Gas and Oil Home Heating Furnaces - gas furnace work for 8 minutes and then stop

I have a 18 years old gas furnace model# 396GAW024050. When I turn the heat on, everything work fine until 8 minutes later the blower stop running. I thought the fan motor is dead so I check it by setting the thermostat switch from "Auto" to "On" and blower work just fine. I also notice the polite light stay on constantly and then about 30 or 40 seconds later the gas burners start to ignite and then shut off by itself. The weird thing is that if I turn off the heat and wait for everything to cool down then everything work fine again but only lasted for 8 minutes. Any help or advice would be appreciated.

Are you saing that when the burners runs, the blower kicks in and runs about 8 min and the blower itself stops, and burners stays on till it goes into high limit?

or

You saying that the bruners will turn off in about 8 min, and then the blower shuts down a few min later?

What happens if you leave the blower ON at the t-stat all time?

The burners and the blower stop at the same time after 8 minutes in operation. If I left the blower on at all time, the burners will not ignite. The only thing that stay on is the pilot light and the blower. However, if I increase the temperature at the thermolstat the burners ignite and then shutdown immediately because the blower didn't kick in.

If I leave the blower on all the time, the burners will not ignite.

That is odd. :confused:
What other work has been done to the furnace?

What type of thermostat do you have?

a faulty high limit switch would make the system think the blower is bad and shut down the burner.

When you say "high limit switch" are you referring to Hi Limit Fuse Safety Link or Limit Safety Switch? I just want to make sure before I order the part.

Here is a link of what I'm talking about:
http://bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?

When you say "high limit switch" are you referring to Hi Limit Fuse Safety Link or Limit Safety Switch?
The fuse link should be in series such that the burner does not fire at all if the fuse is melted. So it must be okay.

I wouldn't rush to buy the part yet. What does your high limit switch look like?
If it is surface mounted, you can readily watch and test its operation.

diagnosing blower problems (http://toad.net/~jsmeenen/blower.html)

(Your link is incomplete, so I can't see what part you have in mind.
Perhaps turn off automatic link parsing when you post.)

Sorry about the link. Here is link again hope it works.

High limit switch (http://bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=LIM1180&Category_Code=)

Ok, that's odd if the burners don't start at all if the fan is switched to "ON".

Are you able to trace back from the gas valve .. safety... limit.. board to see where you have a break in power?

The high limit only should shut off the burners, not the blower.. So I am thinking it's something else as well.

The high limit only should shut off the burners, not the blower.. So I am thinking it's something else as well.

Do you think the relay on the circuit board have anything to do with this?

Do you think the relay on the circuit board have anything to do with this?

maybe..

I'd check to make sure you are getting power at W on the board all times.. If yes, then trace from the board going on up to the gas valve. If you don't get power coming off the board for the gas vavle, then it maybe the board.

After shutting down the furnace for 48 hours, I decided to test it again but this time I turn the blower on first by setting the switch from "Auto" to "ON" at the thermolstat and here is what happen:

The pilot light ignited and then about one minute later the burners start to ignite. About four minutes later, the burners shut off and then about 30 seconds later it ignited again then shut off imediately. The process repeated about 3 times and then the burners completely dead. The pilot light and the blower still running constantly without any problem. If I replace the high limit switch, will this solve the problem?

The hi-lim switch is in series with the thermostat.

When the switch opens, it acts as if the thermostat quit calling for heat.

If you have a voltmeter, read the voltage across the switch.
0 is good. 24V means the switch is bad.

If you don't have a voltmeter and if you are positive that the blower is blowing and there is no overheating (such as during the first 5 minutes of operation), you can pull the leads from this vital safety device and jumper them while you are present and you personally are acting as the safety device to shut the system down if something goes wrong.

If the operational problem persists, you could have loose wiring, a bad thermostat, or something else. Restore the safety switch and move on to the next component.

