I learned something new tonight

[MintJulep]

Although I used to service and set-up gas-fired industrial equipment in excess of 1,000,000 BTUs I must admit that I was blissfully unaware of how the simple gas valve on domestic boilers, furnaces and water heaters worked.

Until tonight, when my heating boiler refused to make any hot.

A quick check revealed no pilot, and my attempts to relight it were rebuffed. Pilot flame as long as I held the little red button down, but the flame went out as soon as I released it.

I knew that the other tube that runs to the pilot was used to prove that the pilot was lit. I figured it held a charge of some fluid that expanded and worked against a diaphragm in the valve.

Turns out that it's a thermocouple, and the voltage pulls in a tiny magnet valve.

Of course the home center is closed now. Guess I know what my first project is tomorrow.

 

[IRstuff]

That's pretty standard for a pilot light. I've had a couple of identical occurrences.

TTFN

 

[imok2]

A single thermocouple is capable of producing about 25 to 30 millivolts dc. Thermopiles are designed to produce 750 millivolts. Other voltages commonly used in the past include 250 and 500 millivolt systems.

 

[rmw]

Well, I've changed them too, but I wasn't cognizant of the science behind it. Kind of wondered, but by the time I got the job done was so glad to get the heat back on (mine were on gas furnaces) and get out of the attic that I didn't linger to study it.

So, thanks for sharing. I learned too.

rmw

 

[Warpspeed]

I have been fascinated by these pilot light sensing thermocouples too.

While the millivolt output does not sound too impressive, they can easily develop a constant 100mA or more. That is quite sufficient to pull in and hold a solenoid valve.

The solenoid coil just needs sufficiently low dc resistance and the ampere turns that a pilot flame can develop will probably astound you.

 

[MintJulep]

Well the heat is back on. Easy enough job, although tight quarters made it more difficult than it needed to be.

In larger industrial gas burners the pilot is proven using a "flame rod", which is bassically a steel rod in an insulating holder (exactly like a spark plug with a long electrode.

A gas flame has the ability to rectify AC current into DC. An AC current is sent to the rod, and a nifty grey box looks for the DC current comming back.

 

[IRstuff]

TOO HIGH TECH...;-)

My old dryer simply had mechanical leverage from the expanding probe to hold the pilot light valve open.

TTFN