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Sears GAs furnace issue with burner running too long

5K views 17 replies 6 participants last post by  how 
#1 ·
A good friend of mine has been having terrible trouble with his Sears gas furnace over the past couple of years. I am not sure of the model number but its about 10 years old, and designed to heat a house of about 1700sq ft.

He has had technicians (3 different guys from Sears) in about 8 times in 2 years and they keep telling him that there does not seem to be anything wrong as the problem is intermittent and cannot be reproduced when they are there.

While I have no intention of trying to fix this problem myself, I would like to try and identify the problem which may help a tech fix it.

Basically, he has the temp set back to 17C at night, in the morning, when the temp automatically sets up to 20C, the furnace burner comes on, and STAYS on for 10 to 15 minutes (the fan in also running) Then, the little Limit Switch at the front (mounted in front of the burners on the plate that runs under them) trips, presumably because the unit has reached some critical temp. Obviously, this then needs to be reset before the burners will come back on.

The issue I see is that the burner runs too long. On my furnace (different model but same type of system) the burner never runs for more that 3min30secs before it shuts off and allows the fan to reduce the heat in the heat exchanger. The burner comes back on after a few minutes if needed.

I can't believe that this furnace is designed to allow the burner to run for so long. On my furnace there is a mechanical limit/fan control switch that visibly moves, as soon as the temp in the heat exchanger hits some preset I hear it click and the burner goes off. I don't see a similar module in the Sears model of my neighbor so I am thinking there must be some other type of maybe electronic module to control this that is not visible (there is a small limit switch on the side of the heat box...maybe this is it?).

The system has no problem keeping the house at a specific temperature all day, I believe this is because the furnace only has to run for short (3-5 min) bursts to keep the temp at 20C.

The house does not have air return issues, problems with blocked vents, dirty filters or any of the common issues that seem to be listed as the reason one of these switches would trip. I am almost positive there is something wrong with the device that controls how long the burner can run for.

Any suggestions to help us point the techs to the problem would help...or if anyone could confirm my suspicion that a burner should probably not be running for almost 15 minutes straight would be helpful.

Thanks In Advance.
D In Mtl
 
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#2 ·
or if anyone could confirm my suspicion that a burner should probably not be running for almost 15 minutes straight would be helpful.
Sorry, but you're not likely to get that.

I've kind of lost track of all your limit and sensor control tracing... but the operating thermostat upstairs is what should be controlling whether the burner is on or not... and that it should stay on until the desired temperature in the living areas is achieved.

After the temp is met, or if the heat exchanger/duct temp gets too high the burner should cut off and the blower will stay on until the heat exchanger has cooled down. This shouldn't take 3 hours... but a 15 minute cycle is not unreasonable.

hth
 
#3 ·
but the operating thermostat upstairs is what should be controlling whether the burner is on or not
Thanks for your reply - with reference to the line above, is it really the Tstat that controls how long the Burner runs? I thought that was controlled by a temp switch in the furnace...that allows the burner to run until the heat exchanger reaches a specific temp....

I understand that the Tstat will keep the furnace running (overall...ie: burner/fanc cycles) until the desired temp is reached...

D
 
#6 ·
Old school and new school will both say that the thermostat commands the call for heat and the length of time the burners run. All the other controls that can interceed on the length of time a burner runs only do so when there is a problem with the furnace system.
Your pictured system is a split fan/limit control. With a thermostat call for heat that particular control measures the temp in the house air part of the heat exchanger. It turns on the fan at one temp, turns off the fan at another temp and only turns off the burners if the furnace is overheating.

PS in my area, the company you mentioned makes every other tech crew look like pro's.
 
#7 · (Edited)
The control your friend is having to reset is located where a roll out switch would usually be positioned. This would shut off the burners if the heat within the heatexchanger isn't all going up the chimney like it suposed to but a portion of it is instead floating or escaping out where the roll out switch is located. Causes can be a faulty roll out switch that is tripping at too low a temp but just about every other cause will put CO into the house. Cracked exchanger/ restricted exchanger/ over gassed/ anything creating low air pressure in the furnace area (competing air equipement like a clothes dryer) etc.
 
#8 ·
First if the puictured control shuts your furnace burners off you have a problem.Your burners should never shut off via this control unless you have a problem.This problem is usually an air flow problem ,meaning you are not moving enough air over the heat exchanger.
If your furnace has been running off your limit for any length of time I can tell you what will happen.You will be without heat on the coldest day of the year and it will bve at night when most companies charge overtime,I know I did.
Now your friends furnace has a differant problem and is potentially much more dangerous.Flame roll out limits only open when the burner box overheats or the flames actually roll out from the burners.This type of problem is usually a plugged heat exchanger or worse a cracked heat exchanger.
Both of these problems put carbon dioxide into the house instead of out the chimney (it also could be a blockage in the chimney or flue.
Now I have had more than 35 years in the business and at least two others with decades of experience have told you the only control that should control your burn times is the thermostat.All the other ciontrols are safties.Got it,I don't care what you think we are right and you are wrong.I can't be more plain than that.
Both of the furnaces you put in this thread are safty controls doing their job.
It is beyond yopur skill level to screw around at this [point because they are safty controls working to save your behind.
Please get help we cannot keep you safe from our keybords.Only you can protect yourselves at this point because Smoky the Bear has retired.
 