I have tested the limit switch and I got 0 reading on a voltmeter so I'm assuming it is good. While testing the limit switch, I also notice the high limit fuse safety link looks a little bit dark but I don't know how to test it to see if it good or bad. My question is if the high limit fuse safety link is dead, will the furnace still operate like normal but shut down after 8 minutes in operation?

In case if you don't know what the hi limit fuse safety link on my furnace looks like, see the link below:

limit fuse pic (http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=FUS286&Category_Code=)

Are you reading O volts on both side of the high limit? Or from one side to ground, and other side to ground?

Is the burner going at the time of the reading?

Are you reading 0 volts on both side of the high limit? Or from one side to ground, and other side to ground?

From one side to the other. Each side back to the transformer should have the same reading. (0V to one side of the xfmr and 24V to the other side of the xfmr.)




[The transformer should be an ungrounded separately-derived system.]

If you isolate the high limit fuse link, it should show 0 ohms if good.

Just to rule out the obvious, be sure the air filters are clean and that you have good air flow from the supply registers in the house.

Ken

I just replace a brand new limit switch today and here is what happen when I turn the heat on:

1) The burners ignite for about one minute
2) The blower start running
3) About 5 minutes later, the burners shut off but the fan still running
4) The burners keep turn on and off for the next 5 minutes until the blower completely shut off for good
5) Now that the blower is off, the burners still continue to turn on and off. Each time the burners ignited, it last for about 10 seconds.
6) If I left this on for an hour, I can feel heat release from the intake vent
7) If I turn off the heat, everything is back to normal except the house is freezingggg.

Now my next plan is to replace the old White-Rodgers thermostat, if that doesn't work I will replace the circuit board.
Could this problem cause by a cracked heat exchangers? By the way, I already did a quick check on the heat exchangers for crack
by putting a candle inside the heat exchangers opening. Next I turn the blower on and the candle didn't went out. Does this mean the heat
exchangers is not crack or what? Should I replace the circuit board first or the thermostat?
:confused:

2) The blower start running
3) About 5 minutes later, the burners shut off but the fan still running
4) The burners keep turn on and off for the next 5 minutes until the blower completely shut off for good


Can you make the blower come on? Like turn the switch to ON?
Is your fan switch a mechanical one that you can watch turning?

Can you make the blower come on? Like turn the switch to ON?
Is your fan switch a mechanical one that you can watch turning?



Yes to both of the questions above, I can turn the blower on by using the switch at the thermostat. If I switch to "ON" at the thermostat, the blower would run all day long without any problem.

If I switch to "ON" at the thermostat, the blower would run all day long without any problem.

I'm talking about the fan control on the furnace that has a bi-metal spring and rotates with change in firebox temperature.

I thought that is what you replaced.

What exactly did you replace?

I replace the safety limit switch that sit between the firebox. Here is a picture of that switch.

limit switch pic (http://bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=LIM1180&Category_Code=)

Do you have a fan switch somewhat like this (http://toad.net/~jsmeenen/images/fanlimit.jpg)?

I just replace a brand new limit switch today
I missed what made you decide to replace it. Didn't it pass all the tests?




1) The burners ignite for about one minute
2) The blower starts running
This sounds good. The blower cut-in switch is working.




3) About 5 minutes later, the burners shut off but the fan still runningCould be that the thermostat is not calling for heat (but I suspect that it is), or your flame sensor doesn't detect fire, or your overlimit switch opened.




4) The burners keep turn on and off for the next 5 minutes until the blower completely shut off for goodThis is indicative of hitting overlimit or of losing flame sense.



5) Now that the blower is off, the burners still continue to turn on and off. Each time the burners ignited, it last for about 10 seconds.
Sounds like loss of flame sense.



Now my next plan is to replace the old White-Rodgers thermostat,
Nothing about what you described really suggests any problem with the thermostat.




if that doesn't work
That's my guess.

I will replace the circuit board.
Not really indicated (but not ruled out) as the problem at this point.



Could this problem cause by a cracked heat exchangers?
Unlikely.


Should I replace the circuit board first or the thermostat?
Neither.
You need to determine what control signal is lost.
If it's the flame sensor, clean or replace that.




