[77newell]

David: the logic in the control boards I've had for my Suburban furnaces goes as follows: fan kicks on - then sail switch checks air flow and timer runs to ensure no fuel is left in furnace - when timer times out the igniter starts sparking and gas valve opens (I can usually hear the sparking when the front of the furnace is open and usually hear the gas valve open) - then the board checks for flame and a timer will shut things off if no flame detected after a short period- furnace continues to run until thermostat circuit opens then everything shuts down.

Butane will vaporize down to about 34 degrees, depending on the mix with propane you may get lower and still get gas.