Old Carrier Furnace, 3-Wire Pilot Assembly, Electrode Sparks Continually While it Runs

Question

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.

Answer 1

I experience this exact same problem (sparking throughout the entire cycle) when the 646AX gas valve assembly failed and had to be replaced. No direct replacement was available but the stated equivalent replacement was the EF32CW.

I soon found out that the EF32CW doesn't actually have true separate PICK and HOLD coils -- it just ties the two connections together to energize a single PICK/HOLD coil. Unfortunately that means that the sparking that is supposed to happen only during the PICK phase now occurs throughout the entire HOLD phase.

My kludgy fix was to disconnect the fake PICK input and route the HOLD to the PICK. So the PICK signal only energizes the spark generator while the HOLD opens the PICK/HOLD valve and keeps it open. Works fine...

Answer 2

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.

Answer 3

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.