"Fred Johnson" wrote in message
news
On Sun, 09 Dec 2018 17:00:04 -0000, wrote:

On Sunday, 9 December 2018 16:40:47 UTC, Bruce Farquhar wrote:
On Sat, 08 Dec 2018 20:33:27 -0000, Rod Speed
wrote:
"Bruce Farquhar" wrote in message
news On Sat, 08 Dec 2018 18:32:08 -0000, Max Demian

wrote:
On 08/12/2018 16:41, 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?

Well they used to have a bimetallic strip to hold the pilot light
valve
open rather than a thermocouple. Or maybe that's when boilers didn't
have an electrical connection at all.

The electricity from the mains supply (on an old basic/system
boiler/furnace) is nothing to do with the pilot light, it just
operates
the main gas valve. The mains connection to my boiler is only on
when the
room stat calls for heat. When the room is warm enough, no
electricity is
given to the boiler at all.

What do you mean "when boilers didn't have an electrical connection
at
all"? Surely they need something to tell them to start burning gas?

Doesn't have to be an electrical thermostat, can be
a bimetallic thermostat or a bulb and capillary tube.

For the internal stat for its own tank, yes. But the room stat needs to
tell the boiler remotely, which is always electricity surely.

pre-electric boilers used non-electrical room stats.

How far do you have to go back to find a boiler which is gas only?

Back to when gas lights were common.