On Sat, 08 Dec 2018 19:24:10 -0000, Clare Snyder wrote:

On Sat, 8 Dec 2018 12:40:57 -0500, 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
Don't know about the UK, but MOST north American furnaces don't use
the thermocouple voltage to operate the valve. The thermocouple
"signal" uperates an electronic circuit that provides power from a
transformer to operate the gas valve for the furnace burner - the
thermocouple DOES operate the "hold" bias forthe(very small and low
powered) pilot valve.

Some gas fireplaces and non fan forced heaters use a
"thermopile" that does operate the actual gasvalve directly - allowing
use without grid power.

My pre-2000 boiler in the UK uses the thermocouple to operate the pilot valve directly, without mains power. Mains power applied to the boiler operates the main burner valve providing the stat in the boiler says its own tank hasn't reached full temperature.