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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.

1 of 3shortssplice

2 of 3 shorts

  • Can you post a wiring diagram for your unit? – ThreePhaseEel Oct 6 '17 at 22:22
  • @petroke If you remove the front panel on the furnace you will see the wiring diagram along with the flashing codes. Post a good picture of that please.. – Ken Oct 7 '17 at 9:02
  • Do you have a heat recovery or a heat transfer installed on this system? – Retired Master Electrician Oct 7 '17 at 14:11
  • I do not have a heat recovery or transfer installed. have pics of wiring diagram just figuring out how to download right now. Seems the pics are too big to upload to sight. have to redo- but @Ken thanks for info on where to get the diagram. Also the flashing codes- the light flashes slowly- which per the code seems to be 'normal'. – petroke Oct 8 '17 at 12:44
  • make that upload to site not sight. – petroke Oct 8 '17 at 12:50
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Final cause is a combination of 2 problems.

  1. After review of the wiring 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 has to be a short in the thermostat wiring between the furnace and my thermostat.

Tracing the wiring - I find that there is a midline splice in the line, that is used to tie in the A/C compressor to the circuit. This splice was just taped with electrical tape to the furnace metal gas line outside the furnace. I found frayed wiring and bare copper showing- thus it appears that that the wiring is occasionally shorting- either from contact to the metal gas line or directly between the wires (why just some of the time I don't know). Correcting these shorts, the blower now does not come on by itself anymore (no more 'ghost' fan starts), only operating with the thermostat in place or if I short the wires. However,--

  1. The thermostat itself is also broken. Possibly due to the shorting that has been going on? It operates the A/C Compressor or heat calls properly based on cycling when the tempature calls for it, but the fan now continually runs, even with the thermostat switched off- unless I remove the thermostat from the wall (prior to fixing shorts the blower would still continue to run, even with thermostat removed from wall).

Thus, all works well except the thermostat seems to have internal shorts that keep the blower fan on at all times regardless of heat or cooling calls, or even when the thermostat is switched to 'off'.

Replaced the thermostat, and now system works fine. So I was trying to solve two problems at the same time- always makes for more fun.

Thanks to comments from 'ThreePhaseEel' for pointing me to the wiring diagram so I could isolate problem, and to 'Ken' for telling me where to find the wiring diagram!.
shorted wires enter image description here

  • Mind editing the details and the final pics into the answer here? It'd come out a bit better organized that way :) – ThreePhaseEel Oct 9 '17 at 22:34
  • Sure, will do. And I should edit anyway since I now have the full causes (yes plural) of the malfunction. – petroke Oct 10 '17 at 13:46

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