# Lennox furnace - no heat



## gregzoll (Dec 25, 2006)

Go look at the board, and post back with the code that it is flashing, then wait for someone to chirp in.


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## Greenlawnracing (Oct 27, 2009)

I should have posted that - the codes are flashing normal.

Is it possible that the gas is not getting ignited?


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## Greenlawnracing (Oct 27, 2009)

Update - there was an HVAC guy at work today who told me I could call him with any questions. I just got off the phone with him - he told me the igniter should be glowing red once the system starts up - but it was not. He said it’s a common failure item and to replace it tomorrow.

Only one thing....he said I could ohm it to make sure it was bad. He also said I could check the lead going into it to make sure it was sending out power.

Ohmed it - there is continuity. Shouldn't it give me no reading if it’s dead?

110 volts are coming out of the igniter power for a couple of seconds - means the board is good...correct?

Finally - I looked at the trouble codes and there is one for "burners failed to ignite".....shouldn't I have gotten that code?


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## Houston204 (Oct 18, 2009)

You want to read ohms not continuity. 
Do you see any visable cracks on the ignitor?


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## Greenlawnracing (Oct 27, 2009)

No cracks, but it has turned white.


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## Greenlawnracing (Oct 27, 2009)

Replaced the ignitor and thats not it. 

When I unplug the ignitor I can get a reading on the power source for a few seconds. Since the circuit is not complete, it shuts off and gives me the incomplete circuit trouble code.

I tried to stick the meter into the wire harness while the unit is running to see if I'm getting power - no reading. I thought perhaps I'm not making good contact, but the harness is pretty open and I'd have to assume I'm making good contact.


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## yuri (Nov 29, 2008)

Post the model and serial # of the furnace. I am a Lennox tech.


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## Greenlawnracing (Oct 27, 2009)

Model G40UH-368-090-02
SN 5801F 41160


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## yuri (Nov 29, 2008)

Where did you get the igniter? A Lennox dealer? Cozyparts.com ? The G40 uses a different igniter than the G50. They also work off a reduced voltage, 83 volts I believe. I'll post you a service manual for it. I got to go now.http://tech.lennoxintl.com/PDFs/0006j.pdf
Back tomorrow.


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## Greenlawnracing (Oct 27, 2009)

I got it from a local parts store - it is an 80 volt ignitor. Is that not correct?


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## Greenlawnracing (Oct 27, 2009)

I was told by a tech today that I need to find out if the pressure valve is good. He tried to describe how to short it our or suck on it, but I couldn't understand him. Can anybody elaborate?


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## yuri (Nov 29, 2008)

The pressure switch is NOT to be sucked on or it gets damaged. If it does not close the board will show an error code. Lets start in the BEGINNING. Does the thermostat start the furnace and does the ventor fan to the chimney run?


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## Greenlawnracing (Oct 27, 2009)

yuri said:


> The pressure switch is NOT to be sucked on or it gets damaged. If it does not close the board will show an error code. Lets start in the BEGINNING. Does the thermostat start the furnace and does the ventor fan to the chimney run?


The thermostat does start the furnace.

Ventor fan to the chimney - do you mean the inducer fan, or the one on the bottom of the unit?


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## yuri (Nov 29, 2008)

The one above the burners and pushes the smoke to the chimney.


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## Greenlawnracing (Oct 27, 2009)

yuri said:


> The one above the burners and pushes the smoke to the chimney.


Yep, that one works.

I thought it was called in inducer fan - is it not?

Edit: although it does run, it only stays on for a few minutes after the furnace attempts to start.


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## yuri (Nov 29, 2008)

ventor fan/inducer fan same fan different techs call it differently. After the ventor fan starts the pressure switch closes and then the igniter is energized. If the switch does not close then the board show an error code. Is that happening? code is on the back of one of the doors. led flashes. Remove the smokepipe from the top of the furnace and look for a dead bird. Yes, I am serious, they fall in and can block the pipe and cause a problem.


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## Greenlawnracing (Oct 27, 2009)

yuri said:


> ventor fan/inducer fan same fan different techs call it differently. After the ventor fan starts the pressure switch closes and then the igniter is energized. If the switch does not close then the board show an error code. Is that happening? code is on the back of one of the doors. led flashes. Remove the smokepipe from the top of the furnace and look for a dead bird. Yes, I am serious, they fall in and can block the pipe and cause a problem.


Just tried that - no bird, just one dead roach. The fan blades appear to be moving properly.

There is no error code, which confuses me. Because the igniter is not being energized and the gas is not being lit - I would have thought that I'd get an error code. 

