Ventilation fan works fine when heating is off but immediately switches off when heating is turned on. Consequently the furnace runs for a minute and then overheats and turns off. Any idea what could be the cause?

  • I would assume you have a limit switch that is not allowing the furnace to turn on the blower. Furnaces have many limit switches to detect faults in the furnace. Maybe the flue is blocked? One of those limits is opening or closing and the computer in the furnace is detecting it and shutting it down. Without knowing what make and model of furnace or showing a pic of the furnace wiring (or indeed even saying what the furnace is, gas, electric, oil, what?) all we can tell you is "it's broken" which you already know. Jan 9 at 18:11
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    What make/model is the furnace, and is this "ventilation fan" the blower in the furnace, or some external fan somewhere? Jan 9 at 18:58
  • Is this a simple heat only type of furnace standard millivolt 2 wire with a 3rd wire for fan on?
    – Ed Beal
    Jan 9 at 21:21

Many thanks to all who have attempted to help. I am afraid the question was based on my mis-interpretation of the symptoms. In fact the furnace was attempting to light, but failing due to an exhaust problem (now fixed) -sorry! Martin

  • Glad you figured out what was going on! Jan 10 at 20:45

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