# Pilot is ON, Furnace won't fire



## Kewilliam (Apr 4, 2012)

Hi All..

So I have an older model natural gas Lennox Furnace which stopped working. The pilot light would not stay lit so I replaced the thermocouple. Now the pilot light stays lit but the furnace won't fire up/light. I switched the knob from pilot to ON, the gas line and furnace switch are on and the heat on the thermostat is turned up. The blower just blows cold air and I smell a faint trace of gas. Am I missing something? Or could it be a faulty gas valve? Any help is appreciated.

Kevin


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## how (Feb 26, 2011)

Age & model of furnace? First four digets of serial no.?

I am assuming that you have an older style lennox with a preheater in the fan/limit control that is now turning on the fan with a call for heat?
You shouldn't be smelling gas except for just a short while during the pilot relighting process.

Check if you are getting 24V to the gas valve? 
Turn off the thermostat and jumper the thermostat connections on the gas valve (often marked TH as opposed to those marked TR that you would meter for the 24V test. If the valve doesn't fire...Change it.
Let us know what you find.


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## Kewilliam (Apr 4, 2012)

First four numbers of the serial are 4691
Model #GS18Q4/5-140-8 from 1991 (I think)

I'm not exactly sure where the gas valve is and I don't know how to check to see if I'm getting 24V to the valve. I'm a newbie to all this so if you could "dummy" down your comments/help that would be appreciated!

Thanks!


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## how (Feb 26, 2011)

Your furnace is younger than I imagined. 

Furnaces will often turn on the fan *and* prevent the main gas valve from operating from blown internal fuses, tripping safety limits, roll out switches or pressure switch controls.

The gas valve is the same control that the thermocouple was attached to.

Considering your last post, a few photos of your furnace with the doors off would help us direct you.


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## Kewilliam (Apr 4, 2012)

*Pics*

Here are some pics..

http://kfurnace.shutterfly.com/pictures#n_5


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## 43harley45 (Apr 5, 2012)

_I'm not a gas man but the gas line from the valve to the burner shouldn't have a kink in it. I'd replace that first before trying any more trouble shooting. Can probably get a piece of steel line at a hardware_ store or Canadian (now Chinese) Tire. If it works check joints for leakage with soapy water. If you don't have a small pipe bender and/or flaring tool.....go to a local garage and ask them to duplicate the pipe...might do it for free or a couple of bucks.


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## ben's plumbing (Oct 6, 2011)

Kewilliam said:


> Here are some pics..
> 
> http://kfurnace.shutterfly.com/pictures#n_5


 this is just a standard gas valve..valve is set to pilot in picture...needs turned to on position...also as mentioned pilot tube is kinked...need to replace or repair this.....may need to trace out 24 volt circuit to see if a safty switch is open....:yes:


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## how (Feb 26, 2011)

Whoaa!
That's one nasty pilot tube kink. In my neck of the woods, thats what gas supply tech's do to pilot tubes when they think a red tag is likely to be ignored. It looks like you have enough spare tube to cut out the kink and reposition it with a new pilot tube fitting for the gas valve.

Hope that the real problem was just the gas valve position even though you said it was turned back to "ON" (unlike what the pic's show).

Your lennox pic looks like a horizontal so you might actually have an older style fan/limit control with that preheater that is turning on the fan 30 - 80 seconds after gas valve gets 24V. If the gas valve gets left in the pilot position, the fan will still come on even if the heat doesn't.


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## GarryE (Apr 3, 2012)

43harley45 said:


> _I'm not a gas man but the gas line from the valve to the burner shouldn't have a kink in it. I'd replace that first before trying any more trouble shooting. Can probably get a piece of steel line at a hardware_ store or Canadian (now Chinese) Tire. If it works check joints for leakage with soapy water. If you don't have a small pipe bender and/or flaring tool.....go to a local garage and ask them to duplicate the pipe...might do it for free or a couple of bucks.


 NO, NO, NO, NO!:no: Use 1/4" aluminum tubing with an alloy brass compression fitting available from any heating contractor. Steel does not belong in any gas line of this type, it is reactive and will corrode. Actually, cut that kink out leaving about 1/2" on either side of the kink, take it to any good hardware store or auto parts store and get a 1/4" brass compression union of the same size and repair the line being sure to seat the tubing all the way into the fitting. Don't overtighten it, just firmly tight using two wrenches and check with a soapy water solution and be sure to rinse and dry it afterwards. Yes, be sure the gas valve knob is set to ON.


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## REP (Jul 24, 2011)

Once you replace that 1/4" alum pilot tube,Then you can turn the gas cock to the on position.That is that little valve lower left in the picture with the red knob.


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## beenthere (Oct 11, 2008)

Wonder if someone tried to clean the pilot. Ended up losing the pilot orifice, and then kinked the pilot line to try and control the pilot flame.


