? doesn't this control On/off on a thermostat ?? the button your referring to is it the piliot test button ? if so I guess you've found a loophole in there test strategy by holding it down for five minutes to get it too stay on ,
what does this heater do when it shuts off on seeing a heated water temp to setpoint (or doesn't it ?) as if the test button is the only way to light it , this button bypasses a few normal safety switches (allthogh I didn't know about the five minute thing) one of the most common safetys that goes on these heaters is the heat exchanger over temp switch , seems the wires get hot and the little connectors fall off it