Gas Furnace Thermocouples

I'll begin this post with its conclusion:

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If you have a gas furnace with pilot light ignition, a good thing to have on hand is a spare thermocouple (the pilot light sensor). That's what this whole thing boils down to.

If you care to read on, here's my pilot light story.

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On Tuesday last, I got a phone call at work from home; it seemed that the furnace wasn't coming on. I dropped everything and drove home.

The pilot light was out. I lit it and let the furnace run to warm the place up. I switched off the furnace before it was entirely done and let it sit. After a few minutes, I checked on it and the pilot light had gone out again -- not a good sign. I lit it again and started sizing up the situation. Calling in a serviceman was an option I'd rather not have had to exercise. I didn't want to hear, "Ya need a new furnace or ye're all gonna die! I've got one on the truck. $3,000.00 installed -- they're on special this week."

The pilot light burned nicely whenever I'd relight it; a healthy little blue flame, not anemic or sputtery. That suggested a thermocouple fault. I noticed there was a screw on the valve body labelled "PILOT ADJ." The visible screw turned out to be just a cap over a little well with the actual adjustment screw down inside it. Here's a close-up of the gas valve -- you can see the location of the adjustment screw's cap.


I removed the cap and tweaked the little screw down inside to make the pilot light flame bigger, and that worked -- and continued to work for the rest of the week.

On Wednesday, I found a furnace parts dealer right near where I work. I went there and they had what I needed. They only carried a single Honeywell universal thermocouple, so it appeared that furnace thermocouples are a highly standardized item, which suited me fine. (The item is a Honeywell Model Q340A 1439, 36" universal 30 millivolt output thermocouple; about $20.00 list price.) Here's a view of what I got in its blister-pack.


On the rear of the package card, there are drawings of three possible mounting arrangements.


My furnace has the arrangement shown at the left in the above photo. I could have used the existing receptacle, but I thought it best to replace it with the new one. The receptacle has a 1/2" A/F hex on it. The valve control end has a threaded collar with a 3/8" A/F hex.

Here's an overall view of the installation.


And here's a close-up of of where the thermocouple resides alongside the pilot light's fuel delivery tube and jet.


It all went fairly smoothly, and saved me some serious money.

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How a Thermocouple Works

First, a word about why it's there. It's a safety feature to ensure that should a pilot light go out for whatever reason, the gas supply to its jet is shut off. So, the 'why' of it is pretty obvious -- it will prevent a sustained gas leak that could have dire consequences. (It's also a good example of what's meant by the term 'fail-safe'. A fail-safe component or system is one in which failure results in a safe condition. A thermocouple's failure can only result in the gas supply being shut off. It's impossible for a thermocouple to fail in such a way that the gas supply would remain on.)

Strictly speaking, a thermocouple is a junction at the ends of two dissimilar metal wires. When heated, such a junction generates a small electric voltage, sufficient in this application to supply enough current to hold open what I surmise to be a tiny solenoid valve. (It's not easy to find detailed information about the innards of furnace gas valves.) The probe end of what's called a furnace thermocouple is the housing for that junction. Bathed in the pilot light's flame, the thermocouple constantly generates the voltage/current required to keep the gas valve open that supplies the pilot light. Should the light go out, the thermocouple cools and the gas supply to the pilot light's jet is soon shut off.

All well and good, but there's an obvious potential for trouble; if the thermocouple's output falls off, or fails entirely, you'll have a pilot light that won't stay lit. I was fortunate in that the thermocouple hadn't failed outright, and turning up the flame bought me time to get a replacement and install it at a time of my choosing. It could as easily have been an outright failure outside of business hours, requiring a costly emergency service call. So, as I said at the outset, a spare thermocouple is a good thing to have on hand.

The length of the unit's lead is not critical; it just has to be long enough. That's no doubt why the furnace parts dealer only stocks the longish 36" item. (There are shorter ones made.) What that copper-tube-looking lead is, by the way, is a two-conductor coaxial cable. The outer copper jacket is one conductor; running down its length inside and insulated from it is the other conductor.^[1]

Maintenance

There really isn't any. I've seen advice to clean the probe, but I don't buy it. My thermocouple was well over a decade old, and there was only a slight sooty coating on it. As long as the probe end is well positioned in a proper flame, and the valve control end is snug, all you can do is replace it if it appears to be at fault. Cleaning it is unlikely to have much, if any, effect.

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Note:

[1] I like to see everything, so I cut the old thermocouple's cable to see how it's constructed. Here's a view of it.

The 'cable' is a copper tube, with a clear enamel-insulated solid copper wire inside it. This is a low-resistance cable if ever there was one.

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