Millivolt generator

49er · Feb 26, 2019

We have a Williams wall furnace and last Thursday the furnace wouldn't turn on. I found the pilot light was off and that's never happened before. I relit the pilot and measured 200 millivolts at the valve while the manual said that around 450 was normal. 200 was enough to keep the pilot on but as soon as the t-stat called for heat the pilot would shut down. I went to 3 different hardware stores but no one had a replacement millivolt generator in stock so I had to order one and it is not supposed to be here until tomorrow.

So desperate times call for desperate measures and here's what I came up with. I got the small 12 volt 7AH battery that I use for my portable fish finder and wired in a potentiometer so that the voltage could be adjusted. I set the voltage to almost zero and then connected it to the valve and slowly dialed the voltage up to 450mv.
We turned on the t-stat and the furnace fired right up and has been working perfectly like this since.

I don't know if I'm an idiot or a mad genius for doing this but we're warm and my wife thinks I'm a hero so that's really all that maters. I do turn off the t-stat and disconnect the battery at night and whenever we leave the house.

Millbilly · Feb 26, 2019

49er said:
We have a Williams wall furnace and last Thursday the furnace wouldn't turn on. I found the pilot light was off and that's never happened before. I relit the pilot and measured 200 millivolts at the valve while the manual said that around 450 was normal. 200 was enough to keep the pilot on but as soon as the t-stat called for heat the pilot would shut down. I went to 3 different hardware stores but no one had a replacement millivolt generator in stock so I had to order one and it is not supposed to be here until tomorrow.

So desperate times call for desperate measures and here's what I came up with. I got the small 12 volt 7AH battery that I use for my portable fish finder and wired in a potentiometer so that the voltage could be adjusted. I set the voltage to almost zero and then connected it to the valve and slowly dialed the voltage up to 450mv.
We turned on the t-stat and the furnace fired right up and has been working perfectly like this since.

I don't know if I'm an idiot or a mad genius for doing this but we're warm and my wife thinks I'm a hero so that's really all that maters. I do turn off the t-stat and disconnect the battery at night and whenever we leave the house.

View attachment 241481

Genius.... until your pilot goes out and gas is still flowing

49er · Feb 26, 2019

Millbilly said:
Genius.... until your pilot goes out and gas is still flowing

That's certainly a legitimate concern but the battery is only connected when we want the furnace to run. With the battery disconnected it's the 200mv from the existing generator that's keeping the pilot lit. If the pilot were to go out for some reason the valve would still close.

wooduser · Mar 1, 2019

About 85 % of the time the problem you describe is with a pilot orifice that needs to be cleaned.

Take a careful look at the pilot burner. It should burn like a small blowtorch, with a sharp blue flame that engulfs the pilot generator, with no yellow tipped flames.

Take the pilot burner apart and clean the pilot orifice ----often a wire from a wire brush does a good job. You'll probably be back in business, and even if you aren't that routine maintenance that needs to be done every 2-3 years.

I like Williams wall furnaces. They are an effective source of heat for a lot of smaller homes. And they are quite reliable equipment, if you clean the pilot burner now and then!

I take it you have a convection heater that doesn't have a fan?

49er · Mar 2, 2019

wooduser said:
About 85 % of the time the problem you describe is with a pilot orifice that needs to be cleaned.

You sure hit the nail on the head with this one!

I installed my nice shiny new generator this morning and it didn't make a bit of difference, it was also reading 200 volts. I removed the pilot assembly and took it apart as far as I could. The pilot orifice is so small I couldn't tell if it was dirty or not but the tube above that was quite dirty. I cleaned it up real good and found a small wire to run thru the orifice. After putting everything back together and lighting the pilot, the millivolts climbed right up to 670. I adjusted the pilot to get it back down to 450 and the furnace works perfectly.
My only defense is that the pilot is tucked up between the burners and is really hard to see without standing on your head. ;em

Thanks a million wooduser for the advice and yes, the furnace has no fan.

wooduser · Mar 2, 2019

49er said:
. After putting everything back together and lighting the pilot, the millivolts climbed right up to 670. I adjusted the pilot to get it back down to 450 and the furnace works perfectly.

I presume you turned down the gas pressure for the pilot burner at the gas valve.

Actually, I'd leave that turned up. The higher millivolts will help insure reliable operation of the furnace when the pilot orifice starts getting dirty again.

Cleaning the pilot is part of regular maintenance on the furnace and if done every 2-3 years will minimize problems with the furnace.

Also, when you have the pilot burner out, the main burners usually come out pretty easily and they can need cleaning too.

49er · Mar 5, 2019

wooduser said:
Also, when you have the pilot burner out, the main burners usually come out pretty easily and they can need cleaning too.

After seeing what a difference cleaning the pilot made I'm sure cleaning the burners is also a good idea. Do you happen to know how the whole valve, manifold and burners assembly come out? It looks like all I would have to do is disconnect the gas piping and remove a few screws.

The reason I ask is that the valve sticks when the t-stat turns off and I would like to replace it. It has never not turned off eventually but it consistently takes anywhere from 1 to 5 minutes before the burners go off. I've had new valve ready to go for a couple of years now but never wanted to tackle it during the heating season and then in the summer it's out of sight and mind.

wooduser · Mar 5, 2019

Usually on Williams wall furnaces, the two burners come off the gas valve as a unit, with the gas valve staying in place.

Take some time with a gflashlight to carefully inspect how the burners are held in place. You'll need to take the pilot assembly off again, then there are usually a couple of nuts to remove, which allows you to pull a couple of hooks up which release the burners.

That's a common methodology, but then again there were a lot of Williams Wall furnaces with varying methods of holding the burners in place. But looking carefully should allow you to remove them without too much trouble.

Have you disconnected the wire at the gas valve to shut off the burner to verify that it's the gas valve that's the problem? Could be, I suppose, that the thermostat isn't shutting off the gas valve properly. You have an electric thermostat, I suppose? (some gas valves have mechanical thermostats.)

I don't recall ever seeing a gas valve that didn't shut off reliably, but anything is possible. Just check out possible alternative explanations such as the thermostat not shutting it off properly.

49er · Mar 6, 2019

Thanks wooduser, I did remove one of the wires at the gas valve as a test. The burners stayed on until I gave the valve a couple of light taps.

JaredLea · Mar 13, 2019

You sure hit the nail on the head with this one!

I installed my nice shiny new generator this morning and it didn't make a bit of difference, it was also reading 200 volts. I removed the pilot assembly and took it apart as far as I could. The pilot orifice is so small I couldn't tell if it was dirty or not but the tube above that was quite dirty. I cleaned it up real good and found a small wire to run thru the orifice. After putting everything back together and lighting the pilot, the millivolts climbed right up to 670. I adjusted the pilot to get it back down to 450 and the furnace works perfectly. I wrote some advices about all of this at edubirdie (link). Check it.
My only defense is that the pilot is tucked up between the burners and is really hard to see without standing on your head.

Thanks a million wooduser for the advice and yes, the furnace has no fan.

You are welcome! Also, be careful with new furnaces. A little inadvertent could cost a life.

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