gas furnace works, then it does not...help

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Old 11-05-05, 08:28 PM
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Question gas furnace works, then it does not...help

ok gang, here is what is going on. i have a 10year old Comfortmaker GNJ050N12A1 Furnace. worked fine for quite a long time, keep it cleaned and all that. just in the last week or so, it will run its cycle fine, then at the end of it reaching the temp, it cycles off like normal, but then will not come back on when the temp drops again. if i turn off the 'heat' switch on the setback thermostat then turn it back on, it will heat up to the programed temp fine, and even sometimes maintain it for a cycle or two, then stops again after the temp comes up, then nothing. and i have to manually turn off the 'heat' switch and turn it back on, and it starts all over again. Ideas?
 
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Old 11-07-05, 05:46 PM
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DanW

It could be any of several problems. The two most likely are either a thermostat or furnace control board problem. When you turn the thermostat to "off" you reset most control boards. The easiest thing to do would be replace the thermostat. I hate to tell people to be parts changers but tracking down the source of the problem could be time consuming for a tech. Unless you have a multi-meter & are comfortable using it, I would not suggest you even try troubleshooting.
 
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Old 11-07-05, 09:32 PM
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thanks for the reply Grady, went there , and put in a new thermostat( try the easist first) and no good, still does it with the new one. i am very comfortable with my fluke, so hint away, where in the control board?

Originally Posted by Grady
It could be any of several problems. The two most likely are either a thermostat or furnace control board problem. When you turn the thermostat to "off" you reset most control boards. The easiest thing to do would be replace the thermostat. I hate to tell people to be parts changers but tracking down the source of the problem could be time consuming for a tech. Unless you have a multi-meter & are comfortable using it, I would not suggest you even try troubleshooting.
 
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Old 11-08-05, 11:33 AM
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i am good with fixing things, just need to know the direction to go to fix this, anyone? thanks.
 

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Old 11-08-05, 03:08 PM
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I would start by going to the furnace and watching it while some one else turns the thermostat on till it lights and runs for a minute, then turn it off again, wait for it to cycle off, turn it on again and wait till it's running, then off again.

Like stated, it could be a number of things.

Most furnaces have a sequence of things they do everytime they come on, and it is easier to figure out what is wrong if you know the sequence, and also where the unit is stopping in the sequence.

Does the furnace have a pilot light that stays lit, or is it electronic ignition, or Hot Surface lit? Is it a power vented furnace?

The Control board would be in the furnace if it has one, and may possible have a diagnostic LED on it.

Are you the original owner of the unit, or was it in the house when you moved in, or did you get it used?

If you bought the unit new, have you ever had anything done to the ignition control?

A quick search for you brand and model number rendered .THIS PAGE that implies that the controls were an issue with some of these units
 
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Old 11-08-05, 06:08 PM
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Not to say Shank's method won't work but if you can catch it in the act of misbehaving on it's own, you stand a better chance of actually finding the problem.
Try turning the stat up just enough to make the heater come on. Let it go thru a cycle without making any changes. Open a window or otherwise let the room cool to the point where the stat is calling for heat again. If the furnace does not come on & you can get to the ignition control without removing a blower door or otherwise disrupting the power to the furnace, remove the wires going to terminals R & W. Check for continuity. If you have continuity, jumper across R & W with a short wire with alligator clips. The furnace should fire up. If so, let it run a while, remove the clip on W, let it cycle off & try the jumper again. If it works every time, I would say you either have a thermostat or thermostat wire problem. If it does not fire up each time, it's likely the ignition module.
 
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Old 11-08-05, 08:13 PM
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opps. By on and off, I meant up and down with the temp setting, not the on/off switch. Though on a gas stat it does the same thing.

You might take the door off ahead of time, and tape the door switch in so you can take it off later without disrupting power.

And instead of pulling the R and W wire off to check continuity, you could check those terminals for 24v (less chance of resetting it and losing your trail). You should show 0v if the stat is calling for heat.
 
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Old 11-09-05, 10:29 AM
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, thanks all for your input. here is what I did and what happened. after doing these type of fixxit projects for many years, one thing I have learned, never assume anything and check the easy stuff first. soooo with that in mind and researching how the system worked, I pulled the panels off. after not seeing anything obvious, pulled all the cables connections apart(one at a time) and back together a few times(with power off BTW) to clean the connections, as that is something i have found to be the problem in other areas more than once. then powered it all back up and tested it for function. mmmm seems to work now....let out slow held breath....ok lets see if it will go a few cycles....end of day it still is ok. ok now overnight and will it heat up in the morning like it should, yep, woke up and house is toasty. thanks guys, looks like all is well.
 
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Old 11-09-05, 07:13 PM
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Thumbs up Fixed, Cool Beans

Ah yes, the old dirty connection can drive you nuts. Good Job.
 
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Old 11-10-05, 05:07 PM
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Originally Posted by danw2002
ok gang, here is what is going on. i have a 10year old Comfortmaker GNJ050N12A1 Furnace. worked fine for quite a long time, keep it cleaned and all that. just in the last week or so, it will run its cycle fine, then at the end of it reaching the temp, it cycles off like normal, but then will not come back on when the temp drops again. if i turn off the 'heat' switch on the setback thermostat then turn it back on, it will heat up to the programed temp fine, and even sometimes maintain it for a cycle or two, then stops again after the temp comes up, then nothing. and i have to manually turn off the 'heat' switch and turn it back on, and it starts all over again. Ideas?
To test to find out if your thermostat is not resetting properly,this is an easy test for anyone who knows how to test continuity with a voltmeter. You simply access the furnace cover and get to where the two skinny thermostat wires screw onto the terminals. Unscrew just one of the wires as to eliminate a possible feedback scenario. Then check continuity of the thermostat wires when the thermostat should be calling for heat.

If there is no continuity, you have a bad thermostat. But if you do, you have more than likely a bad ventor relay switch. The ventor must run before any of the ignition control board (some call this a brain box) can be activated to tell the furnace to fire up. Without that ventor motor/fan relay coming on, the furnace will be dead quiet. I just had to replace one of those about a week ago and that fixed the very problem like you are having. These can fail especialy on furnaces that have a history of short-cycling, which means the contacts in the relay are seeing 'double-time'.

Also, always make sure your botom fan blower compartment cover is on properly as if it isn't, the safety switch on this door will cause the furnace to not run at all, as that is wherd the initial wiring goes through. If a furnace ever stops working, that is one of the first things you should check.
 
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Old 11-10-05, 05:10 PM
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Originally Posted by danw2002
, thanks all for your input. here is what I did and what happened. after doing these type of fixxit projects for many years, one thing I have learned, never assume anything and check the easy stuff first. soooo with that in mind and researching how the system worked, I pulled the panels off. after not seeing anything obvious, pulled all the cables connections apart(one at a time) and back together a few times(with power off BTW) to clean the connections, as that is something i have found to be the problem in other areas more than once. then powered it all back up and tested it for function. mmmm seems to work now....let out slow held breath....ok lets see if it will go a few cycles....end of day it still is ok. ok now overnight and will it heat up in the morning like it should, yep, woke up and house is toasty. thanks guys, looks like all is well.
What you did is always a good idea, especially for furnaces in damp/dank basements. Also, you can find those connectors to be expanded/lose. I remove them and squeeze them together a little and make sure when i slide the connector back on that I feel a resistance..that it's good and snug.
 
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