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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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i have a lennox g40uh series gas furnace that wont ignite.

Resolved Question:

i have a lennox g40uh series gas furnace that won't ignite. The diagnostic codes based on the led lights (LED #1 - Slow Flash / LED #2 - ON) indicate the Primary or secondary limit switch open. Limit must close within 3 minutes or unit goes into 1 hour Watchguard. Can you tell me what this means and whether it requires a service call to address?
Submitted: 3 years ago.
Category: HVAC
Expert:  Matthew replied 3 years ago.

Matthew :

Hello, My name is XXXXX XXXXX I will do my best to help you today.

Customer :

ok

Matthew :

a limit switch senses the heat inside the furnace. If your furnace isn't not pushing the air out sufficiently these limits will trip causing the burners to go off and to run the blower all the time until the limit resets. This is caused by a dirty filter, dirty evaporator coil underbelly, blocked returns, a malfunctioning blower motor, or a malfunctioning limit. Most often the limit is not bad it is doing it's job.

Matthew :

You do have roll out limits which are manually reset by pressing the small red button in the middle of them. These will trip if the burner flames fire backwards out of the furnace due to a blocked exhaust or a cracked heat exchanger.

Customer :

So are any of these things that a homeowner can typically resolve, or does it require a service call to resolve?

Matthew :

Full Size Image

Matthew :

You can check these roll out limits which I just sent you a picture of. If these are triped then run the furnace as normal and if it does it again you may need a service call. Sometimes wind will push against the exhaust and cause these to trip.

Matthew :

Change your filter and make sure none of your return registers are blocked by furniture or rugs. Other than that you most likely will need a service call.

Customer :

the image didn't come thru

Matthew :

Full Size Image

Matthew :

Did that one?

Customer :

nope - do you have a url by chance?

Matthew :

http://bestbuyheatingandairconditioning.com/Merchant2/graphics/00000001/LIMD25303217.jpg

Matthew :

urg

Customer :

ok i see the image with the red button

Matthew :

those are your roll outs all of these can be reset if they are tripped by pushing the button down so try all those first and see if it resets the fault code.

Customer :

so these are inside the back panel that can be removed, or where are they located?

Matthew :

The main limit is auto reset or you can reset it by turning power off to the furnace, but if the limit is working like it should it will not take long to reset since the heat inside the furnace is still there.

Matthew :

They will be all around the burners compartment and maybe on on the exhaust motor housing.

Customer :

ok - i'll go check and see

Matthew :

The main limit is a 3"x4" square plate on the face of the heat exchanger with 2 wires going to it.

Customer :

will be back in a couple of minutes

Customer :

ok

Matthew :

If you take these two wires and touch them together this bypasses the limit and if this is the limit triped the flash code should go away.

Customer :

Nothing seems to change the LED diagnostic codes so it doesn't appear that i'm getting the limits reset.

Matthew :

There may be one other limit on your furnace. Sometimes they put an aux limit on the blower compartment for furnaces that can go horizontal. They are very hard to reach and if your furnace is in the upflow position you can just cut the wires and tie them together to bypass this limit on the back of the blower compartment. Take the bottom door off and see if there are two wires goign back behind the compartment. Not the wires going to the blower motor these limit wires will be the same color as each other and should be looped in series with the other limits of the furance.

Matthew :

This limit should reset with the power being turned off to the furnace so if these is the limit that is tripped then it is bad if it doesn't reset.

Customer :

i appreciate your help, but think i've reached the limits of my comfort level. I'll wait for the repair tech tomorrow.

Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience: 14 years experience in the HVAC field of service, installation, Manual J & D calculations
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