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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A Huge Rusty Spoon

One finds the oddest things in one's backyard at times. Last summer, a freakish sequence of wind gusts brought down a huge old Manitoba maple at the bottom corner of my yard. Before the tree removal crew came to deal with the thing, I went down and relocated a stack of fence boards that I'd piled near the tree some time ago, so the crew would have unimpeded access to the stump area. While I was rummaging around down there, I found this rusty old spoon. The thing is huge. 'Looks like something you'd use to give cough syrup to a hog. (Do hogs get coughs?) I'll see how well I can clean it up. It's in pretty rough shape.

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MONDAY, FEBRUARY 15, 2010

After

It's still ugly as sin, but in a clean sort of way.

I wire brushed it on a 6" wheel, scoured it with a scouring cleanser and gave it an application of GM Blue Coral Preservative Sealer. And it turns out that I have something of an antique item in the workshop (see below).

It would take extraordinary efforts to get this item to a truly pristine condition, so I'll leave it at this. I've made a place for it to hang on one of my toolboards. If I ever have to administer cough syrup to a hog, it'll be right at hand.

GM Blue Coral Preservative Sealer

It's a paste wax, basically. It has a blue colour in the jar. I don't know when it was last available. I must have bought the jar pictured in the late sixties for the chrome on my Austin-Healey Sprite. Most of it is still left.

There's someone in Kernersville, North Carolina who has an empty jar of it on Craigslist -- asking price is $1.00.

There is an outfit known as Blue Coral that makes car cleaning products, but they don't appear to have any connection to this GM Blue Coral.

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Scissor Sharpening

Scissor sharpening is a chore that I never much liked doing before I acquired an upright belt sander that takes 1" x 42" belts. The machine makes many sharpening jobs easier, scissor sharpening notably so.

The finest belt I've been able to get so far for the machine is 120-grit, which is just barely fine enough for the task. The aluminum oxide abrasive works well on carbon steel, not so well on stainless steel but well enough. I find that the belt 'grinds' with much less heating effect than one gets from a grinding wheel -- a big bonus.

It's helpful but not essential to have a pair of scissors taken apart for sharpening. The latest pair that I did had a nice screw-and-nut pivot that came apart easily.

Present each scissor to the belt with its operative face down, so the belt is leaving the cutting edge corner rather than approaching it as it goes by. Make smooth, moderate passes from pivot point to tip until you've turned a 'wire edge' the entire length of the cutting edge. At that point, the machine has done its work. Hone the operative face of the scissor with a pocket stone just enough to rid it of the wire edge effect and you're done.

Use blue threadlocker when reassembling the pivot. Once you've got the tension to where you like it, set the scissors aside for the threadlocker to cure for an hour. Lubricate the pivot with light mineral oil.

The only part of the operation that demands a bit of skill is presenting the scissor's edge to the sanding belt consistently at the same angle for every pass made. I'm thinking of making an oversize table for my sander. Then I could fabricate some manner of 'sled' fixture that would pretty much deskill the process entirely.

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A Cautionary Tale

I was reminded of this story from decades ago while I was rigging an electric chainsaw motor for a bench test; securing it in my woodworking vise so its start-up torque reaction could cause no trouble.

One day at work, one of my co-workers didn't show up in the morning. I soon got the story from my boss of what had happened to him.

It seemed he'd been repairing an electric lawnmower the previous evening. He had it upside down on his workbench, unsecured. He hit the switch to start it, and start it did. The torque reaction of the motor and blades sent the mower careening off the edge of the workbench, the blades raking his thigh as the mower descended. From what I was told of the aftermath, the words 'cut badly' don't begin to describe the injury he sustained. I can't recall if he ever returned to work, but I don't think I was there much longer after that. (The place was a hell hole, but I digress.)

Anyway, there's a story to take to heart and file in your 'don't let this happen to you' folder. The start-up torque reaction of powerful universal motors is a force to be reckoned with. Never be lackadaisical about protecting yourself from it.

