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Our 29 year-old Carrier furnace relies on a 3-wire pilot with an intermittent spark igniter. It sparks probably around 30 times before the gas valve opens and the furnace fires up. That's a lot of sparking, and surely not good in and of itself, but the real problem is, it just keeps on sparking continually throughout the cycle, up until exactly the moment when the blower stops.

I would estimate it sparks twice per second throughout the cycle duration, roughly twice as frequent as the sparking that happens prior to the blower coming on.

Please note this furnace does not have a flame sensing rod. This is the bimetal type. Any suggestions on troubleshooting are appreciated.

Sparking about 120 times per minute during furnace operation

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These 3-wire assemblies are crap. They sometimes act as you've described, other times they spark and never ignite the pilot. You can try taking the assembly out and cleaning it up, or just replace it.

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    I would also check the ignition control box. If the pilot is sparking, I would assume the control box has not received a signal that it is lit. Could be either the pilot or control. – Jeff Cates Mar 5 '18 at 17:29
  • @JeffCates in most cases, I'd totally agree with you. However, these pilot assemblies are awful, and often exhibit this behavior. – Tester101 Mar 6 '18 at 1:19
  • In all hope, the ideal part would be the pilot that has a short. – Jeff Cates Mar 6 '18 at 12:34
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The following is a sequence of operation for that style of Bryant/Payne/Carrier furnace that uses the three wire bimetal safety switch:

  1. The thermostat calls for heat.

  2. 24 volts goes to the HOLD coil in the gas valve and to the 3-wire pilot switch.

  3. The 3-wire pilot switch sends 24 volts out through the “cold” contact to the spark module that then produces the high voltage spark and 24 volts is also sent to the PICK operator.

  4. With 24 volts now to the PICK and the HOLD coils on the pilot gas operator in the gas valve, the PICK operator opens and gas flows out to the pilot and internally in the gas valve down to the inlet of the MAIN operator.  (To open the pilot operator, both the PICK and the HOLD coils must be energized, but to keep the pilot operator open it takes only the HOLD coil staying energized.)

  5. With the spark going [see step 3] and the pilot gas flowing [step 4], the pilot now lights.

  6. The bimetal in the pilot switch assembly is now heated by the pilot flame and the bimetal bends causing the switch to toggle.  The “cold” contact is now opened and “hot” contact is closed.  This shuts off the 24 volts to the high voltage spark box and to the PICK operator in the gas valve. 24 volts is now sent to the MAIN operator coil. The MAIN operator opens to send gas down the manifold to the main burners.

  7. The pilot light provides the source of ignition for the main burner gas and the main burners light.

  8. As long as the thermostat keeps calling for heat and the bimetal switch works correctly ("hot” contact stays closed), the HOLD and the MAIN operators stay energized until the thermostat is satisfied.

  • I have been experiencing intermittent non ignition with my "old" carrier. Sometimes it never gets to the spark stage, especially when outside temps are below freezing. But you can see by the sequence, it should stop sparking as soon as the pilot ignites. – Jeff Cates Mar 5 '18 at 18:28
  • Usually that is a sign that the flame sensor is dirty or kaput. Typical first order maintenance is to clean it with steel wool or a fine emery cloth. – DaveM Mar 5 '18 at 20:25
  • Neither of our systems have flame sensors. They are bi-metal switches, that can get dirty, but that usually does not affect this issue. And the bi-metal switch is very hard to "clean" without disassembling the entire pilot system, in which case, you are better to replace. My is not that issue either, as I suspect neither is their issue. The issue with the OP is either the pilot of the spark control. Somewhere along the sequence it is not telling the spark control that the pilot is lit. Could be 1 of the wires, or the switch has rusted and won't open the loop (rusted closed). – Jeff Cates Mar 5 '18 at 21:46
  • To troubleshoot, I suppose you could use a multi-meter to check voltage across the wires as the furnace completes the ignition sequence. Another way would be to remove the pilot and set your meter to ohm and check for resistance across the wires while pressing the switch gently by hand. If the resistance moves from wire to wire as it should, then the spark control could be the issue. But could also be a wire not hooked up correctly to the gas valve. – Jeff Cates Mar 5 '18 at 21:50

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