Gas Furnace Question

[CWLee]

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.

[Tony Hwang]

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

[clare]

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.

[Tony Hwang]

And this furnace has a pilot?

[CWLee]

No filter.

Thanks.

[CWLee]

wrote

No fan.

Thanks.

[CWLee]

Yes.

Thanks.

[The Daring Dufas]

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

[HeyBub]

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.

[Jules]

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

[clare]

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

[Jules]

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

[zimpzampzormp]

TROLL!!!

[Jim Elbrecht]

I suspect that the above d*****ad is *not* Bob, but is trying to get folks to harrass him. He just started trolling in here a short time ago- has nothing to offer but wisecracks.

Jim

[Stormin Mormon]

Cheerfully, and with smiles. Be sure to mention you think it's a bad thermostat.

[Oren]

I'm a "pro d*****ad". Give me your GPS and I can slap a few knots on your head, faster than you could rub 'em off.

I spent 25 years dealing with turdheads like you. All mouth!

Is Drexal a school of hard knocks?

[Soundhaspriority]

My fame isn't virtual, it's real.

Bob Morein [phone]

[Bob Villa]

Let's assume the doctor means "Get a pro, d*****ad!"

bob_v

[CWLee]

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.

[Bob F]

It sounds to me like someone at tha hvac group got bored and decided to go be a jerk somewhere else.