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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?

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Any possibility to 'tweek' the physical relationship of the pilot orifice to the spark gap? –  HerrBag Oct 8 '13 at 12:24
    
@HerrBag No. It's a single assembly. –  Tester101 Oct 8 '13 at 12:47
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3 Answers

Blowing on a gas pilot or burner to get it to light generally means that gas pressure is too high or the air/fuel mixture is too rich. Most pilots don't have an air mixer because the fuel flow is so low that one is not needed, so that leaves high pressure as the problem.

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gas pressure too high or the air/fuel mixture is too rich maybe? –  mac Oct 8 '13 at 16:36
    
@mac you're right, thought typo. thanks! –  longneck Oct 8 '13 at 16:38
    
Gas pressure is at 6 WC, well within the range of 4.5 - 13.6 specified on the furnace. –  Tester101 Oct 9 '13 at 9:26
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Looks like sufficient gas isn't getting into the path of the ignitor. That could be be due to partial blockage of the burner hole nearest the ignitor. When was the last time you brushed off the burners, or at least blew the junk out of the burn box w a reversed vacuum cleaner? They build up crap over the course of years, which can cause problems like you're experiencing.

-While you're in there. check whether the blower motor bearings need oiling. The manufacturers sometimes hide that info on a sticker you can't see until you actually open up the furnace.

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There is no problem with the main burners, only the pilot. The ignitor does not light the main burner in this model. –  Tester101 Oct 8 '13 at 13:48
    
Main burners may look like they have no problem, but if there's a fleck of rust over the burner hole nearest the ignitor, the ignitor won't be getting any gas. You won't be able to see that by noting that the burner itself lights. –  Wayfaring Stranger Oct 8 '13 at 13:54
1  
The main burner and the pilot assembly are completely separate. How would a dirty burner affect the pilot in this case? –  Tester101 Oct 8 '13 at 13:57
    
Maybe on your unit... On mine, there is definitely a separate pilot assembly. The main gas valve (which feeds the main burner), does not open until the pilot has proven (closed a bi-metal switch). Some models may indeed use a spark gap to light the main burner. Mine, however, does not. –  Tester101 Oct 8 '13 at 14:49
    
Video appears to show main burner coming on with application of flame further into burner. Regardless of gas source, something is blocking gas flow to your spark region. –  Wayfaring Stranger Oct 8 '13 at 14:49
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up vote 0 down vote accepted

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.

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