Does your control board have a diagnostic LED that flashes a count for the number of the error?

No, the fan doesn't have that limit switch on it. It had five wires attached to it red, blue, yellow, black,and white. Wire black, white, and red connect to the circuit board. The control board have no LED at all.

So what does it have for a flame sensor?

So what does it have for a flame sensor?


I look all over and I don't think it had a flame sensor. I thought the safety limit switch that I replace earlier today is part of the flame sensor because it sit between the two heat exchangers to detect heat. Correct me if I wrong on this.

Hey bolide

Thanks so much for helping me out this far and everyone here as well eventhough the problem is not fixed yet. :D

I look all over and I don't think it had a flame sensor.Possible. I am lost as to what you do or don't have.
Does it have a control board? If so, it probably has a flame sensor (can't think of one that doesn't).



I thought the safety limit switch that I replace earlier today is part of the flame sensorNope.




Flame sensor detects fire (hundreds of degrees) not just high temp (in your case, I think 180 degrees).



If your pilot stays lit while the burner is firing, and you don't have a flame sensor, then that's not the problem.




I also notice the pilot light stays on constantly
Does your furnace have a standing pilot?

So what does it have for a flame sensor?

I look all over and I don't think it had a flame sensor. I thought the safety limit switch that I replace earlier today is part of the flame sensor because it sit between the two heat exchangers to detect heat. Correct me if I wrong on this.

Please disregard my previous post because after I check the control board wiring diagram, the furnace does have a flame sensing unit. According to the control board wiring diagram description, the flame sensor is near the safety pilot which located next to the burners.

Does your furnace have a standing pilot?
Yes it does.

I replaced a new high safety limit fuse link for my furnace today and the old problem still exist. I thought the old fuse link might be burnout because it look really dark when compare to a new one. The problem is that the burners keep shutting off and on in a cycle exactly after 8 minutes of operation. Each cycle last about 15 seconds. I am suspecting that either the gas valve, circuit board, or the flame sensor is bad. Can some one out there help me figure out which one of these components is likely the cause of the problem.
I could buy these three components and test it one by one but the parts is too expensive to do this experiment and I also can not return it once installed.

I replaced a new high safety limit fuse link for my furnace today and the old problem still exist. I thought the old fuse link might be burnout because it look really dark when compare to a new one.
No, if it were melted the burners would not work at all. I told you this back in message number 7. I guess you didn't believe me.



The problem is that the burners keep shutting off and on in a cycle exactly after 8 minutes of operation.
Is the fan running?




I am suspecting that either the gas valve,
Why?

circuit board,
Why? What make and model?

or the flame sensor is bad.
Why?



Can some one out there help me figure out which one of these components is likely the cause of the problem.
Maybe none.



I could buy these three components and test it one by one but the parts is too expensive to do this experiment and I also can not return it once installed.
So it makes more sense to figure out the actual problem first.


Previously you posted:
1) The burners ignite for about one minute
2) The blower start running
3) About 5 minutes later, the burners shut off but the fan still running
4) The burners keep turn on and off for the next 5 minutes until the blower completely shut off for good
5) Now that the blower is off, the burners still continue to turn on and off. Each time the burners ignited, it last for about 10 seconds.
6) If I left this on for an hour, I can feel heat release from the intake vent
7) If I turn off the heat, everything is back to normal except the house is freezingggg.

Obviously this is incomplete and perhaps inaccurate.

You leave out too many important details like the standing pilot, the thermostat setting, and the ambient temperature.

Frankly, I still don't know what your furnace has or doesn't have or what works or doesn't work.

So if you want help, you need to answer questions.

Most recently you mentioned that your burner fired for 8 minutes. But you didn't say a word about whether the blower was running. So sure your burner should shut off it the blower didn't come on. You can't leave out important details.

O.K. let me start this over again.

