CWLee wrote:
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.

Dunno. But an air pressure sensor is not too difficult of a device -
sometimes it's just a vane attached to a switch. It's not too hard to get it
clogged or jammed either.