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Billy
Billy, HVAC Technician
Category: HVAC
Satisfied Customers: 19649
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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i have a lennox g12 series furnace. the pilot light is lit and the blo

Resolved Question:

i have a lennox g12 series furnace. the pilot light is lit and the blower is working, however, the air is cold not hot.
i noticed the 4 burners are not lighting. any solutions?
Submitted: 2 years ago.
Category: HVAC
Expert:  M Oglesby replied 2 years ago.

M Oglesby :

When is the last time the filter was changed?

Customer:

august 2011

M Oglesby :

Check to see if the filter is dirty.

Customer:

ok

M Oglesby :

Filter should be changed every 3 months.

Customer:

would a dirty filter prevent the 4 burners from lighting

M Oglesby :

Yes. A dirty filter will cause the Main Limit switch to open and not allow the burners to light.

M Oglesby :

The Main limit switch will open because a dirty filter causes the furnace to overheat.

Customer:

changing filter now

M Oglesby :

The main limit switch is usually a rectangular or square looking device that has two wires attached to it and is usually located inside the furnace cabinet on the back panel wall behind the gas valve. Locate it and let me know when you find it.

Customer:

hammock style filter.. on one side very dirty

M Oglesby :

That is probably the problem.

M Oglesby :

Clean it

Customer:

ok standby

M Oglesby :

k

Customer:

changed filter- turned on heater- air is still not hot- burners are still not lit. i have located the main limit switch. any other suggestions

M Oglesby :

Do the main limit switch have a number such as this written on it, L180 - 40F ex...?

Customer:

it has this on the front face of it FALTS57E-13T-110-1160 and on the bottom side it has this number 13B8501

Customer:

sorry correction it is FALTS57E-13T-110-160

M Oglesby :

I dont beleive that is the Main Limit switch.

M Oglesby :

Give me some more description of it?

Customer:

small black box behind gas valve on wall of furnace. it looks like it has also something to do with fan controll. It has fan off and temperature settings from 80 to 110 degrees and a small dial which is on 110 degrees

M Oglesby :

Yes that is the fan limit switch. I will opt out so another expert can assist you with this because I don't know how to bypass the fan limit switch.

M Oglesby, HVAC Technician
Category: HVAC
Satisfied Customers: 2038
Experience: Universally Certified HVACR Technician
M Oglesby and 5 other HVAC Specialists are ready to help you
Expert:  Billy replied 2 years ago.
Hello,

First of all is this a standing pilot lit all the time or a spark pilot that lights when it is time for heat?

Customer: replied 2 years ago.
standing pilot lit all the time

Expert:  Billy replied 2 years ago.
Thank you,

Very easy test first and we will go from there...

First turn off power and tap the FALT control in the center about 4-5 times then turn power back on.

Then tell me if you have 2 or 4 wires attached to your gas valve
Customer: replied 2 years ago.
when you say turn off power you mean turn off thermostat or the gas valve
Expert:  Billy replied 2 years ago.
The power switch on side of furnace

Customer: replied 2 years ago.
OMG THE 4 BURNERS ARE LIT!!! AND I HAVE HOT AIR BLOWING FROM MY VENTS
CANT BELEIVE THAT WAS ALL THAT I NEEDED TO DO...TAP THE FALT!!!!
YOURE A GENIUS. COULD YOU EXPLAIN THE TAPPING OF THE FALT
Expert:  M Oglesby replied 2 years ago.
Tapping on the Fan Limit switch can reset the limit switch or close its contacts. Make sure to keep the filter changed so it does not overheat again.
Expert:  Billy replied 2 years ago.
What happened is the unit overheated and the limit opened...when it cools it is supposed to close. Yours being older needed coaxing to close. I am very glad it worked out.

I see you clicked for the first expert who opted out to get the accept. Could I trouble you to switch this if I forward to customer service?

Billy
Billy, HVAC Technician
Category: HVAC
Satisfied Customers: 19649
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
Billy and 5 other HVAC Specialists are ready to help you
Customer: replied 2 years ago.
ABSOLUTELY!!! YOU SOLVED THE PROBLEM, HE TRIED BUT DID NOT
Expert:  Billy replied 2 years ago.
I thank you...glad your warm.

Billy
Customer: replied 2 years ago.
I MUST HAVE ACCEPTED BY MISTAKE. 1ST TIME ON THIS WEB SITE. WASNT SURE HOW IT WORKED

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