Limit switch reaches temperature too quickly


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Old 12-17-11, 09:34 AM
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Limit switch reaches temperature too quickly

Hi, I have an Olsen furnace from around 1993... It seems to only keep the flame on for five minutes or so, then shuts down for about another five minutes before coming on again. This seems to be due to the temperature limit switch reaching the high cutoff quickly. It is set to the highest setting, about 190. My fans all seem to be running good, so does this mean my heat exchanger is clogged and that's why its reaching the temp limit so fast? It manages to heat my house eventually but it can't be good for the furnace with it going on and off like this so frequently. Is there a way to test that this limit switch is working properly?
 

Last edited by sluge; 12-17-11 at 09:36 AM. Reason: wanted to add more details
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Old 12-17-11, 09:46 AM
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I probably shouldn't comment...no Pro...but when you say limit switch...do you mean the box with the probe that extends into the combustion area?

It may not be that your fan isn't running right or the HE is bad (though that seems likely on an old system) but that there just isn't enough airflow for whatever reason.

Are all filters clean? Do you have any intakes or vents blocked? Are the fan blades themselves clean? A dirty blower won't move the amount of air it should.

What are your fan on and off settings?
 
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Old 12-17-11, 09:50 AM
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Gunguy's advice is the right place to start. Check that stuff first --- rather often it will solve the problem.
 
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Old 12-17-11, 09:58 AM
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Thanks for the quick reply! Yes I mean the box with the probe that extends into the combustion area... it is a honeywell, not sure what model, with the metal disc about 1.5" diameter... the low temp off is set to about 70, on is about 110, and the high temp off I set to the highest setting at about 190. when it comes on I can watch the disc turn and reach the cutoff temp in a few minutes. The air filter is fairly new and clean; I tried it with the air filter out and the cover off the compartment that contains the blower to try and eliminate this as a cause. Not sure about the blades, can't really see them but they haven't been cleaned in the 7 years I have owned this house. How can I tell if the proper amount of air is going through? The flame is blue and seems to be good...
 
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Old 12-17-11, 10:01 AM
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Did you check to verify that the cool air return and warm air vents are open?

Is the fan motor turning on and does it appear to be coming up to speed?

Does the fan turn freely if you turn it by hand?
 
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Old 12-17-11, 10:01 AM
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Just to add...I think sometimes the "fan on" setting is set too high. People don't like the cooler air so the On setting gets bumped up. Then the HE overheats and the Hi Limit is hit.

What is the actual cycle events and time between them?

Burner on, fan on, set point reached, burner off, fan off (with varying times between) is normal.

If it's taking 2 min for the fan to kick on...then something prob isn't right.
 
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Old 12-17-11, 10:46 AM
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Fan motor (at bottom of furnace where the air goes in) turns freely by hand, although it does seem quite dirty, got a fair bit of crud on my fingers when turning it manually. How difficult would it be to get in there and clean the blades?
Turned main switch off... waited 5 minutes, turned main switch back on
Fan on bottom of furnace turns on immediately
after 45 sec igniter heats up
at 1:20 burner on
at 2:15 another fan turns on, I'm assuming when limit switch hits the 'on' temp of about 110 (I guess this is the one that actually moves the heat through the house? Where is it located?)
at 3:40 the temp limit switch hits off limit and flame goes out, bottom (intake?) fan still running...
after a few more minutes it comes on again and repeats the whole process.
 
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Old 12-17-11, 11:00 AM
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Ummm. Fan blades with a lot of dust and dirt on them suggest that the equipment has been operated extensively without a filter.


Do you have air conditioning? If so, there should be an AC coil upstream of the furnace which is subject to being plugged with dust and dirt and is another common cause of overheating problems.

You need to inspect the side of the coil that's towards the furnace to see if it is plugged with dust and dirt.
 
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Old 12-17-11, 11:11 AM
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Yeah I thought it was weird too that there was dust and dirt on the fan blades... I have of course always operated it with a filter, but the furnace is 18 years old and I have only owned the house for 7 so I can't say if the previous owners did, although I can't fathom why anyone would run it without the filter! Maybe its just because of many years using crappy ones?
I do have AC, but not sure what 'coil' you are talking about but I will try and find it and figure out how to clean it.
 
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Old 12-17-11, 11:33 AM
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Ok I looked up a diagram of the coil and see what you mean, it's the evaporator coil located right on top of the furnace, with the cooling lines going in and a drain line coming out. Problem is, this seems to be fully sealed into the plenum, with the lines in and out and all the duct joints sealed with silicone! So, it looks like it would be difficult to get in there and see if it needs cleaning. Maybe I should just cut an access hole in the back of the plenum and re-cover and seal it up when I'm done?
 

Last edited by sluge; 12-17-11 at 11:34 AM. Reason: bad grammar
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Old 12-17-11, 12:28 PM
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You must get in to be able to inspect it, one way or another. Cutting a hole you can later patch by screwing in a piece of sheet metal is a possibility.

Using a flashlight and an inspection mirror is a good practice.

Even the cheapest filters do a fine job of protecting equipment from dust and dirt. Probably run without filters would be my supposition.
 
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Old 12-17-11, 02:11 PM
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Pulling the blower involves removing the entire assembly, taking the motor/capacitor off and washing the blades.

It's not possible to clean a blower properly in place.

If the furnace is not high efficiency and the heat exchanger doesn't curve, you might be able to see the underside of the evap coil with the blower removed.

If it's high efficiency, the secondary heat exchanger could be clogged.
 
 

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