. I have seen where the thermostat is actually calling for heat, but the coach has run out of propane.....and the fan continues to run.
It definitely could be caused by heavy rain flooding the furnace. This also caused the AC to stop working. The solution was to replace the breaker. Replacing the breaker was easy, but finding the right breaker was not. It is duplex 30/20 breaker. Buying them seperately might have worked in a pinch.
BTW - I turned off the furnace in the outside panel. The AC would not work without turning it back on.
Well glad you fixed your issue.......but I am not sure how replacing a 120VAC breaker would fix your furnace fan (12vdc) from always running?
Probably the reason why your air conditioner was not working when the external power switch was off inside the furnace is because the furnace usually supplies 12vdc to the thermostat which uses that voltage to control calls for heat and cooling. So “switch off” inside furnace should result in no call for heat or cooling via the thermostat.