Overtemperature switch tripping

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Old 11-12-05, 01:06 PM
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Overtemperature switch tripping

I have a Rheem gas heater, probably about 15 years old, and since I turned it on this season, the overtemperature switch keeps tripping after a couple minutes after the heat comes on. I changed the filter, cleaned out the fanbox, and oiled the bearing of the fan (which I've never done in the 5 years I've been in this house). Is there anything else simple I could check into myself before it gets cold? I was thinking of just replacing the switch, I don't know if they go bad or not.
 
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Old 11-12-05, 02:09 PM
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that is an airflow problem.
 
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Old 11-12-05, 02:13 PM
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What is the model # of the furnace?
 
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Old 11-12-05, 03:43 PM
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Originally Posted by BriCK
I have a Rheem gas heater, probably about 15 years old, and since I turned it on this season, the overtemperature switch keeps tripping after a couple minutes after the heat comes on. I changed the filter, cleaned out the fanbox, and oiled the bearing of the fan (which I've never done in the 5 years I've been in this house). Is there anything else simple I could check into myself before it gets cold? I was thinking of just replacing the switch, I don't know if they go bad or not.
Is this like one of those vertical vented gas space heaters? You don't have bird's nests or something in the vent, do you ?

But then, you never said if the blower actually comes on. If the blower is not kicking on, depending on the type of heater it is...it could overheat and then shut itself down.

We all need more information about this heater, I think. You need to tell us what it looks like...what you know about it.
 
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Old 11-12-05, 05:31 PM
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I don't have the model number, I for some reason couldn't find any identification plate or anything on it. I'll have another look at it, but all I could fing was "Rheem". The blower does come on. In fact, I had been running it continuously over the summer, so I was hoping it was just that the bearings needed oil and the blower was running too slow. The chimney is capped, so I don't think there's any birds in there. I figured if it's not just an over-sensitive switch, then it's an airflow problem, but I didn't know what to check next (i.e. where could the constriction be?)
 
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Old 11-12-05, 06:43 PM
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Is this thing a space heater instead of a furnace?

And do you know the history of the filter changings? Maybe the heat exchanger is all plugged up, and not allowing the blower to transfer away all the heat that it should be.

If the fan is blowing when the burner is on, for those several minutes before it trips out, does the heat feel extra hot? Or, not hardly hot at all? if it feels hot, maybe the limit has become overly sensitive. If it blows out cooler heat, then you probably have a transfer problem.

Can you see the limit device and can you remove it? You could remove it, set it somewhere and use a hair dryer and an oven thermometer set next to the high limit so that you blow the hair dryer on the high limit device and the oven thermometer equally at the same time. (I have done this test) to see at what temperature the limit switch closes down (technically they call it "open"). And it may have stamped on the device what that temp is.
 
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Old 11-12-05, 07:45 PM
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why don't you just check the temp rise?
 
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Old 11-12-05, 07:52 PM
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The ID plate should be inside the burner compartment.
 
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Old 11-13-05, 06:41 AM
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Sorry for the delay. Thanks Shank, I found the ID plate. It's a Rheem 100,000 BTU natural gas furnace, model # RGDA-100E-GR. The limit setting is 180 F and temperature rise of 60 to 90 (I don't know what this last bit means, but tinner73 suggested check the temp rise).
I have been fairly good with the filter changings, though I probably could have been more diligent with that.
With this info, I was thinking maybe reset the switch, put an oven thermometer in the burner compartment, and run it until it shuts off again. If it shuts off below 180, then I need a new switch? If it does rise above 180, is cleaning out the heat exchanger something I can do? I guess it's obvious I'm no HVAC guy.
 
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Old 11-13-05, 07:04 AM
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To do the temp rise test use "one' thermometer "stick type prefered". Dril a hole a into the return duct a foot or so before the furnace and another in the supply after the furnace. Take a temp reading at both points with the furnace running. This will give you your temp rise.

It sounds more like your exchanger needs cleaned and on this furnace it's really not DIY.
 
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Old 11-13-05, 09:07 AM
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Finally dug up the parts blow up of this unit, and if you are talking about the switch over the burners you should look and make sure your heat exchangers aren't cracked. Look close as they can be difficult to spot some times.

Is it running on LP gas, or Natural?
 
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Old 11-13-05, 09:29 AM
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Originally Posted by BriCK
Sorry for the delay. Thanks Shank, I found the ID plate. It's a Rheem 100,000 BTU natural gas furnace, model # RGDA-100E-GR. The limit setting is 180 F and temperature rise of 60 to 90 (I don't know what this last bit means, but tinner73 suggested check the temp rise).
I have been fairly good with the filter changings, though I probably could have been more diligent with that.
With this info, I was thinking maybe reset the switch, put an oven thermometer in the burner compartment, and run it until it shuts off again. If it shuts off below 180, then I need a new switch? If it does rise above 180, is cleaning out the heat exchanger something I can do? I guess it's obvious I'm no HVAC guy.
Oh. A furnace eh? Threw me a curveball. You mean you have ducts and registers too?

