Gas Furnace Question

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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.
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CWLee
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CWLee wrote:

Hi, First is the switch keeps tripping if reset? Is the filter CLEAN?
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wrote:

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.
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snipped-for-privacy@snyder.on.ca wrote:

Hmmm, And this furnace has a pilot?
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> And this furnace has a pilot?
Yes.
Thanks.
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No fan.
Thanks.
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On Sat, 19 Dec 2009 22:45:10 -0800, "CWLee"

OK - no "pressure vent" means it is a stack temperature situation. Call a furnace repair service and be prepared to replace the furnace - you could very well have a cracked heat exchanger
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CWLee wrote:

TROLL!!!
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No filter.
Thanks.
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CWLee wrote:

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
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Cheerfully, and with smiles. Be sure to mention you think it's a bad thermostat.
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I don't think the thermostat is bad. I think it is a thermostatic vent safety switch that is bad, a different device.
Does that make sense to you?
Thanks.
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CWLee wrote:

Just don't go to the hvac group unless you like being abused.
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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.
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Yes, it makes perfect sense. Now, the questions. Is there really a venting problem, or is the switch growing weak? Does the switch read pressure, air flow, or temperature?
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That's exactly what I'm trying to determine.

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 degrees.
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.
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Many snap disk have a temperature number on there, some where. Maybe the wall heater manufacture can sell you a new one, and be done with it.
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CWLee wrote:

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.
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On Sat, 19 Dec 2009 21:04:35 -0800, CWLee wrote:

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...
cheers
Jules
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On Sun, 20 Dec 2009 09:22:44 -0800, Smitty Two wrote:

I think you just got my favorite post of the day award :-)
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