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M Oglesby
M Oglesby, HVAC Technician
Category: HVAC
Satisfied Customers: 2038
Experience:  Universally Certified HVACR Technician
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I have a lennox furnace/ A/c combo unit located outside the

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I have a lennox furnace/ A/c combo unit located outside the home. The furnace will run for about a minute, the gas valve opens, then everything shuts down. I replaced the roll out switch, with out any changes in the symptom. The board on the unit blinks 3 times. What could be wrong?

M Oglesby :

Hello I am here to assist you.

M Oglesby :

Remove the furnace door panels and look on the back and front of them to find out what the 3 flash LED troubleshooting code is indicating?

M Oglesby :

Let me know what it says please.

Customer:

there is no code telling me what it says

M Oglesby :

Ok. If you set the thermostat to the fan on position, does the fan come on?

Customer:

yes

Customer:

we switched out the thermostat though

M Oglesby :

Try turning on the heat with the fan in the on position and tell me how long the heat will run

Customer:

thats not possible any more with the new thermostat i cant control the fan by itself any more

M Oglesby :

ok

M Oglesby :

Is the board blinking right now?

Customer:

no because its a gas system i have it shutoff from the breaker box

M Oglesby :

Turn it back on and tell me what happens.

Customer:

ok this will take about a minute to a minute and a half

M Oglesby :

ok

Customer:

ok turned it on from the breaker box after about ten seconds the system fired up started working for about a minute then it stut itself down the green light on the board countinuious blinks now after the unit shut itself down i came back inside and shut it off at the breaker box

M Oglesby :

Did the blower motor ever come on?

M Oglesby :

Also it is not necessary to keep cutting it off at the breaker.

Customer:

i cant be sure the blower motor is inside a different compartment however the a/c fan does come on

Customer:

also we are assuming that the system is running normally accept for the shutting itself down part

M Oglesby :

It sounds like the limit switch is opening its contacts and shutting the furnace down. That means that the furnace is overheating and the limit switch is shutting it off for safety purposes.

M Oglesby :

I am suspecting that the reason the furnace is overheating is because the Fan blower is not coming on in the heat mode.

Customer:

the tech changed the limit switch and a roll out switch

M Oglesby :

That means that they are both good. They are doing their jobs.

M Oglesby :

They are supposed to open their contacts when the furnace overheats.

Customer:

how can it overheat in a minute?

M Oglesby :

When the heat is turned on, the burners are allowed to heat the heat exchanger to a certain temperature before the fan is called on. This is to keep the furnace from blowing cool air into the conditioned space. But I beleive that the blower fan is not being called on and the furnace is overheating the heat exchanger.

M Oglesby :

So turn the breaker back on and let it stay on. Turn the heat back on and let it shut down. After it shuts down, let me know if you feel any air coming from the vents. This will tell us if the fan is coming on or not.

Customer:

ok

M Oglesby :

Let me know what happens.

Customer:

ok turned it on did not light this time i hace left it on at the breaker also i opened the compartment that houses the blower motor so if it does come on again i can check to see if the blower is coming on

M Oglesby :

Is the door safety switch pressed in?

M Oglesby :

Sometimes there is a door safety switch that the cover holds in.

Customer:

i do not /did not know there is one however ever time the unit has lit the doors have been off

M Oglesby :

When you turned it on, what did it do?

Customer:

first time nothing just went out and did it again

Customer:

and the system turned on normally but the blower motor'

Customer:

did not come on then i came in the house and turned it off

M Oglesby :

YOu will need to call a technician out to change your blower motor. That is your problem. The blower motor is not coming on and the furnace High Limit switch is not allowing the furnace to stay on.

Customer:

one more question if i may please/

M Oglesby :

Please hit ACCEPT if you are satisfied with this answer. If not, I will continue to assist you. Bonuses and positive feedback are very much appreciated!

M Oglesby :

Go ahead with your question.

Customer:

thank you why didnt you lean towards a bad heat exchanger/

M Oglesby :

I did not hear of any symptoms that would point in that direction.

Customer:

thank you very much for your time you have been extremely helpful

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