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.


Is this a wall furnace? Would the model number start with "GWT" instead
of "GW"? If it is what I think it is and the vent safety switch is not
defective, the vent cap on the roof could be messed up. Of course it
would help to know if it's a gravity vented wall furnace.

TDD