Gas furnace does not try to light, Carrier


  #1  
Old 10-10-03, 05:20 PM
bigdog
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Question Gas furnace does not try to light, Carrier

Carrier gas furnace, model unk (name plate gone), about 20 YO, with draft inducing fan and electric ignition does not try to light. I'd suspect some safety interlock switch has tripped, but no reset buttons or switches are apparent. Cover plate decal reads "Super Saver." Unit worked last season, and blower worked well all cooling season.

Draft inducing fan runs, and in fact keeps running regardless of where thermostat is set (heating, cooling, temp high or temp low). This seems strange; why doesn't it cut off when thermostat calls for cooling, or when heat setting is set high temp?

I've tried turning off main power, then back on. No effect. I've tried turning off the "Sqare D" type switch in the furnace room. No effect on inducer fan running.

Are there any obscure reset switches? Any steps to try before incurring an amazing service charge? Any suggestions on reliable service companies in Northern Virginia, should push come to shove?

Clue for dogged detectives: the model number on the gas valve
reads SX345NSX-12.

I read some earlier posts a few minutes ago, and have every expectation that you'll give me some practical advice. The most significant fact to me is that this unit is not TRYING to light. What could have happened over the summer?

bigdog
 
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Old 10-10-03, 05:47 PM
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Relay stuck

Sounds like the inducer relay is stuck.
The furnace will not attempt to start unless it sees a change in status from the inducer switch.
 
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Old 10-10-03, 05:50 PM
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The fact that the inducer fan keeps running is an indication that it is a safety lockout. That means that you have power but something is preventing the ignition sequence. The first safety control is the fan prover. Now you have to understand that there are 86 different types of furnace design so my description may not be excatly that same as your furnace.
The fan prover could be found by following a small rubber hose from the inducer fan housing to the control. You could try tapping on that control to dislodge it if it has stuck over the summer. If not you could jumper the terminals on startup to see if the sequence progresses. The best way would be to check with an ohmmeter. If you have one, proceed that way. There should be continuity when the inducer is running if the air flow is sufficient. If the vent is blocked, the control may be doing its job. If th vent is clear and you can feel the air coming out of the vent outlet, maybe the fan prover switch is bad. It then progresses to the ignition circuit, ignitor, flame sensor, etc.
 
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Old 10-13-03, 09:01 PM
bigdog
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Talking Have you heard the joke about the guy with his hammer?

You two were bang on!

Thank you, 54regcab and KField for your quick replies. Massive thanks to you, KField, for the step-by-step directions as well as the accurate diagnosis. I trust you've already guessed that the furnace is now working.

There is great satisfaction in fixing a balky machine by banging on it with a hammer. I banged on the fan prover, and several other parts for good measure, then replaced the blower compartment cover. The inducer fan started running --- but soon stopped. After waiting several minutes, I raised the thermostat temperature setting, and waited some more.

And then the pilot lit! Like Christmas in October! Soon, the burners lit, but very shortly went off. So I raised the thermostat to max temperature. To my great relief, the burners soon came back on. I reduced the thermostat setting in small steps until the burners shut off, then nudged it up. In a little while, the furnace came back on, and has since been operating like a furnace should.

I will tell this story with gusto to lots of people, including fellow senior citizens who also are tying to stretch fixed incomes. I'm sold on this forum; I'm sorry it took me almost 10 days of trying all kinds of Google searches before I lucked onto it. And it goes without saying that I'm deeply appreciative of your detailed help, KField. I hope that in some way I can return the favor, to you or to someone else in need on this forum.

How about I buy you a huge box of Crayola crayons at what these days they're calling the Crayola factory? At one time I had an idea to include the factory on a trip for seniors, until I learned it's not really THE factory.

If you read anything which raises any concern in my description of the sequence of events in getting the furnace back up, I'd be glad to hear your advice.

bigdog

PS: In case you haven't heard the joke, it goes something like this:

A guy brings his car to a mechanic after getting no help from three other repair shops. The mechanic, for whatever reason, works on the car while the owner is still in the shop.

He takes a hammer and whacks the engine in a very precise location. Then he starts the car, and it purrs just right.

The mechanic presents his bill: $205. The guy is very upset, having watched the repair process. He protests the bill, saying a hit with a hammer isn't worth $205.

The mechanic says, "It's $5 for the blow with the hammer, and $200 for knowing where to hit."
 
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Old 10-14-03, 05:27 AM
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Success brings a warm feeling to heating professionals and customers alike.

Good Job
 
 

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