I Turned Off My Home Hvac System but the Fan Continues to Run

My TRANE XE 90 blower fan has a mind of its own. Even with the thermostat removed, it randomly turns on. It has gone a day without coming on (thermo off the wall the entire time) then just cycles itself on. Just blower, no heat or AC.

Will not cycle off unless I remove power to the entire HVAC unit. Then, when power restored, fan stays off. Until it doesn't - which could be an hour or a day.

I left the front panel open and taped the panel safety switch closed (thus allowing the system to cycle on with panel removed.). When the blower ghost cycled the fan on, I removed the tape and the fan stopped (as it should). But when I re-closed the safety switch, the blower fan again started. Thus, cycling power to just the fan motor does not cure, even temporarily, the problem. But turn power off to the entire unit- then the fan stays off for some random amount of time. Usually measured in many hours, although not always.

Very hard to troubleshoot like this.

I was convinced it was the thermostat, (have ordered a replacement- thus thermo has not been connected while this has been transpiring over past few days). I did test short the thermo wires coming from the HVAC unit and all works properly, again which made me believe it was the thermo itself. But not now.

Wondering if there is a limit switch or relay that could trip or short to the ON position, then a momentary power interuption 'resets' it until some later time.

I find it interesting that NOTHING is being done to or near the system when the blower ghost turns the fan on. Not even walking by. The outside temps have ranged from low 60s to upper 70s during this period- pretty benign.

I do not have a heat recovery or transfer system.

EDIT: Thanks ThreePhaseEel for pointing me to the wiring diagram, and thank Ken for telling me where to find the diagram. After review of the diagram, there appears NO way power can get to the blower motor except via the 24 volt transformer voltage, which only goes thru my thermostat. There is no other way to get power to the blower. Thus there likely has to be a short in the thermostat wiring between the furnace and my thermostat. Tracing the wiring and reviewing the colors of the wires attached to the furnace output with the wires coming into the back of my thermostat- I find that they Do Not Match. Different color wires exist at the thermostat versus those screwed into the furnace control block! There as been a splice, which I found, that spliced in power/control to the outside compressor and also changed some wire colors midstream. This splice, alongside the outside of the furnace, was just taped with electrical tape to the gas line coming into the furnace. And it is there that I found small cuts in the wiring exposing the copper of at least 2 of the wires. Since they were taped without shielding directly to the METAL gas line, I am sure that potential for a short exists- whether thru the gas line or just in direct contact. You can see the exposed copper in the last picture I have posted.

I will fix the short potential, and also trace the wiring to find out what is really connected where. Also, I did have an HVAC guy out in the spring of this year who changed out the 24 volt transformer and did a service check, and he was messing with the wires at the furnace thermostat block for some reason. I need to make sure he didn't mistakenly 'fix' something based on the colors he saw,not knowing about the downstream splices. It may be possible that this 'ghost' fan problem has been going on all summer, and we may not have noticed since in Atlanta the AC runs quite a bit during the summer, and only now that it is cooler does the blower fan bring attention to itself by running at the wrong time. Regardless, I feel I have isolated the problem. Thanks for help.

shorts

sullinsglinte.blogspot.com

Source: https://diy.stackexchange.com/questions/124656/hvac-blower-comes-on-by-itself-even-after-thermostat-has-been-disconnected-from