Lennox G12

[vlink55]

Hi all,

I'm new and stumble upon this great forum. Hopefully I'll get my heater back on w/ the help from all of you

Well I have a Lennox G12 Gas furnace, 24+ yrs. , The pilot lights up fine, I can see the gas flame, but there's no HEAT. All it blows is cold air. I dont have AC, this is just a gas furnace.
I changed the Thermocouple and nothing. Still no heat All it kicks out is cooooldddd air. Any ideas as to what is wrong? Oh yeah I can't determine where the blower is or what it is. I have some pics that I've taken, maybe somebody can help me on this.

thanks alot
vlink55

[Troy]

Welcome to the forum!

Here are some pictures that might help someone answer:

[16x80]

vlink55

You could have a couple problems.

You have already ruled out the thermal couple, It is possible that you are getting less than 24v to the gas valve which would allow it to keep the pilot light lit but not enough to open the gas valve!

You could also have a bad main high limit (located on the heat exchanger area with two wires going to it) (one of them is the red one on top of the gas valve) This limit should be normally closed (reading continuity) with no wires hooked to it!

Question is the pilot light getting bigger when you make a call for heat or does it stay the same size regardless?

16x80

[vlink55]

16x80

Do you mean the red and yellow wires running to the block shown in picture #1?

Now w/ the "bad main high limit", Is there a way to repair/replace just that? and where is this? I'm sorry for asking too much- I just want the heat back and running.

The pilot light appears to stay the same. I could hear the fan spining but no heat.

[16x80]

Get me a picture of the wiring diagram shown in pic 4, I will be able to help you more with the eletrical road map.

16x80

[vlink55]

Hey you guys...

So no one can help me out here w/ the Lennox G12? I am clueless as to what is wrong w/ it. No heat and winter is near.

[a46geo]

The gas valve is not opening to allow for the main burner(s) to ignite.
Thermostat, low voltage wiring, transformer, and the gas valve are all common problems that result in what you discribe.

A less common but possible problem could be a gas regulator or the gas pressure itself.

There isn't room here to explain all of the hows & whys, but before you start trial and error replacement, it might just be cheaper to call in a expert.

An expert can also make sure that the thing is operating safe. Carbon monxide leaks can and do occure with age.

At 24 years of age, I wouldn't spend a whole lot of money on it. Ask the utility company to do a return on inverstment analysis for you. If a new furnace will substantually cut your heating bill, that may indeed be your best choice.[?]

[Troy]

Here are some more pics:

[rkelder]

Hope it's OK to jump on this thread as my problem is about the same.

I've got the same system, pilot light kicks up when the thermostat calls to it, ignitor clicks away and then resets. But not all the time, system works in the morning every day, but hit and miss in the evenings. Did first step and put a new thermostat in, what should be my second step?

Thanks in advance

Ray

[Muggle]

I know its a little bit off topic, but it appears that you have no filter in that unit. In the long run, that can lead to damage. That frame in the blower compartment should have filter media in it.

[gronos]

I also have a Lennox G12 furnace. The furnace appears to be working correctly, pilot lights, burner goes on, blower works, etc. I have a service contract on it with a local company. However, the amount of heat it is putting out seems much lower this winter than last. It takes 4-8 hours to heat up a room that took 1-2 hours last year at the same approximate temperature. Is there anything that can be causing this? Thanks.

[The Home Smithy]

Hi Guys;
Just a thought that might help with the heat not getting the place as hot as fast as it used to.
Check your ducts for holes, insulation that has come loose, ducts that have come loose at the registers etc.
Tighten them up and then add some insulation over the ducts. Youd be suprised at how toasty you get after that.
Hope this helps. Best; Smitty

[HayZee518]

that's all taken into consideration "Smithy" but the problem is the main burner is not firing up. OK, the pilot is staying lit that eliminates the millivolt generator which you have pictured. next is to check you are getting 24 volts to the main gas valve which you also have pictured with the yellow and red wires. use a multimeter set for 200 volts and measure the voltage at those terminals when the unit is calling for heat. if you have 24 volts but the solenoid didn't open then replace the gas valve solenoids or the whole valve. shut the gas off before you disconnect any piping.