My furnace fan sometimes stays on and runs continuously. I can stop it by flipping the breaker for the furnace on and off. It then stays off. When heat is called for, the fan comes on again and sometimes stays on and sometimes will cut off as it should. Is this the fan limiter switch (Honeywell L4064W 1106). Could it be anything else? The "fan on" control at the thermostat was disabled when the current fan limiter switch was installed.

  • It does sound like the fan limit switch is failing. – isherwood Dec 7 '15 at 15:04

It could be a problem with the limit switch, or the limit switch operating properly.

If the furnace is overheating, the limit will open and cause the blower to continue until the heat is dissipated.

It could also be that the limit is going bad, and opening when it shouldn't. Or that the limit is opening due to overheating, but then not closing again as it should.

  • I replaced the limit switch and the problem is still there. The fan came on when I tested it with the automatic/manual switch on the new limit switch, and the fan would not go off, even though there was no call for heat and the system was cool. Is there a separate relay or control that could be keeping the fan on? Sometimes the fan will go off and cycle as it should. Other times it will stay on. This furnace is quite old. I cannot find another relay in the heating system. – WArthur Dec 11 '15 at 19:18
  • There should be other limits (rollout, pressure, etc.). Check the schematic, it will show all the safety switches. – Tester101 Dec 11 '15 at 20:04

Ok, the HVAC guy came out and found the problem. When AC was added, many years ago, for some reason they rewired the low voltage power supply for the furnace to come from the AC transformer, which is located outside the house, some distance from the furnace. The 24V source at the furnace transformer was abandoned. I had turned off that transformer to save electricity. So now everything works as it should.


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