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Any precautions needed in soldering to a millivolt gas control system?
February 18, 2013 9:46 AM   Subscribe

A gas stove in our family room uses a thermocouple, millivolt valve and manual switch to control the gas to the stove. I need to solder a new switch to the leads that go to the old switch (the total wire length is about 4 feet). What precautions do I need to take to ensure that I do not blow out the millivolt gas valve when I do the soldering?

I have done lots of electronics soldering in my lifetime, but never to a millivolt system. I plan to use a soldering pencil, and electronics solder (resin core). I am not lengthening the wire or adding a second switch.
posted by mbarryf to Home & Garden (7 answers total) 1 user marked this as a favorite
If you are concerned about static damage, the thermocouple is not a problem. It is not a static sensitive part, so solder away. It is intended to withstand high temperatures. This is assuming that the thermocouple has wire leads that are already welded to the thermocouple itself.

Likewise, assuming your gas valve is passive, like a water heater valve, and without external power, the pilot switch is simply a magnetic coil powered by the thermocouple and also is insensitive to static damage.

What you want to make sure of is that the connection is good and of very low resistance because the thermocouple only puts out a few millivolts. So make sure the wires are clean and twist them tightly together before soldering. Don't depend on the solder for the electrical connection. You want copper to copper. The solder is there just to keep the wires from coming apart.

If you don't do it correctly, the worst that's going to happen is that the valve will close and the gas shut off when you release the pilot button.
posted by JackFlash at 10:46 AM on February 18, 2013 [1 favorite]

I've never done soldering on a millivolt gas system per se, but I've done soldering on systems that were pretty sensitive (i.e. wouldn't be okay with more than a few mV of seepage). The general recommendation is to ground everything that you can, and then use a grounded soldering iron. Most good irons designed for electronics work (not crafts use or something) will be grounded and have the heating element leads isolated from the tip.

You want to make sure that your grounds are all good, of course (that the soldering iron ground is ~0 ohms away from the chassis ground of the equipment you're soldering, etc.) or else it's all for nought. A bunch of test leads with alligator clips should fix this if it's not the case already.

Also I assume you're going to have the gas turned off and bled out of the line downstream of the cutoff valve, right? Because that seems wise as a general precaution. And if you can cleanly disconnect the thermocouple from the rest of the millivolt system, particularly the gas valve, I'd do that. Generally the thermocouples are removable via solderless connectors, at least on the hot water heaters and one gas pool heater I've ever had to open up, I assume because they are consumable parts / tend to fail. (The one time I thought I was going to have to do soldering inside a HWH, it turned out that there were solderless connectors on either side of the part to be replaced. Unfortunate if this is not the case in your system.)

Overall I'd be more concerned with static electricity than the soldering process itself, and that's all a grounding issue. That and not standing on a wool carpet or something while you're doing the job, if you can avoid it...

On preview, you say you're planning on using a "soldering pencil" which might mean one of the little butane ones like this. If that's the case you can ignore my comments about ensuring the iron is grounded, but I think it makes using a grounding strap more important.
posted by Kadin2048 at 10:46 AM on February 18, 2013

Oooops. I think that I was not clear enough. The pilot light is on, and stays on. A thermocouple generates a few (maybe 500-750) millivolts. The switch that I am planning to solder is the one that is in the circuit to turn on the main gas burner on the stove. The switch is mounted on the external casing of the stove for the user to turn the main flame on or off.

I am using an electric soldering pencil, and will ground the soldering pencil when I solder the wires.
posted by mbarryf at 2:44 PM on February 18, 2013

Neither the pilot nor the main gas valve contain static sensitive components. They are just springs and electromagnetic wire coils. The thermocouple (pilot valve) and thermopile (main valve) are just bimetallic metal assemblies. You can't hurt them with less than a hammer. The low voltage doesn't mean they are particularly sensitive devices. It just means then have low efficiency. As I said previously, just make sure that you make good connections. 500 millivolts isn't a lot to work with so you need low resistance. Current is what makes the electromagnets work and any resistance reduces the current and the valve will not stay open. It is a fail-safe system.
posted by JackFlash at 3:11 PM on February 18, 2013 [1 favorite]

A thermocouple doesn't generate much voltage, but it does need to drive enough power into the electromagnet or bimetallic strip inside the gas valve to make it overcome spring resistance and pull the valve open. There's no other power source in the system; all of it comes from the thermocouple.

That means that the DC resistance of the driven device needs to be relatively low. The only way you're going to damage a low-resistance non-electronic device is by pushing enough current through it to overheat it, which there is absolutely no way you are going to do unless your soldering iron has a gross electrical fault. As long as you don't see arcs and flashes as you ground the iron to the frame of your heater, you can't possibly blow anything up.
posted by flabdablet at 4:09 PM on February 18, 2013

JackFlash is right. You pretty well can't hurt the millivolt components with a standard soldering iron. The problem is in making a very good connection. Clean the wires at the connnection really well before soldering and make doubly sure you aren't using an acid flux or acid core solder. Grounding isn't required.
posted by Mitheral at 6:42 PM on February 18, 2013

Those millivolt gas valves are a thing of beauty -- bombproof and fail-safe. They do it all with just springs, a couple copper coils, clever aluminum casting and drilling, require no outside power and will operate unattended for decades. Rivaled in exquisiteness only by the Holley four-barrel carburetor which is much more finicky.
posted by JackFlash at 8:19 PM on February 18, 2013

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