If so, you have to make sure that not too many are closed down. This willl not allow the main duct manifold, where all the branch ducts join in, to remain at a steady temp below the high limit point. If ducts are closed, the blower does not know that and tries blowing the same force of hot air. And without the air being able to move swiftly past the heat exhanger(s) and out the registers, the exchanger will just get hotter and hotter.

I have had similar problems where I have relieved the problem you are having by opening up the register on the main manifold, in the basement. This helps two ways. It relieves some of the trapped heat, and also warms up the basement. Besides warming up the basement for when pewople come down and do their clothes, it also keeps the first floor's floor warmer.
 
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Old 11-15-05, 07:17 AM
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UPDATE- more info

I found the notation "L 230" on the overtemp switch, which I assume means it's supposed to trip at 230F, although this sounds kind of high to me, so mayber I'm misinterpreting. I laid a temp probe on the switch and ran the furnace and found it trips at about 190, so this suggests that the switch is bad. I think my next step should be to try a new switch.
As far as the heat exchangers, it does look somewhat rusty and ugly in there, but it does blow warm air from the vents, and the main duct coming out of the furnace gets pretty hot to the touch, so I think heat is being exchanged pretty well. I haven't checked the rise but could do so if you think I should.
Finally, I opened up the exhaust ducts and they look clear. I can see daylight when I look up at the top of the chimney.
Any thoughts? Does this sound like a sound plan? Thanks!
 
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Old 11-15-05, 08:15 AM
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Originally Posted by BriCK
I have a Rheem gas heater, probably about 15 years old, and since I turned it on this season, the overtemperature switch keeps tripping after a couple minutes after the heat comes on. I changed the filter, cleaned out the fanbox, and oiled the bearing of the fan (which I've never done in the 5 years I've been in this house). Is there anything else simple I could check into myself before it gets cold? I was thinking of just replacing the switch, I don't know if they go bad or not.
One of the key issues lies in your quoted OP. You said it limits after just a "couple of minutes"?? Literally?

You can TRY the new limit switch, but I wouldn't be surprised if this don't fix it, with the sudden rise you get to trip it at 190. Its POSSIBLE that the climb could slow down between 190 and 230, but, all I think what will happen is that you will increase the limitiing time form a couple minutes to a couple minutes longer. A properly tuned furnace shouldn't rise to 190 that quickly, I don't think.

Is this problem a new problem you are experiencing, or, because perhaps your newly gained knowledge of furnaces, you are just perhaps discovering a problem that this furnace may have had? Are you certain the problem is new to this season? Maybe the furnace ducting was never done right? I have been a witness to this, with a furnace man besides me, analyzing such problems. Maybe you will find closed down damper controls on several of the ducts, and you don't know it. Often times you will find these damper controls in the duct, beSIDES having controlable registers. You might want to take a look.
 
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Old 11-15-05, 08:28 AM
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A couple of minutes isn't exactly true. I sat and watched it go up today, and I think it was probably on the order of about 15 minutes (VERY rough estimate, I didn't think to time it). I've never checked the temp before, but I can say that at least the overtemp switch never tripped in the last 5 seasons.
 
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Old 11-15-05, 10:02 AM
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To clarify on my last post...

A cracked heat exchanger will not stop the exchange of heat. It also will not stop the exchange of CO from the combustion chamber to the circulated air.

The only times I have ever run into that switch doing what it is doing is because of a cracked heat exchanger.

What happens is when the fan comes on it blows a small amount of air into the heat exchanger and forces the flame to roll out of the chamber.

A test for you:

Turn the unit off, and set a candle, or hold a lighter by the burners.
Have someone turn the fan on and see if it alters the flame of the candle or lighter. And do this by all the burners.

You are talking about the manual reset switch in the plate abocve the burners right?
 
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Old 11-15-05, 10:03 AM
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And, You DO have carbon monoxide detectors in your house RIGHT?
 
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Old 11-15-05, 10:29 AM
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I do NOT have a CO detector, but that did become a top priority when I saw the heat exchanger. For now, the furnace is off, but I'll get detectors before I turn it on. Incidentally, I can't remember, does CO rise or fall? I want to say it falls. One of the things I was planning on doing before I permanently install the detector was to hold it at the registers to see if I'm blowing CO.
Turn the unit off, and set a candle, or hold a lighter by the burners.
I haven't tried that test, but I can see the burners through the grate in the combustion chamber cover, and the flames don't seem to roll out of the chamber.
You are talking about the manual reset switch in the plate abocve the burners right?
Yes, that's the one. Thanks again!
 
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Old 11-16-05, 05:19 PM
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Originally Posted by shank
To clarify on my last post...

A cracked heat exchanger........

A test for you:

Turn the unit off, and set a candle, or hold a lighter by the burners.
Have someone turn the fan on and see if it alters the flame of the candle or lighter.
What's wrong with just observing the burner flame at the moment the fan kicks on?...to make sure it does not change from how they looked before the fan came on?
 
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