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Old 01-02-2012, 07:19 PM   #1
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Sears GAs furnace issue with burner running too long


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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Old 01-02-2012, 07:35 PM   #2
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Sears GAs furnace issue with burner running too long


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Originally Posted by dmanmtl View Post
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

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Old 01-02-2012, 07:43 PM   #3
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Sears GAs furnace issue with burner running too long


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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
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Old 01-02-2012, 07:55 PM   #4
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Sears GAs furnace issue with burner running too long


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Originally Posted by dmanmtl View Post
...is it really the Tstat that controls how long the Burner runs?
I'm kind of old school w/r/t systems so I'll allow that the newer furnaces will ALSO have other components and whatnot in the mix... but yeah, that thermostat is "Captain of the Ship" (so to speak). So if the house hasn't gotten to 20C yet that switch will remain closed on the control circuit until it does.

Quote:
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
Those are safety and limit controls (which are important in their own right) but they're not operating controls.
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Old 01-02-2012, 08:05 PM   #5
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Sears GAs furnace issue with burner running too long


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Originally Posted by TarheelTerp View Post
I'm kind of old school w/r/t systems so I'll allow that the newer furnaces will ALSO have other components and whatnot in the mix... but yeah, that thermostat is "Captain of the Ship" (so to speak). So if the house hasn't gotten to 20C yet that switch will remain closed on the control circuit until it does.

Those are safety and limit controls (which are important in their own right) but they're not operating controls.
Agreed with you that the tstat will continue to call for heat if 20C has not been reached but I am fairly certain that the burner run is controlled in the furnace itself - my unit has the device pictured below that I can actually see moving up to a certain temp - when it hits that temp, the burner goes off but the fan stays on - no matter how far the tstat tempt has to go to get to its target.
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Sears GAs furnace issue with burner running too long-montr-al-20120102-00098.jpg  
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Old 01-02-2012, 08:55 PM   #6
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Sears GAs furnace issue with burner running too long


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.
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Old 01-02-2012, 09:11 PM   #7
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Sears GAs furnace issue with burner running too long


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.

Last edited by how; 01-02-2012 at 09:13 PM.
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Old 01-02-2012, 09:46 PM   #8
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Sears GAs furnace issue with burner running too long


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.
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Old 01-02-2012, 10:30 PM   #9
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Sears GAs furnace issue with burner running too long


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
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Old 01-03-2012, 12:30 AM   #10
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Sears GAs furnace issue with burner running too long


If it helps any I believe that your friend's kenmore furnace is a Heil furnace rebranded for sears.
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Old 01-03-2012, 09:58 AM   #11
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Sears GAs furnace issue with burner running too long


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...
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Old 01-03-2012, 10:39 AM   #12
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Sears GAs furnace issue with burner running too long


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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...
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?
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Old 01-03-2012, 11:11 AM   #13
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Sears GAs furnace issue with burner running too long


Test this by bypassing the thermostat..

Jumper (R & W) on the main furnace board (Thermostat wire connection points) or on the thermostat base to bypass the thermostat. Do the furnace burners now stay on for as long as it is jumpered?

Last edited by how; 01-03-2012 at 11:17 AM.
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Old 01-03-2012, 11:28 AM   #14
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Sears GAs furnace issue with burner running too long


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Test this by bypassing the thermostat..

Jumper (R & W) on the main furnace board (Thermostat wire connection points) or on the thermostat base to bypass the thermostat. Do the furnace burners now stay on for as long as it is jumpered?
By R & W, do you mean red and white? or do R and W stand for something else?
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Old 01-03-2012, 04:13 PM   #15
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Sears GAs furnace issue with burner running too long


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Originally Posted by how View Post
Test this by bypassing the thermostat..

Jumper (R & W) on the main furnace board (Thermostat wire connection points) or on the thermostat base to bypass the thermostat. Do the furnace burners now stay on for as long as it is jumpered?
There is no need to run a jumper if the limit is hitting high cutoff. The problem is airflow IMO. Are their any dampers not fully open? Sure the blower wheel is clean?

The temp of the heat exchanger is to high and will cause issues if not addressed. I would ask the service tech to measure the static pressures of the supply and return after verifying the blower wheel is clean and there is no restrictions from the filter ( 3M ultra allergy for example ).

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