Johnson Ultra SX90 Air Ease gas forced air furnace


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Old 12-16-10, 01:00 PM
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Johnson Ultra SX90 Air Ease gas forced air furnace

Johnson Ultra SX90 Air ease furnace is about 13 years old (original to house). Over the course of 8 years we have had three (3) different occasions when we have been out of town and furnace does not shut-off. This causes the temperature in the house to reach well over 100 degrees. Not sure how hot the temperature got but large candles melted, corks popped out of wine bottles and ceramic flooring so hot you could not walk on it without shoes -- also cold water in pipes was so hot you could not stand to touch it. Each time, the thermostat was set at around 58 degrees. Each time the heating repairman replaced the digital thermostat. I have spoken with numerous heating contractors and no one has ever heard of this happening. The Air conditioner is also connected to the thermostat and we never have trouble with the AC not turning off when set temp is reached. Again this has happened 3 times in 8 years and we are now reluctant to leave the house for extended vacations for fear that the house will burn down. The last time this happened was the end of October 2010 and again a new digital thermostat was installed. I find it hard to believe that the thermostat always the problem. I would appreciate any insight you my have on this problem. I await your response.
 
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Old 12-16-10, 02:42 PM
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I agree that the problem is not caused by the thermostat.

Two possible causes would be:

1) a defect in the thermostat wiring that occasionally allows two wires that control the furnace to short together and keep the furnace on.

2) a defective furnace circuit board. Usually the thermostat closes a relay on the furnace which causes other circuits to operate.

Relays can stick in the closed position, which would mean keeping the furnace turned on.


I'd start by carefully checking the thermostat wiring for any signs of cuts, wear, missing insulation or bare wire.

While an unpleasant thought, rats gnawing on wiring behind walls could cause a problem you would be unlikely to observe. Have you EVER had a problem with rats or mice in the house?

Supposing that the problem was the thermostat was not unreasonable ---- many digital thermostat have relays that can stick, too. But not with three different thermostats.


Perhaps you can post the model number of the furnace which can be obtained from the rating plate in the burner compartment of the furnace.
 
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Old 12-17-10, 12:35 PM
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Thanks for your input. The model and serial numbers are GUK97501-3A and 8499A11921. Since my husband and I are complete novices when it comes to electricity we will probably call an electrician to come check the wiring from the thermostat to the furnace and vice versa. As far as rats and mice are concerned, I am 100% positive there have been no rats (house is 13 year old quad level and I have not ever heard of anyone in our subdivision having a rat problem. Mice are a possibility, but I have never seen any indication of mice (no droppings anywhere in the house) but who knows. In remembering the past episodes, they have always happened right after I have reprogrammed the thermostat. Since we have replaced the thermostat 3 times I agree that its quite a coincidence that the could all have the same problem. I have spoken with several heating contractors (all from difference companies) and none have ever heard of this problem. They all said that they would have to witness the event to accurately tell me what the cause is. My regular heating repair guy felt that it could not be the thermostat and that it may be the circuit board. But when he came to make the repair, he said that the thermostat was bad and he again replaced the thermostat (without doing anything to the circuit board). Is it possible that if it is a nick in the wiring that when the wires touch it could ruin the thermostat? I appreciate any help anyone can give me. We are at our wits end.
 
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Old 12-17-10, 06:42 PM
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A damaged thermostat wire MIGHT cause the furnace to run continuously, it MIGHT cause a fuse to blow on the circuit board (if there were one) or it MIGHT cause the transformer to overheat and burn out.

If you have had a digital thermostat, none of the problems are likely to damage the thermostat. Do you know WHAT specifically was wrong with the thermostat?

Personally, I'd favor a furnace repairman over an electrician to inspect the thermostat wires. A furnace repairman probably should have more experience looking for the kind of problem that might exist and an electricians skills really don't add anything to what a furnace repairman can bring to the job.

(I had to ask about rats--- I've seen furnaces replaced because they wouldn't shut off and being unable to identify the problem. When the problem recurred after the furnace was replaced, gnawed thermostat wires behind the wall were finally identified as the problem. It would be negligent not to ask)

It's not quite true that a repairman has to wait around for a furnace to display a fault before it can be diagnosed.

Firstly, can you ever recall an instance or instances where your house began to overheat while you were there? The odds are that while you found exceptionally high temperatures while you were gone on three occasions, there were other episodes where this defect occurred while you were home.

Did you ever notice something like that happening?

Secondly, since the thermostat is ruled out as being the likely problem, a sticking relay or on the furnace circuit board or ignition module is the likely problem.

If I were the repairman, I'd sit in front of the furnace with the cover off and turn the furnace on ---- oh, 500 or 1,000 times to give the furnace the opportunity to fail so the problem can be identified for sure.

The odds are reasonably good that a sticking relay will be observed during such a test. A sticking relay may stick, but then open eventually, shutting off the furnace when it does. That's why you MIGHT have noticed occasions when the furnace overheated the house temporarily but then went back to normal.

If that kind of torture test didn't allow me to observe and verify the problem, I might still recommend replacement of the circuit board or ignition module. That would be the likely problem and you need to go after the likely problem even if you can't observe the problem directly in order to verify this theory.

That's what I'd do, anyway.


I invite our other repairman/posters to discuss what they would do with this problem.


(intermittent problems are the bane of repairman. How you deal with them is a real measure of a repairman's skill)
 
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Old 12-18-10, 07:47 AM
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No, I have not noticed unusual temperatures when I am home. Of course, I'm not really paying that much attention either. We have several sky-lites on the main level of our home (not near the thermostat) and when the sun is shining it can get warmer than the setting on the thermostat. Starting immediately, I plan to make checks of the thermostat throughout the day and evening over the next several weeks. What you suggest makes a lot of sense. Again, I really appreciate your input as I plan to pass this information along to a heating contractor after the holidays. At this point, I am not satisfied that the new thermostat that was installed October 2010 has solved my problem. If anyone else has suggestions, I will be happy to hear from you.
 
 

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