Lennox G26 forced-air gas furnace error code!

[AWMP]

    Rt Led:    Slow flash

    Left Led: On

Sticker on back of front panel reads:

Primary or secondary limit switch open.

Looking for a service manual and/or diagram that shows the location and

function of this switch.

Or can someone give me an explanation & location of said switch?

Any help is appreciated.

[hvacdrd]

The limit is typically located about center of the cabinet when you take off the cover. Look for something mounted to the front wall about 2" x3" with two wires if it is a newer furnace. It looks like a flat piece with two terminals from the front. The secondary limit would be on or near the blower housing if it is a downflow furnace. Numbers on the limit would be something like L180 - or post a picture and we will tell you exactly where to look.

Dirty filter?

[AWMP]

Filters have been changed.

Furnace was installed in 1997.

I will try to get a pic as soon as possible.

What is the function of this (or these) switches?

[hvacdrd]

To shut off the burners if the air flow is restricted, failed blower motor, dirty filter, etc. - operate as a safety circuit.

Here's a picture of a common limiit...

[AWMP]

Error on my part:

                            Left Led: Slow flash

                            Rt Led:     On

Sorry!!

Error message remains the same.

Here is pic of top half.

Are you referring to the inside air flow or the outside air intake from the 3" pipe? There is another error code for "Blocked inlet/exhaust vent" for the burner chamber.

[dkpd1581]

The other error code about a blocked flue could be a problem with the 3 inch pvc exhaust vent.  This pipe carries  cool flue gas and vapor away from the machine.  Two things generally happen.

First is that the plastic tubing that is designed to drain this vapor condensate away and into the condensate pump or 3/4 inch pvc drin line will get clogged.  The liquid builds up and causes the blocked flue error to appear.  If the line itself is not blocked, your condensate pump itself may be plugged up and the over flow safety switch is locking out the machine.  The condensate is a product of combustion and as such it carries fine soot and particles which looks like mud or sludge in the drain lines and/or condensate pump pail.

The other and less frequent cause for the blocked flue concerns the 3 inch drain line having a sag somewhere in the horizontal run as it exits the house.  The line is supposed to have a pitch to it to keep the condensating vapor from building up.  Some installations have dips in them which causes the condensate to build up to the point where the gasses canot get by.  The machine senses this fact and shuts down.  Typically the water build up is where the 90 degree elbow turns up if you cannot see and obvious  dip in the  horizontal runs. 

If you do in fact have this second problem, heat will build up in the pvc exhaust and eventually back up into the exchanger, this in turn will give you the limit switch error code.

One other check to do is get on the roof and verify that there are no obvious obstructions where the 3 inch pvc breaks through the roof line (squirel or bird nests).

I have run into several units that had the issue of the sagging or improperly pitched 3 inch pvc.  It can be fixed...it just depends on how bad the sag is, where it is located, and how accessible it is.  Remember most of this line is behind dry wall and ceilings so that makes it a painful process to locate and fix.

Lemme know and we can go from there...

[hvacdrd]

I am referring to the inside air flow...The limit is located right in the center of the picture you posted. A different style than the one I posted. It is the black disc with a white label about the size of a quarter with the blue wires connected to it.