#9 ·
thanks

I do appreciate all the replies - even the blunt ones - nothing beats "directness".

As mentioned, we (my friend and I) have NO INTENTION of trying to fix this ourselves, we are simply trying to provide the service people with some sort of direction due to the fact that this is an intermittent problem and has already cost him in excess of $500 with no resolution. He insists on calling Sears because it is a Sears unit and he feels that they should know best (he is an older gentleman and a bit hard headed).

My assumption was that since my unit worked as described and his seems to run the burner for so much longer, there was some sort of problem with a part that controls the burner in the furnace. Obviously that is not the case. The thermostat has been changed (twice, I think) and the service guy has been there at least 7 times, I believe he has not been charged for all these visits (some were two days in a row) but most of the time, its $120 just to show up.

My next course of action will be to convince him to call a different service company to come out and look at this, however the trend here seems to be that techs only like to work on the types of equipment their firm installs and stocks parts for.

Thanks again for your replies!
D
 
#11 ·
what's the middle "ON fan" pin set at on your Honeywell limit control there? once the stat calls and the burner heas up the Heat-X and that limit turns to bring the fan ON the HI-LIMIT should never be reached and the burner should continue till the stat satisfies.the supply fan is the last thing to come on with the start of a heating call and the last thing to go off...you shouldn'y be cycling the main burner during a heating cycle...your over heating that Heat-X check the filter and those rounds on that squirrel cage F the supply fan...the only way your tripping into hi limit is a lack of air across the Heat-X you and your neighbor...
 
#12 ·
Now you have me thinking my system is not working correctly. To confirm, the furnace is clean, filter is changed regularly. But my heating cycle goes like this when I raise the temp 3C:

Chimney Fan starts> Blower starts (yes the fan blower starts before the burner)> Igniter clicks a few times in fast succession and lights a little pilot > Main burner lights from the pilot and runs for between 3 and 4 minutes, then shuts off > Fan continues running > about 3 or 4 minutes later the Burner lights again, runs for 3 to 4 minutes then shuts...cycle continues until I reach the Tstat temp.

When the burner shuts off, I now realize it seems to be triggered by the Thermostat because I hear it click (its a honeywell programmable)

The furnace is cleaned and tuned annually. it is a Heil, about 15 years old.

Do I have an issue with this?

D In mtl?
 
#16 ·
Sorry dmanmtl.

The R & W jumper approach was to eliminate the thermostat..BUT
I'd forgot that you mentioned on an earlier post that you watched your burners turn off when they reached a certain temp on your fan/limit control. If the burners turned off because the fan/limit control dial turned clockwise past the third temp nib, that indicates an overheating furnace or a fan/limit control that is faulty.
I take it you don't have an led light that is blinking a high limit code on a mother board in your furnace. Actually the only thing I know about your furnace is the older style fan/limit control you posted.
Age/ brand & model would help.
 
#17 ·
My machine is a Heil Ultimate 2 installed in about 1993/1994.

When I say that the burner is being turned off by the high limit switch, I may be mistaken, now that I am more cognoscente of this, I believe the tstat is actually click faintly right as the burner ****s off...I did not realize the tstat actually controlled the burner, I thought it just said "HEAT" and the furnace took over.

At this point I am really curious to understand how all this works. With regards to airflow, I am 99.9% sure I don;t have an issue as I had a new heatpump installed 2 years ago and they deal all the fancy tests then.
 
#18 · (Edited)
If you watch the fan/limit control that you posted all the way through a heating cycle you will know what's going on and whether it's the thermostat or the fan/limit control that's shutting off the burners.
Turn the thermostat at least 5 degrees above the present room temp.
The fan limit dial will turn clockwise as the furnace heats up. The small indentation that lines up with the middle of the control gives you the supposed temp on the dial, of the house air stream in the heat exchanger. On your picture it's exactly where the first nib is.
The dial normally passes by this indentation with the first nib and when it gets to the second one, the fan will turn on. If the dial then continues to rotate up to the third nib, then the burners will shut off as the limit cuts off the electricity to the gas valve. You will probably hear the click of this control when it limits out. This cutting off of electricity to the gas valve circuit may be why your thermostat makes a clicking sound just as the burners turn off.
This will mean that the furnace has just overheated or that the fan/limit control is no longer properly calibrated to the real temp.

Poke a small hole in the furnace plenum, just beyond the line of sight of the heat exchanger, stick a thermometer in the hole and read it just as the furnace limits off, now subtract the room temp from that thermometer plenum reading and compare it to the recommended temp rise that is listed on your furnace plate.
Let us know what your measured temp rise is compared to what is recommended.

.
 
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