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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.
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    We need the make and model# to be helpful. But it may be a hot surface igniter issue
    – Ed Beal
    Commented Oct 30, 2019 at 13:16
  • Is the thermostat still calling for heat when this happens? If you switch the thermostat off, how long does the fan continue after the furnace goes off.
    – Phil G
    Commented Oct 30, 2019 at 13:47
  • can it only ignite if the blower is off, or does that not matter?
    – dandavis
    Commented Oct 30, 2019 at 16:49

2 Answers 2

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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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  • I believe some forced-air systems run for a minute or so to ensure all the heated air is delivered (reduce wasted heat). If so, either a timer or a thermostat may be dead.
    – Carl Witthoft
    Commented Oct 30, 2019 at 17:24
  • @Greg Hill, yes, there is a power switch which I did try to turn it off and back on. This got the furnace temporarily back to normal cycle: ignition->heat->blow-a-few-minutes->off. After a few heat cycles it goes again to blowing non-stop. I will try your steps and will update on the details. Any clues on how to check the inducer blower state? Visually, sound, vibration? Thanks a lot.
    – sasha
    Commented Oct 30, 2019 at 18:28
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    @sasha Rest a hand on the inducer blower, watching out for any bare electrical terminals or parts that might move or be hot. If the thermostat is calling for heat the inducer should come on quickly after the furnace power is switched on. The big blower would come on later, after the burner is lit. Hopefully you'll be able to distinguish between the vibrations and sounds of the two.
    – Greg Hill
    Commented Oct 30, 2019 at 20:08
  • @Greg Hill, I added the observed sequence to the original question. Indeed there appears to be a problem with the heat (blue) flame staying on, so after several attempts the blower starts and never stops. Does this help to identify the problem?
    – sasha
    Commented Oct 31, 2019 at 1:09
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The bright orange ignition light is called the "hot surface ignitor."

The observed sequence of HSI heating, the gas ignites, and then the gas shuts off after a couple seconds usually indicates a "flame proving" problem. The furnace controller wants to sense that the gas is actually burning, ie that it's not just filling the room with gas waiting to explode. When it doesn't sense or "prove" the flame it closes the gas valve and tries again a few times.

Through some clever physics the ignitor element is used for sensing flame too. If a connection gets dirty then this starts failing.

You could try removing and cleaning the ignitor. Turn off the furnace power first, and be gentle with the ignitor. It could be brittle and break if it's struck hard. Don't contaminate the part that glows with body oils from your fingers either. Blow off any dust. I'm not sure whether to suggest scraping away heavy scaly deposits (if any).. If the mounting hardware is dirty or rusty, clean that too. The mounting screw may be part of the electrical circuit that senses the flame.

While you're in there look at the electrical connections, especially the connection to the bare or green ground wire from the electrical supply and any green ground wires inside the furnace. Clean and tighten any that are dirty or loose.

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    I cleaned the flame sensor (I guess it depends on the furnace brand, mine if York, which kind of sensor they use - a metal rod or an igniter as you suggest). After that two consecutive heating cycles went fine. Continue to monitor, will update if this took care of the problem. Thanks for your time and the detailed explanation.
    – sasha
    Commented Oct 31, 2019 at 13:50
  • So indeed the problem was resolved by cleaning the flame sensor rod. It was hard to figure out initially because the sensor intermittent malfunction produced enough heat (mild cold outside). The suggestion from @Greg Hill to video record the sequence revealed some consecutive failed cycles and the blower was running as a protection from the gas build up.
    – sasha
    Commented Nov 4, 2019 at 19:07

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