On Sunday, December 9, 2018 at 11:36:53 AM UTC-5, Bruce Farquhar wrote:
On Sun, 09 Dec 2018 02:18:42 -0000, Clare Snyder wrote:

On Sat, 8 Dec 2018 22:59:22 -0000 (UTC),
(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. I thought the "press and hold" was just to keep the
valve open until the thermocouple warmed up. So I'm providing the
effort to open the valve with my thumb. 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.

A lot closer to 65 than 250 for MOST natural gas pilot lights (about
60 cu meters per year) Some of the really old ones may have used 4
times as much - - -
Nothing available today (in a furnace) (at least in Canada) has a
standing pilot. All have electric ignition as part of the "high
efficiency"standard.

I doubt it's as high as either of those, or your gas bill would be astronomical. Plus the boiler would feel pretty damn warm to the touch even when not using the main burner. Imagine leaving a 60W lightbulb running inside a metal box and think how hot it would get.

I could run a 60W bulb 24/7 for a month here for $5 using electric.
Natural gas is about half the cost of electric or less, so figure ~$2
a month for a 60W pilot light. That's not astronomical nor would
60W going into a boiler produce much in the way of raising it's temp
significantly. Most of that small amount of heat probably goes right
up the flue with the natural draft that's there, unless it's a modern
one that limits that. But those don't have pilot lights.