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Old 01-19-2012, 04:24 AM   #1
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Furnace burners will shut off after 10 mins


I have a Lennox G16Q75 with an electric spark ignitor where it will initially start and all burners engage for approx 10 minutes or so but then the burners shut down. The ignitor will light the pilot but it will go out in less than 5 seconds. The burners will not engage and it just keeps repeating the spark, pilot lights for less than 5 secs but no burner light. This repeats again and again but the burners do not engage.

The pilot does appear to dance abit for 3 seconds before going out. When the burners do light briefly, the burners do not dance at all.

The board is an older Honeywell S8610U which keeps repeating the sparking of the pilot which does light but only for a few seconds but the burners fail to in turn light. Pilot goes off gets sparked again. Repeat.

I do note that a wire is not connected but hanging there from the dual rod sensor and an open pin on position 6 for the 24V. I believe that this was not supposed to be connected as there are jumpers at the dual rod posts

I have replaced the flame sensor with no effect. Replaced the dirty filter. Vacuumed

If I turn the thermostat down or off for a short period and then turn it up, the burners will engage but will shut down after 10 minutes. I seem to be able to only get the temp upto 12 degrees C before it shuts the burners

Any suggestions please. Thanks in advance


Last edited by Liberator; 01-19-2012 at 04:32 AM.
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Old 01-19-2012, 09:09 AM   #2
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Furnace burners will shut off after 10 mins


do you have an induced draft motor with a round 4" silver pressure switch right above the burner with 1/4" tubes off it.do you have AC ..if so check the A coil for blockage sounds like your going OFF on hi limit knocking the burner down you need to eyeball the coil area right above the heat exchanger

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Old 01-19-2012, 12:14 PM   #3
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Furnace burners will shut off after 10 mins


Thanks for the post. I do not have AC. My up flow Lennox G16Q75 is built around the mid 80's. I am not sure how to access the induction motor area above the heat exchanger but will look again

The induction motor kicks in right away when I call for heat

Thanks
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Old 01-19-2012, 12:48 PM   #4
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Furnace burners will shut off after 10 mins


the motors not hidden was just wondering if you had one sounds like you don't i'm not talking about the furnace supply fan.check the squirrel cage on the supply fan make sure it is clean on the fins...if this just happened with the burner dropping out doubt it's the fins but check them anyway
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Old 01-19-2012, 04:02 PM   #5
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Furnace burners will shut off after 10 mins


The S86 may be failing due to age. That old furnace has a 95% chance of a cracked heat exchanger and parts are going obsolete. I sold hundreds of them and if we sold a hundred about 5 are still running. Not worth spending a lot of $$ on IMO.
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Old 01-19-2012, 04:11 PM   #6
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Furnace burners will shut off after 10 mins


Need honey- well ignition spark control board but very expensive. Note may need check ground too. If you have ground problem that board will never work or go out as soon you replace it.
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Old 01-19-2012, 05:10 PM   #7
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Furnace burners will shut off after 10 mins


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The S86 may be failing due to age. That old furnace has a 95% chance of a cracked heat exchanger and parts are going obsolete. I sold hundreds of them and if we sold a hundred about 5 are still running. Not worth spending a lot of $$ on IMO.
Ditto. Before you put one cent towards parts get the heat exchanger inspected. Not many of them lasted 20 years so the chances of yours being good at nearly 30 years old are very,very slim.
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Old 01-19-2012, 08:52 PM   #8
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Furnace burners will shut off after 10 mins


Thanks for the info.

The burners when they are lit look pretty good... blue with no dancing. What are the signs of a bad exchange besides dancing flames? Color?

How do I check for cracks and holes?

I think I saw a new Honeywell S8610U listed for $290

Is there any other DIY tests I may do before breaking the bank on a new furnace?

Thanks everybody
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Old 01-19-2012, 10:24 PM   #9
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Furnace burners will shut off after 10 mins


The watching of the burner flames to change when the blower fan comes on is a useful test. You can make it more effective by turning the control on the gas valve towards the pilot. Turn it until the main burner flames become small and soft which will make them move more visably to a pressure change that might blow through a cracked exchanger when the fan starts up.
Visually inspect through any access plates in the plenum above the furnace (made by duct cleaners) or a plenum heat register or cut your own hole that you can then
cover later. Turn off the power and do the same inspection with a bright flashlight and mirror in the burner chambers.
There are at least 25 different ways of checking exchangers. Every tech seems to use a different collection of methods. I prefer the visual with custom high intensity lights & mirrors, a flexible cam scope and an expensive yearly calibrated CO detecter but the reason there are so many methods is that no one technique trumps all the others.

Another thing to check before you consider expensive repairs is that your pilot orifice may be dirty and not keeping the pilot flame strong enough to keep engaging the pilot rectifyer. A weaker pilot flame will also be drawn away from the rectifyer by the updraft of heat through the exchanger.

Last edited by how; 01-19-2012 at 10:48 PM.
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Old 01-20-2012, 02:30 AM   #10
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Furnace burners will shut off after 10 mins


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Another thing to check before you consider expensive repairs is that your pilot orifice may be dirty and not keeping the pilot flame strong enough to keep engaging the pilot rectifyer. A weaker pilot flame will also be drawn away from the rectifyer by the updraft of heat through the exchanger.
How, thanks for this tip. I noticed that the pilot stays lit when the burners are on. Is the pilot supposed to stay lit whenever the burners are on with an intermittent spark ignitor?

Would I just use a pipe cleaner or should I consider replacing the pilot? The pilot when lit, does dance quite abit

Thanks
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Old 01-20-2012, 10:23 AM   #11
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Furnace burners will shut off after 10 mins


A weak pilot (if dirt related) can be from a dirty orifice or dirt in front of the orifice. Cleaning or replacing an orifice involves taking apart the pilot assy and is more than many might consider a DIY project but it might just be loose debris in front of the orifice which can be easier to deal with.
Take a large screwdriver and rap the metal base of the pilot assy when the pilot is running. Use as much force as you would to knock on someones door. Don't hit anything that might be damaged like the aluminum pilot tube/ igniter or the pilot sensor. The hope is that the vibration combined with the gas pressure might move the debris out of the way. If the pilot becomes stronger and harder...stop and see if that has sorted out your problem.

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