How does a "vent safety switch" work?
I'm trouble-shooting a friend's gas furnace (Empire Comfort Systems Model #GW-25-6). I have isolated the problem to what the instruction booklet calls a "manual reset vent safety switch." It says this safety device will switch off the gas flow if "the ventilation is not proper" or similar wording. I'm trying to figure out if the switch is defective, or if there is a ventilation problem. I'm satisfied that the pilot and burner mechanism work properly, and that the thermostat works properly. Nevertheless, the furnace shuts off unexpectedly, but will restart promptly if the vent safety switch is reset, meaning a button on it is pushed.
What variable does the switch sense in order to determine if the ventilation is proper?
My first thought is that it is temperature, and that if the temperature of the air around it is above some threshold the switch cuts off the electric current that is keeping the gas valve open.
My second hypothesis is that the switch senses air flow (how would it do that?) and if the speed of air passing the switch is below some threshold the switch cuts off the electric current that is keeping the gas valve open.
Enlightenment, explanations, elaborations, etc. appreciated.