Here's where I was going when I mentioned the thermo-couple sensing the drop in temp....
"A thermo-couple sensor detects when the temperature of the water drops and sends an electrical impulse to the thermostat control. The thermostat causes the burner to light and bring the temperature of the water back up to the temperature setting on the thermostat."
The "piezo" switch needs mechanical input in some way, that is, if we're dealing with a piezo SW per-se. However, it's quit possible, there is an electronic ckt that triggers the ignition or lighting of the gas, instead of a piezo, upon receiving the call for heat?
After giving this more though, I'm just trying to rationalize the process.