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
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
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.
Former slayer of dragons; practice now limited to sacred
Nothing to do with the filter. I assume this is a pressure vented
furnace (has a burner fan). If so, any restriction in the "stack" will
cause a pressure build-up which will trip the pressure operated safety
switch. DO NOT RUN THE FURNACE if there is an exhaust restriction.
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.
You havn't been on alt.hvac? I call it alternating havoc.
It's been a bit slow, lately. But mentioning a thermosat
there. It's like throwing bloody chum to starving
alligators. Take a couple Valium before reading the replies.
An off-line continuation of this thread with one of the
posters leads me to believe that it is a temperature sensing
device. It may be called a thermodisk or thermodisc. I'm
told the red dot on it signifies that it trips at 195
One of my ideas is to remove it, take it to a
heating/plumbing supply store, and see if I can match it.
Once I have it removed I may be able to do some bench
testing to see if it is working. My off-line advisor said
the switch should "click" when it is reset. So far, I can't
get it to click, but access is difficult so I'm suspending
judgment until I have it off the furnace.
Thanks for your continuing assistance.
Well, how do exhaust combustion gases leave the property? If the
exhaust is partially jammed then I can see that giving the behavior
you're getting - the switch will trip because with the furnace running
gases don't clear the exhaust fast enough, but as soon as the furnace
is off (because the switch trips) the gases "slowly" leave the exhaust and
pressing the reset lets it run again.
I'd check the exhaust first, make sure it's clear of any obstruction
(dirt, leaves, ice, dead critters, bug nests etc.) and only worry about
whether the switch is defective after you've ruled the exhaust out.
*don'* bypass the safety switch, obviously :-) If you want to rule out the
switch, buy a replacement and do it that way...
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