On Sat, 08 Dec 2018 16:41:17 -0000
"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?

When I lived in America, I was confused that what Honeywell called a
Thermocouple was actually a bulb and capillary tube.

See:
https://www.amazon.co.uk/Honeywell-Q.../dp/B01H2SOZSK

They do not use the differential metal concept, but rather pressure due
to expanding liquid.

--
Davey.