Why is the furnace blowing cold air


My HEIL 7000 natural gas furnace has stopped working what are the most likely causes, and how to verify faulty parts?

Normal operation routine

  • Thermostat sends "need heat" signal.
  • Exhaust blower starts.
  • Electric sparker lights gas.
  • Once operating temperature is reached, circulation blower turns on.

Current operation routine

  • Thermostat sends "need heat" signal.
  • circulation blower turns on and blows cold air.

Things I have checked.

  • House has gas (hot water tank is working).
  • Followed reset procedure as described in owners manual.
  • Looked for obvious signs of electrical short/arcing (black marks, melted wire sheathing, etc)

Best Answer

Turned out to be the High Limit switch. The switch had burnt out, which caused the burner not to fire. Turns out when the limit switch senses the heat box is too hot, it shuts down the gas and forces the blower to continue so it will clear the excess heat. When the switch failed it always told the system that the furnace was over heated, so the burner would not come on and the blower would run constantly to try and clear the excess heat.

To trouble shoot this issue I used an ohm meter to test each sensor, to make sure it was open/closed as it should be. Another way to test would have been to use jumpers to bypass the sensor all together, until the faulty sensor was located.