[jamesgangnc[_3_]]

On Dec 20 2009, 12:04*am, "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.

--
----------
CWLee
Former slayer of dragons; practice now limited to sacred
cows. *Believing we should hire for quality, not quotas, and
promote for performance, not preferences.

If it's naturally vented (no fan in the vent system) then it's
probably a rollout switch. If so then you have draft problems. Hold
you hand near the switch when the unit starts. Feel hot air? If so
that's exhaust gases that are not going up the vent. If you don't
feel anything then replace it.

[CWLee]

"jamesgangnc" wrote

If it's naturally vented (no fan in the vent system) ...

Yes, that is the case here.

then it's
probably a rollout switch.

Although the problem has been solved, I'd like to know just
what a "rollout switch" does. Can you explain how it works?
I've Googled "rollout switch" but all I learned were places
to buy them, not what they do or how they work. Is it a
switch that does anything more than shut off a circuit if
the temperature sensed by it exceeds a specified level?

Thanks for any enlightenment you can provide.

[The Daring Dufas[_6_]]

CWLee wrote:


"jamesgangnc" wrote

If it's naturally vented (no fan in the vent system) ...

Yes, that is the case here.

then it's
probably a rollout switch.

Although the problem has been solved, I'd like to know just what a
"rollout switch" does. Can you explain how it works? I've Googled
"rollout switch" but all I learned were places to buy them, not what
they do or how they work. Is it a switch that does anything more than
shut off a circuit if the temperature sensed by it exceeds a specified
level?

Thanks for any enlightenment you can provide.




From what I've read, it appears that the problem was a vent safety
switch which is similar but not the same as a roll-out safety switch.
On a forced air furnace one or more roll-out switches are located
around where the burners take in combustion air and air for the draft.
If the flames "roll out", ie, come out backwards from the burner
assembly, the safety switch(s) detect this and shut down the furnace.
Roll-out switches are usually manual reset like most safety sensors.

TDD

[CWLee]

"The Daring Dufas" wrote

From what I've read, it appears that the problem was a
vent safety
switch which is similar but not the same as a roll-out
safety switch.
On a forced air furnace ...

The furnace in question has no fan to "force" the air flow -
does that change your response?

... one or more roll-out switches are located
around where the burners take in combustion air and air
for the draft.

The switches that were apparently defective, and replaced,
were at least 3 feet above the pilot and burner area - does
that change your response?

If the flames "roll out", ie, come out backwards from the
burner
assembly, the safety switch(s) detect this and shut down
the furnace.
Roll-out switches are usually manual reset like most
safety sensors.

It does sound like a good idea to have such switches. Are
they normally easy for the consumer to reach - or are they
like the switch in question in this case, only accessible
after removing 6 machine screws and a large panel?

Thanks for continuing my education.

[The Daring Dufas[_6_]]

CWLee wrote:


"The Daring Dufas" wrote

From what I've read, it appears that the problem was a vent safety
switch which is similar but not the same as a roll-out safety switch.
On a forced air furnace ...

The furnace in question has no fan to "force" the air flow - does that
change your response?

... one or more roll-out switches are located
around where the burners take in combustion air and air for the draft.

The switches that were apparently defective, and replaced, were at least
3 feet above the pilot and burner area - does that change your response?

If the flames "roll out", ie, come out backwards from the burner
assembly, the safety switch(s) detect this and shut down the furnace.
Roll-out switches are usually manual reset like most safety sensors.

It does sound like a good idea to have such switches. Are they normally
easy for the consumer to reach - or are they like the switch in question
in this case, only accessible after removing 6 machine screws and a
large panel?

Thanks for continuing my education.



No problem my friend. I responded to your original post after I guessed
it was a wall furnace which is so much simpler than a central forced air
furnace. My responses wouldn't change because I was making comparisons
between the forced air and the gravity furnace. The safety switches
usually have a little reset button between the electrical terminals
and when pushed in will make a little click sound when reset. When I
check a malfunctioning furnace, I will go through and press all the
reset buttons until I come across one that clicks. From there I can try
to determine what tripped the safety. If I had a small house/cottage or
an efficiency apartment, I think I would prefer a gravity wall furnace
because of the simplicity and the fact that it would keep a home warm if
the power was lost during a severe winter storm.

TDD