I did connect my multi meter to the pressure switch. The left lead reads 27v, and the right lead reads .8v.


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## yuri (Nov 29, 2008)

You probably have a bad circuit board. It definetley should show you an open pressure switch code. Try cozyparts.com for Lennox parts or a Lennox dealer.


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## Greenlawnracing (Oct 27, 2009)

yuri said:


> You probably have a bad circuit board. It definetley should show you an open pressure switch code. Try cozyparts.com for Lennox parts or a Lennox dealer.


Is there a way I can test the board to confirm its bad before I buy a new one?


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## yuri (Nov 29, 2008)

Nope. All the codes should be on the sticker and they never fail to show up on a board. Especially the pressure switch code.


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## Greenlawnracing (Oct 27, 2009)

Fixed!
I went to the supply house to get a new tube...they didn't have the exact tube that I broke, just a generic one that wasn't nearly as stiff and had a larger interior diameter.
I had a guy come over to look at it tonight. Before I put the new tube on he had me run a wire into the inducer fan....guess there had to have been something tiny plugging it up. I didn't feel or see anything, but it fired right up.
The high heat tube that Lennox used is different than the one I bought today...the one I got kinks easily and required me to reposition the pressure sensor. Stupid replacement tube from Lennox is $25!

Still baffled as to why I never got any trouble codes.


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## yuri (Nov 29, 2008)

That is very strange. Good to know in case I run into one. Probably a bad batch of boards from whomever made them.


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## thehvacguy (Oct 30, 2009)

*What do you mean about the tube?*

What kind of tube are you talking about that you replaced? I don't think i have ever heard of a high heat tube.


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## Greenlawnracing (Oct 27, 2009)

thehvacguy said:


> What kind of tube are you talking about that you replaced? I don't think i have ever heard of a high heat tube.


The orange tube is the original one that I tore while removing. It was basically molded to fit perfectly. I replaced it with the black tubing, but it isn't nearly as thick and kinks easily. I had to move the pressure sensor to make it work, and I'm worried that it will melt as I don't know if it can withstand the heat.

I'll spend $25 if I have to...but can you get a replacement somewhere else? $25 from cozyparts just seemed pretty high for a little piece of rubber.


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## yuri (Nov 29, 2008)

It is SILICONE hose and not rubber and the stuff costs me about $10/foot. Buy it or it will melt. Too bad you don't live nearby, I have lots of those leftover from other jobs.


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## Greenlawnracing (Oct 27, 2009)

Any place I can buy something similar? Like an autoparts store?

Any chance you want to ship me a piece? :laughing:


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## thehvacguy (Oct 30, 2009)

So pretty much the tube allows the heat from the vent to flow to a limit switch?


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## yuri (Nov 29, 2008)

If it is the right diameter and made from silicone it will work. The Lennox one is better because of the square shape/prevents kinky/kinking.LOL.:thumbup:

The tube goes to a pressure switch which proves the ventor fan is running by the vacuum the fan draws.


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## thehvacguy (Oct 30, 2009)

The pressure switch is normally open or normally closed?


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## yuri (Nov 29, 2008)

Open. Study this:http://tech.lennoxintl.com/PDFs/0006j.pdf.
Quiz later


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## Greenlawnracing (Oct 27, 2009)

Thumbs up to hvacpartsshop.com! I emailed them asking for a replacement tube....they only sold them in big packages for $20. Being the cheapest that I could find, I was going to order a pack and then found out they were local. When I went to pick it up, they simply gave me a few pieces since I didn't need an entire pack. Seemed like a great place to buy parts from!


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## cindy015 (Jan 20, 2011)

yuri said:


> Post the model and serial # of the furnace. I am a Lennox tech.


Hi,

I'm having problems with my lennox furnace. Air is flowing through the vents but no heat. And there's no light on . No code showing either.


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## cindy015 (Jan 20, 2011)

Does anyone know why a furnace's burner is not igniting? Could it be the humidifier with added to the house?


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## diyorpay (Sep 21, 2010)

Better to start a new thread but...

Besides checking flame sensor 'connection', try cleaning sensor by one of the burners with emery cloth, fine steel wool or sandpaper. 

For expert techs post model # of unit

Tell what point you are at:

1. Combustion fan/induction fan/ventor fan comes on
2. Pressure hose jumps, air pressure switch closes
3. Ignitor glows 
4. Gas valve opens, burners light
5. Blower motor starts


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## xuzhang (Oct 27, 2011)

*Lennox furnace - no heat.*

It is fixed. Thanks.


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