paulmedd said:
Hi Nester-kelebay
Found the thread mentioned and comment from you re solder joints but would be interested to see your mentioned comment on how a gas valve works. Have just started a thread with my problem
Regards Paul
Paul:
Sorry, I guess I posted the wrong thread. The correct thread is entitled:
"Thorn M Boiler - Problem maintaining the pilot flame alight"
posted in this Plumbing & Central Heating forum by davidnicole on Jan. 30
Basically, in a nutshell, what I say is:
a) a gas valve contains of two electromagnetically operated valves arranged in series so that no gas will flow completely through the gas valve unless both electromagnetically operated valves are open.
b) The first electromagnetically operated valve get's it's power from a thermocouple or thermopile sitting in a pilot light.
c) gas must flow through this first electromagnetically operated valve before it can flow up the pilot tube to the pilot light, so if the pilot light is burning, that means the thermocouple or thermopile is producing enough voltage to operate this first electomagnetic valve.
d) the second electromagnetic valve is operated by the voltage produced by a thermopile or from a transformer, and controls the flow of gas to the main burners of the appliance.
e) a thermopile is typically used on hot water heaters which don't require a household power to run a circulating pump or blower, and is simply a whole bunch of thermocouples connected in series within one component.
f) the voltage produced by both thermocouples and thermopiles will drop in time, and they periodically need to be replaced because of this.
So:
1. if a pilot light won't stay lit, the most common cause is that the thermocouple or thermopile needs to be replaced because it's not producing enough voltage anymore.
2) if the pilot light is burning, it means the thermocouple or thermopile is producing enough voltage to keep the first electromagnetically operated valve open. And, my understanding is that if there's enough power to keep that first electromagnetically operated valve (called the "safety electromagnet") open, then there is enough power to operate the second main valve to the burner trays.
3) On an appliance that has household voltage available to it to operate a circulating pump or blower, then the gas valve will use the power from a transformer to operate that second electromagnetically operated valve. So, if the pilot light is burning on a boiler or forced air furnace, but the gas valve isn't opening, check for the correct voltage at the wires going to the gas valve. If there is voltage between those two wires, then the problem is likely to be that the gas valve needs to be replaced. If there is no voltage there, the problem is that voltage is not getting to the gas valve to open it, possibly because of a safety device stopping the voltage from getting to the gas valve.