On 08/12/2018 22:59, Andrew Gabriel wrote:
In article ,
Clive Arthur writes:
On 08/12/2018 17:51, Bruce Farquhar wrote:
On Sat, 08 Dec 2018 17:40:57 -0000, wrote:

On 12/8/18 11:41 AM, Bruce Farquhar wrote:
On older boilers (furnaces if you're American), when the heating isn't
actually running (eg. the thermostat says the house is warm enough),
there's no power to the boiler, so how does the pilot light valve stay
open with the tiny voltage (40mV?) and current from the thermocouple?

To *hold* the valve open only requires a small voltage & current. To
*pull* the valve open would require a larger voltage. That's why you
have to "Press & Hold" the manual knob to restart a pilot.

See here for more detail:
https://en.wikipedia.org/wiki/Thermo...pliance_safety

I see, thanks.Â* I thought the "press and hold" was just to keep the
valve open until the thermocouple warmed up.Â* So I'm providing the
effort to open the valve with my thumb.Â* That link states 0.2-0.25A - do
you really get that much current off a thermocouple?

Yes, it's a very low impedance source, a metal to different metal
contact. 10mV 200mA is 50 milliohms. It's only 2mW, but that's a very
small proportion of the pilot flame power.

Indeed - a pilot flame is typically around 250W.

Interesting. I thought that sounded like quite a lot. A quarter of a 1
bar fire? Half the output of a big halogen flood? Found a web site that
says a tea-light is about 50 watts (4 grams per hour), so I guess it's
probably about right.