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Natural Gas Lennox furnace problems


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forklord forklord is offline
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14-Apr-2011, 06:12 PM #1
Natural Gas Lennox furnace problems
Hey guys just checkin out this forum, lotsa helpful stuff here!! anyway, i have a lennox furnace which is prob 15 years old cuz thats how old the new house i bought is. The problem is that it tries to ignite and you can hear the igniter clicking and doesnt light. then it tries again. nothing. sometimes it lights first time and works normal, or the second try. but today its going all wacky. Now it lights, the ingiter keeps clicking, then turns off then it goes out, then tries to light again and fails, then lights again. and on its next cycle it may work fine. a few months back it was also banging/mini explosions due to gas build up maybe? but that hasnt happened since i cleaned it. weird stuff. Just FYI i am a journeyman electrician but furnaces are not my area of expertise....Thanks in advance!!

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14-Apr-2011, 06:26 PM #2
What sort of ignition does it have ? (hot surface? spark to pilot? etc.). Any control lights that blink?
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14-Apr-2011, 06:30 PM #3
a red led blinks once at a time and i cant find a diagnostics chart on it anywhere. im not sure what type of ignition it is. spark to pilot i think. theres a gto/spark plug type wire going from the board to underneath the burners.
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14-Apr-2011, 06:57 PM #4
Could be a number of things.

Not familiar with the Lennox but I'd look at the flame prover first of all. Get a bit of very fine sandpaper and rub the metal part that is in the flame. If that eases the problem I'd nevertheless get a new one and install it. They can also crack with age so look at the flame pattern.

Then there could be an imbalance in the pressure switch.

Check that all wiring connections are good and sound. It could, for instance, be that the signal for the gas valve to open is not sufficient to open completely, the ignition spark thus not meeting enough gas to ignite.

For better understanding, usually a call for heat will send a low voltage to the ignition module which in turn sends power to open the pilot part of the gas valve and, at the same time, high power to the ignitor to create spark. Once the pilot flame is established the signal goes back to the module which then turns off the ignitor and opens the main burner valve (and gets the burner going, of course). That'll keep burning as long as the pilot flame is lit, respectively call for heat (thru thermostat) remains.

So if it's not the flame prover you have a lot of ground to cover.
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15-Apr-2011, 05:46 AM #5
It sounds like you have a DSI (direct spark ignition). The ignitor is also the flame sense. Make sure that all the burner tubes and slots are good and clean. The ignitor is also the flame sense. If it is too far into the flame it will not sense. You want the electrode about 3/16 above the burner or in the middle of the flame. If the porcelain is cracked, as stated by buffoon, the burner may light but it will not flame sense. i would do all of the above and if you still have problems, start by replacing the ignitor, as it is the least expensive. next would be the DSI board.
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15-Apr-2011, 05:42 PM #6
Check the pilot to make sure it`s burning nice and blue, and extending out far enough to hit the sensor/thermocouple. If it`s burning yellow and looks like a bent finger instead of straight , and not keeping the thermocouple hot the gas valve wont stay on even if it does light. Around here we have a problem with little tiny spiders building an almost invisible web in the pilot tube, blocking airflow and causing it to burn rich/yellow. You can see light through the tube and it looks clean, but the web is blocking enough air to cause a problem. Causes problems with furnaces, water heaters, and propane/nat gas heaters of all types. Just a thought, something simple to check before you start tearing it apart and pulling hair.
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15-Apr-2011, 09:57 PM #7
Most manuals suggest clean the pilot regularly
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16-Apr-2011, 08:23 AM #8
This might help- I just replaced my furnace last summer then immediatly broke it by leaving the door open on the coldest night. The exhaust condense line froze and flooded 2 miniswitches controling the lighting schedule. The diag codes did corectly identify the problem, but it cost me $200 for a mechanic anyway because I didn't want to mess with a new item and screw it up.
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20-Apr-2011, 08:49 PM #9
Check the exhaust fan
I'm very familiar with the banging you heard. Couple this with a 15 year old burner and I'd say check for cracks in the burner and exhaust ducting. I've seen this banging blow apart the plenum and exhaust ducts usually at the seams. I'm not that familiar with Lennox, but most of these systems have a 115 volt exhaust fan that draws the right amount of air across the flame. A faulty fan or air entering from cracks could cause the symtoms you're discribing. I believe the flame sensor is looking for a blue flame...when it sees a yellow flame it immediatly shuts off the gas valve.

Hope this helps.
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