After blowing heat the fan won’t stop, continue blowing cold air for prolonged period of time (30+ min)

furnace

Not always, but often the furnace continue blowing cold air after blowing heat. It may blow cold air for more than 30 minutes. Under this conditions I also see the error code (8 red flashed) "Ignition lockout due to recycles".

Is there anything that I can easily adjust/a part to replace? Here is how the furnace looks: enter image description here
Adding observed sequence. After a day thermostat on OFF, I switched it to heat and the very first sequence was fine – inducer blower first, then bright orange ignition, then click and steady blue gas flame, then blower fan, then after several minutes all off (and the diagnostic LED slow green "all normal"). However the second sequence, several minutes after the first sequence, revealed the problem:

  1. Thermostat is on heat
  2. Inducer blower ON (diagnostics LED slow yellow)
  3. Bright orange ignition light ON
  4. A click, blue flame ON, but after a few seconds it's OFF
  5. The steps #3 and #4 repeat about four or five times
  6. Orange ignition light OFF, inducer fan OFF, blower fan ON, the diagnostic LED 8 red flash "Ignition lockout due to recycles". Blows continuously for until I turn the furnace power off.

Best Answer

The blower remaining on after a heat cycle can be a symptom of a failing safety sensor (but we don't have enough information yet to confirm whether that's happening here). The logic is that if the controller can't confirm the gas burner has shut down then the blower is kept on as a safety to avoid the furnace literally burning itself up. In this case the inducer fan (the large black rectangle in the middle) would also remain on.

You'll need to carefully observe the furnace and report back in order for us to help further.

  1. Adjust the thermostat to call for heat
  2. Turn off the power to the furnace for about 30 seconds, then turn its power back on. Hopefully there's a power switch (looks like a light switch) on the exterior of the furnace probably on its right side; if there isn't you'll have to have an assistant flip the circuit breaker.
  3. Standing at the furnace while the power is switched on observe what happens. Make notes or record a video. Listen for clicks, notice when the main blower and the inducer blower come on (the inducer blower is the black rectangle in the center of the photo), watch for an orange glow to appear in the burner area and for the gas to turn on.

Edit the question to include the sequence you observe. We don't need to have the video uploaded (I'm not sure whether SE even supports that); a written description will be fine.

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