Why isn’t the spark gap lighting the pilot on the furnace

furnacepilot-light

I've got a Carrier 58GS natural draft furnace, which has a spark gap for igniting the pilot. Unfortunately, the spark gap has not been successfully doing its job. The spark clicks away as the gas rushes past, but the fuel never ignites.

Normal Ignition Sequence:

  • Thermostat calls for heat.
  • Pilot valve opens.
  • Spark gap produces spark.
  • Pilot ignites.
  • Bi-metal switch heated by pilot.
  • Pilot proves.
  • Main valve opens.
  • Main burners light.
  • Blower starts after time delay.

Current Ignition Sequence:

  • Thermostat calls for heat.
  • Pilot valve opens.
  • Spark gap produces spark for 90 seconds.
  • Pilot does not prove.
  • Pilot valve closes, and furnace shuts down.

I've lit the pilot manually using a grill lighter, and once it's lit everything else works fine. If the pilot is lit manually, the furnace cycles properly. It has even cycled on/off for a whole day after being lit manually, but will not ignite when cold (not used for extended time).

This morning I noticed the thermostat was calling for heat, but the furnace was again in the shut down state. I reset the thermostat, and removed the access panel from the furnace. I let the spark gap spark for a bit, to see if it would ignite the gas on its own. After about 45 seconds of failure I gently blew on the spark, and what do you know, a flame danced into existence. The flame started off very orange as I blew, but settled to a stable blue flame quickly.

Here is a YouTube video of a similar model furnace exhibiting similar behavior. Note: My spark gap does not sound erratic, and has a steady tic-tic-tic.

Here's what the pilot assembly looks like:

Carrier 58GS Pilot Assembly
Click for larger view

Here's the schematic for my setup:

Carrier 58GS Wiring Diagram
Click for larger view

I'm assuming the furnace is original to the house, which would put the installation around 1990. Other than this issue the furnace is in good repair, so I'm not looking to replace the entire unit. I'm just baffled by what could be causing the pilot to not ignite?

Best Answer

I finally found some time yesterday to take a closer look at the furnace.

Gas Pressure

I started by measuring the incoming gas pressure using a gas pressure gauge. I found it to be 6 WC (inch water column), which is well within the acceptable range specified on the furnace.

Gas pressure gauge

Dirty Gap

Next I took a closer look at the spark gap itself, and watched closely as the spark was generated. I even recorded the spark with my video camera, which allowed me to zoom in fairly close. This is when I noticed a bit of carbon buildup on the electrodes, and wondered if it could be that simple...

enter image description here

Cleaning the Gap

I grabbed a bit of 220 grit sandpaper, and cleaned the electrodes up a bit. steel wool is probably the tool of choice for this job, but I didn't happen to have any on hand at the moment (not yet unpacked).

Success!

After testing the pilot ignition a few times, the problem seems to be fixed. We had a cold night last night, and the furnace fired up fine all night. As I write this, the furnace has just cycled successfully yet again.

Update

I found this YouTube video from Grayfurnaceman that describes the problem exactly, and says that the only solution is to replace the assembly. I'll be replacing the pilot assembly once the weather warms up, so we'll see if that solves the problem.

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