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LED Flashlights

My son gave me a small LED flashlight two Christmases ago, and when I started using it in the workshop I had one of those, "Where have you been all my life?" moments.

The uniform, icy-white light beam from the thing makes it the most amazing little inspection light for the nooks and crannies and orifices of machinery I've ever had the use of. Battery life is pretty good. Don't hesitate to get yourself one; they're not a gimmick.

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Removing Labels From Pill Vials

Whether for the sake of information security, or to have the use of an empty vial sans label, there's an easy way to get the labels off.

Get an empty metal coffee can with a snug-fitting, plastic lid. Wet the label completely with WD-40 and stand the vial in the bottom of the can. Close the lid and leave it overnight. The next day, the label will peel away nicely. The muck it leaves behind cleans away with methyl hydrate and a paper towel. Neither the WD-40 nor the methyl hydrate has any ill effect on the plastic vial.

SUNDAY, AUGUST 15, 2010

Addendum

I tried the same thing with Varsol instead of WD-40, and that worked just as well and with less residue left behind to clean away. One of the vials I did was a perfectly clear one, and the Varsol fogged the bottom of that vial after its overnight treatment. That vial may have been made of polystyrene. The vials that come clean so well without harm from the solvent have all been the amber/orange coloured ones made of polypropylene, recycling number '5' (PP).

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CA (Cyanoacrylate) Adhesive

Commonly known as Krazy Glue or Super Glue, CA adhesive is indispensable in the workshop. Following are a few brief points about it:

a) It bonds skin instantly. The stuff can be dangerous.^[1]

b) Dollar stores carry it. I've found the dollar store stuff to be perfectly good for my purposes, and considerably less expensive than the brand name products.

c) There's a gelled version available, but I really can't imagine why.^[2] One of the adhesive's biggest virtues is that it's extremely runny. It will wick into the tiniest gap or crevice.

d) Runny though it is, it can be made to 'build' with repeated applications. That can be very helpful in the restoration of rusty old machinery; small unwanted crevices can be filled with it. The resultant fill can be filed or sanded, is impervious to water or oil and can be painted.

e) Further to its use as a filler, I've read that you can fill a larger gap with baking soda, then apply drops of CA. I've yet to have tried this myself. I suspect that pungent fumes result when it's done. Some types of applications result in fumes, and this is likely one of them.

f) It's not terribly heat resistant. LePage's website advises that immersion in boiling water can soften cured adhesive, so it's likely unsuitable for any application that will subject it to temperatures approaching 100° C. That said, you can safely use it as a threadlocker, or for any machinery assembly application that may need to be taken apart again. Just as with threadlockers, sufficient heat will soften it.^[3]

g) Its water and chemical resistance is impressive. I've read that acetone will soften it. Apart from that, though, the stuff seems to stand up to just about anything.

h) Keep it in the fridge. Heat, light and humidity all tend to thicken it. Kept in the fridge, a tube of it will stay fresh and runny; you'll get the use of all of the tube's contents.

i) I've read that if it gets on cotton, it generates a lot of heat as it sets, enough heat to be dangerous.

j) There are many specialized versions available, but you have to go to industrial supply houses for them. So far, I've gotten by fine with the common variety. There's much more information about the material available on the web.

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Notes:

[1] There's an upside to this.

If you're plagued, as I am, by splits in fingertip skin through the winter months, CA adhesive is a therapeutic. The instant you acquire a fingertip skin split and notice the attendant pain, seal the split with CA adhesive. The pain stops. The split is cleanly sealed against infection and can heal from beneath undisturbed.

I've read that some individuals may find the adhesive irritating and consequently useless for this, but it works for me.

[2] A Bit Of New Information -- SATURDAY, DECEMBER 7, 2013: Since this post was first written, I've learned what the gelled version of the material is good for -- it's a great boon to model makers. The gel's thickness helps to hold tiny parts in place until the adhesive 'takes'. That feature can greatly simplify many steps in the construction of a model.

[3] This post illustrates a useful machine shop application.

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