| Not sure about outdoor boilers, but on indoor gas-fired water heaters that I've been involved with has a device called a thermocouple. It has to be exposed to a serious amount of heat to allow gas to flow. Normally, it is set so that the sensor is immersed in the pilot light flame. If the flame goes out, all gas is cut off in the space of a second or two. In fact, to relight it, one has to push and hold a special button while one lights the pilot - then once it's lit, you have to hold the button until it heats up enough to hold the gas supply open (normally about 30 seconds to a minute). |