Here is what I have:

Natural Gas Furnace

Make : Payne
Model: 396GAW024050
Airflow: Downflow
Ignition Type: Electronic spark
Polit: Automatic intermittent ignition
Last maintainance date: 2002 by professional
Filter condition: brand new

Thermostat

Make: White-Rodgers
Model: LR27935
Heating: .15 to 1.0 AMPS
Cooling: 1.5 AMPS Max.
Max 30 Volts A.C.

There are 3 switches on the thermostat:

Switch #1 Lable "FAN" and it had 2 options "Auto" and "ON"
Switch #2 Lable "SYSTEM" and it had 3 options "Cooling" , "OFF" , "Heating"
Switch #3 no lable but it is use for controlling the tempurature, slide up to increase temp. and slide down to decrease temp.


Furnace problem as of right now

1) I set the three switches on the thermostat to the following setting:
FAN = "Auto", SYSTEM = "Heating", Set the tempurature to 70*F

2) I ran quickly to the furnace to record what happen.

3) I hear a few clicks from the spark and then the pilot light up.

4) 30 seconds later the two burners ignited.

5) 1 minute later the blower turn on.

So far everything work fine at this point. Now here is the problem

6) Exactly 8 minutes later the two burners shut off but the blower keep on running.

7) About 15 seconds later, the two burners light up again for about 15 seconds and then shut off. This process repeated in a cycle.

I left the furnace run for about an hour to see what happen next.

8) While the two burners keep doing the on and off trick, the blower continue to run for half an hour and then boom the blower shut off.

9) I looked at the pilot light and it still on and the two burners still doing its on and off trick again.

10) Now the blower can not kick in again because the two burners keep turn on and off in a 15 seconds cycle which caused the firebox to cool down.
As a result, the 180*F high safty limit switch that sit between the two firebox also get cool down causing it not to trigger the blower to run again.
However, if I switch the FAN to "ON" instead of "Auto" at the thermostat, the blower would running continuously while the two burners continues its on
and off.

11) To stop the burners from turning on and off, I can either lower the tempurature at the thermostat to something like 60*F or lower so that the thermostat will not call for heat again. The other option is to turn off the heating. I also notice the pilot light goes out when one of the action above is performed.

Here is a list of things that I did to fix the furnace but did not work:

1) Replace new air filter.
2) Look for any obstruction in the vent.
3) Check for loose wires in the circuit board and gas valve and anywhere that have wires going through.
4) Clean burners and heat sensor with small brush very gently.
5) Replace Hi Safety Limit Fuse Link and 180*F HI Safety Limit Switch with a brand new one.
6) Check the thermostat for any loose connection.
7) Check the two relays on the circuit board for any burnout by using my eye balls.
8) Use air compressor to blow all the dust in the blower.
9) Press the Manual-Reset Auxiliary Limit Switch on top of the furnace.
10) Check for crack in heat exchanger.
11) Check the 24 volt transformer locate near the circuit board with eye balls because I'm afraid to touch it. The transformer look very brown.

I hope this will clear up some of the confusion.

8) While the two burners keep doing the on and off trick, the blower continue to run for half an hour and then boom the blower shut off.
Boom?


10) Now the blower can not kick in again because the two burners keep turn on and off in a 15 seconds cycle which caused the firebox to cool down.

How long do the burners stay on?




As a result, the 180*F high safety limit switch that sit between the two firebox also get cool down causing it not to trigger the blower to run again.
This doesn't even make sense. I think you are just guessing.


However, I'll buy that the temperature is below the blower cut-off temperature.


However, if I switch the FAN to "ON" instead of "Auto" at the thermostat, the blower would running continuously while the two burners continues its on and off.

For an hour or more?


[/QUOTE]3) Check for loose wires in the circuit board and gas valve and anywhere that have wires going through.[/QUOTE]
The problem repeats too perfectly to be a loose wire.



4) Clean burners and heat sensor with small brush very gently.
What kind of heat sensor do you have?

What kind of control do you have?

Does it have any lights on it?

You have gone over a week without a functioniong furnance. I think it might be time to call a service guy. Pick his brain when he arrives to fix it.