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## how (Feb 26, 2011)

One of my first service jobs was a simple tune up to a furnace that was apparently doing well. After I finished my servicing, the furnace started over heating and I can't figure out what I did wrong. The op came downstairs after I spent 30 minutes longer than I stated it would take. He just reached over my shoulder and turned the knob on the gas valve half way from "on" to pilot.

Said he'd had it adjusted that way for years ever since he eliminated much of the ducting. That first day was a rude awakening to just how far the ideal & the actual might be in the field.


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## Kewilliam (Apr 4, 2012)

OK I'll work on replacing the alum pilot tube. One question..would the kink in the tube prevent the whole system from firing up?

Still trying to figure out why the pilot light won't fire it up?

Thanks for all the help!

Kevin


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## 43harley45 (Apr 5, 2012)

REP said:


> Once you replace that 1/4" alum pilot tube,Then you can turn the gas cock to the on position.That is that little valve lower left in the picture with the red knob.


Kewilliam ....... You forgot to read this part.


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## Kewilliam (Apr 4, 2012)

The red valve in the lower left of the picture is the main gas valve. The pilot light won't light without that turned on. My pilot WILL light and stay lit - it just won't catch fire to the entire furnace (not sure what exactly to call this). Make sense?


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## how (Feb 26, 2011)

If your pilot does light and stays lit on it's own then can you test if the gas valve is getting 24V ac when there is a call for heat?


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## Kewilliam (Apr 4, 2012)

I can - if you tell me EXACTLY how to do it.
Pretend you are speaking to a small child though.:laughing:


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## how (Feb 26, 2011)

The picture shows your gas cock off and the gas valve lever in the pilot position. Obviously both of these need to be on.
If you are able to light the pilot and have it stay on with the red button allowed to rise back up on it's own.... then test if the gas valve is getting 24V ac *when there is a call for heat*.


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## how (Feb 26, 2011)

Sorry for the duplicate posts.
You have not yet confirmed that the control on your gas valve that is pointing to "pilot" in the picture has now been turned to the "on" position>

Take your electrical meter, turn it on & set it to measure Volts ac above 24V, unplug the white & orange wires from the gas valve (remember where they were connected so you can plug them back in the right spot afterwards) and put your two meter leads on the ends of those wires & turn up the thermostat.
Let us know what you read.


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## GarryE (Apr 3, 2012)

This has been going on way too long. You must have a volt-ohm meter to test with. If you have a standing pilot, lets go from there. Turn off the power to the furnace and test the valve circuit. Picture 3 shows the gas valve and the wires leaving it going back to the control board. The orange wire goes from the valve to a pair of terminals on a black component mounted on the side of the burner compartment. This is a secondary limit switch, it must be closed. Take both wires off, set your meter to ohms and test across the terminals. Your meter should read at or very near zero Ohms, like when you touch your test leads together. If you get infinity or no reading the limit is open. If the limit has a little red button in the middle, press it. If it clicks, it was tripped and you have just reset it. Reconnect the wires and try a call for heat to the furnace. You may need an assistant for this to turn up the stat while you watch it light. If it lights, watch the entire process through the startup and running of the blower. It may work but the problem has not been solved. The issue is why did it trip? I don't remember if this unit has a draft inducer or if it is natural draft. Either way the main flame should light smoothly and evenly and have little to no blowback. Be especially watchful of the flame when the blower starts there should be no deflection of the flame, no lifting off of a burner. These are ribbon burners and the flame should be even and steady the full length of the burner top and a nice blue color with bright light blue centers, no or very very little yellow at the top. If that does not do it turn off the call for heat, switch your meter to read in the 24 volt range, pull the leads off of the gas valve and hook your test least to the wires, bring on a call for heat and read the volts. It should read 24 volts plus or minus about 10%. If you have 24 volts coming in and it doesn't fire the gas valve is at fault. Now, before condeming the gas valve the incoming gas pressure should be tested with a manometer to be not more than 8" w.c. for natural gas and 14"w.c for LP. Values higher than that the valve will lock out. The valve will still likely be shot but that gas pressure will have to be corrected before installing the new valve or you will have the same problem and yes, regulators do fail. Then the manifold gas pressure will also have to be set to the correct value after installation of a new valve. That must be set to whatever is specified on the data plate on the unit itself. Back to flames. If you have any major movement of the flame in any of the chambers the heat exchanger has failed and at the age of that unit it is time for replacement. Do this stuff up to the point that you are comfortable and then stop. If you feel like you are over your head you probably are. Don't just blunder on you are messing with a bomb and your life and house is on the line. If you need help call a company that has NATE or APHH certified service technicians. Yeah, they usually cost more but they know what they are doing and have taken the time and expense to get the certification to prove they have the training and experience to do the job right.


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## carmon (May 8, 2010)

well said...:thumbsup:


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