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Repairing gas water heater (third time's a charm)

Hedges

I See Electromagnetic Fields!
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Mar 28, 2020
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I had picked up a gas water heater with circulator pump at ReStore (Habitat for Humanity's thrift store), planning to use it for solar thermal water storage. So, it was on-hand when I just needed a replacement water heater. The Kenmore that came with my house when I bought > 21 years ago finally failed. Water leaking from a hose fitting so I replaced that, but still leaking; tank had failed. Two leaks at once? My guess is the stainless braided hose failed, and dripping water rusted through the glass-lined storage tank on the outside.

After removing door frame to shoe-horn in the replacement 60 gallon tank, I hooked everything up. I did have to mess with tank support and exhaust pipe to fit things correctly. This one has a piezo ignitor, no more looking for a match. It lit, but then I realize that after gas valve opened, it never shut off. This unit had been stored outside for years, and something gummed it up.

Water heater gas thermostats are an available item, $80 to $150 or some much more.


These are interchangeable for most models. In fact, the one on the Kenmore had been working fine ... Just three smaller fittings and rotate the body to unscrew 3/4" pipe thread from tank. All the fittings matched, and water heater rumbled to life.

That was a few weeks ago. Yesterday, shower didn't warm up enough. Water heater was off. I re-lit it and it rumbled to life again. Later, off again and not hot enough. So I googled the topic. I've had a bad thermocouple elsewhere in years past, but due to more sealed assembly, wanted to test this one before replacing.

I was aware of 750 mV thermostats (for furnaces) and larger thermocouples which power them. But looking up water heaters, turns out these produce just 20 to 30 mV. That's enough to drive a solenoid?!


I first tested thermocouple from old water heater (Besides transferring the thermostat, I pulled out the burner, pilot, and thermocouple assembly before junking it. Did I say I'm a pack rat?) On the stove, sure enough, I got 27 mV. Current setting, a quarter amp! Not a lot of watts, but impressive amount of current.

Testing "new" water heater's thermocouple in situ, similar voltage but only 60 or 70 mA.

For this semi-sealed unit I ended up removing four torx screws an sliding burner assembly out part way to access thermocouple body, and twisted it back and forth until I worked it out. Installed old thermocouple as replacement and reassembled. It now heats the tank properly and stays lit.

Here's the faulty thermocouple being tested on stove:

thermocouple 27.7mV IMG_1997.jpg thermocouple 62mA IMG_1996.jpg
 
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Nope not enough to pull in an armature but enough to keep one pulled in.

Don't know 'bout that. It is obviously able to turn on the gas valve; that's all these water heater thermostats have available to them. No active circuitry, no other power source (except gas pressure; I would guess these have a solenoid controlled pilot valve similar to sprinkler valves.)
Measuring resistance of coil in the old one I pulled, it is below 0.1 ohm scale.

At work, we were measuring currents around 0.25 or 0.5Arms induced in chassis by by leakage field from 1 or 2 kVA power transformer. Two chassis connected together front and back as if in a rack. When I opened the circuit, it showed 0.7 mVrms. Low enough resistance, you can get plenty of current at low voltage.
 
I think the gas valve is mechanical no electricity is needed. The thermocouple is used only for a loss of flame.
 
While one could use bimetalic switch to open/close a valve (open when cold, close when hot), that wouldn't implement the safety function of shutting off gas when no heat from flame. I think the gas valve is a solenoid, and thermocouple powers it.

(I used to thing they were a bulb with fluid that expanded with heat, but that's not the case. Temperature-compensating water valves do work that way.)

It appears my gas range will allow gas flow to burners regardless of flame, but all other appliances (including oven) have a thermocouple in flame and are shut off by lack of pilot.

I think the thermostats are electromechanical, with manual adjustment knob, bimetallic strip, electrical contacts. That completes the circuit, allowing current generated by thermocouple to reach solenoid gas valve and operate it.

Most of those, thermocouple has a screw fitting which connects it to thermostat/valve body. Some furnaces (which require no external electric power source) use 750 mV thermocouple wired to a remote thermostat. Of course, modern furnaces are generally forced air. Mine has glowing plate ignitor and 24V transformer to thermostat, which also controls relay in A/C.
 
750mv is a thermopile
30mv is a thermocouple
check the connections and eco switch
al else fails replace gas valve and thermcouple and be done with it
 
al else fails replace gas valve and thermcouple and be done with it

And that is what it took to fix it.
Replaced water heater with second hand one, but determined gas valve was bad.
Replaced gas valve (just threaded fittings) with old one from failed water heater.
That worked for a while, but then stopped holding gas on.
Took measurements, replaced weak thermocouple with stronger one from failed water heater.
Working now.

The thermocouple on replacement second-hand water heater was a bit less accessible than what I've seen before, because a cover plate carrying burner had to be removed to access it. Those I've worked on before had pilot light requiring lighting with a flame, so had a large readily accessible opening.
 
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