Sorry, perhaps I didn't explain this well enough. I meant having multiple thermocouples, each of which would be wired to its own valve. The valves would be plumbed side-by-side, in such a way that gas can flow to the burner even if only one of them is open. (i.e. gas is shut off only if all valves are closed). Of course, this solution means having multiple (probably expensive) valves, and also more pipe fittings that can leak.
Ahh I see, paralleled valves. That's a fair bit of pipe-work, but I do see what you mean. The regulator is of course in series so it wouldn't matter how many valves in parallel, the flow will be limited. I got a bit carried away with the whole '3 thermocouples, 1 valve' thing (sounds like a very dodgy internet video..).
So far the solutions, in order of preference (and ease):
- Give a single thermocouple adequate thermal contact to the shield. One thermo, one valve. Hope it can maintain temperature to hold valve open.
- Three thermocouples, power-OR circuit, to a single valve. Relatively easy circuit, but requires power (+9v), and annoying insulating connectors to prevent shorts.
- Three thermocouples, one valve, solenoid has 3 commoned windings. Annoying forcing 3 thermocouples to one valve connector, requires modding the solenoid (not too tough, #22 enameled wire).
- Three thermocouples, each with their own valve, plumbed in parallel. Expensive, requires pipe work, but *possibly* the most reliable solution.
The only other advantage of an active circuit is that is makes it much easier to add other forms of cut-off from other sensors. A tilt switch could easily be used with the power-OR, and if its a circuit that drives the solenoid directly, one could add a delay between a sag in thermo voltage, and tripping the valve. Its an overkill solution though.