"Bruce Farquhar" wrote in message
news
On Mon, 10 Dec 2018 17:53:15 -0000, newshound
wrote:

On 10/12/2018 14:32, Bruce Farquhar wrote:
On Mon, 10 Dec 2018 11:16:54 -0000, newshound
wrote:

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. 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.

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.

Common sense tells us it can't be. A quarter of a 1 bar fire would make
the water in the boiler pretty warm! Now remember that at this point
the pump isn't running, so that heat can't easily move to the
radiators. The boiler would get piping hot pretty quickly. My medium
sized gas boiler is rated 4-12kW (depending on the gas pressure
setting), which would mean it's still using up to a 16th of the gas when
turned off! That would be truly absurd. The only thing as stupidly
inefficient as that is a Sky satellite TV box which uses precisely the
same 38W when in standby as when switched on!

Oh I don't know. In the dim and distant past, my boiler which had a
pilot light was floor standing and about 0.6 x 0.6 x 1 metre high.
Giving it a surface area of about 2.4 square metres. Much bigger than a
modern boiler I know, but modern boilers don't have them. So, taking a
convective heat transfer coefficient of 10 W/m^2K the surface
temperature rise to dissipate 250 watts if it does not go anywhere else
is about ten degrees C. Warm to the touch but not huge. And I bet 90% of
the energy used to go out of the flue.

In the mid 70's I had a multipoint gas water heater which, IIRC, was 22
kW. The exposed surface was probably only half that of the boiler in the
calculation, but that had a fairly lively pilot light and the casing was
always perceptibly warm to the touch.

I once calculated my gas usage for the pilot light to be £10 a month (at
3.5p a kWh) on my 12kW boiler, as it was the only thing running over the
summer. That's a ridiculous cost,

But now most gas suppliers have a fixed charge even if you don't use any gas

no wonder they use electric ignition now. Not sure why they didn't always
do that - gas cookers have had spark ignition for at least 40 years, if
not more.

But those werent automatic, they required someone to pull the trigger.

Even if they didn't have a spark that could operate millions of times, it
could always light a pilot and extinguish it if not in use for a certain
amount of time.

Some did do that with house heating only boilers.

My 'maiden aunts' used to have a gas hot water
system for their bath. Had to be manually lit.