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ComfortZone
ComfortZone, HVAC Technician
Category: HVAC
Satisfied Customers: 516
Experience:  10 years in the HVAC industry.
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Why is my furnace running but no heat is coming from it?

Customer Question

My heater is running, but no heat is coming out, just cold air. It is not igniting, the the fan will not turn off, even when you turn the thermostat off. I had to use my emergency switch on the wall to turn off the furnace.

Submitted: 7 years ago.
Category: HVAC
Expert:  ComfortZone replied 7 years ago.

Check to see if the igniter glows red. If not, then you have a bad igniter or a pressure switch issue. But if the blower comes on, that is usually an igniter problem.

There is a control panel on the bottom of that furnace that will have a LED and it will flash an error code. The error code descriptions are on the door of the furnace or on the board itself. Turn the furnace on and see what happens. I need to know what works.

Customer: replied 7 years ago.

The blower sends out cold air, I have taken the panel off of the bottom, and the LED light blinks 4 times and stops then blinks 4 times again and stops. The panel has instructions that if it blinks 4 times, the Main switch or the AUX switch is not working. I looked for a reset bottom and I am not sure where it is. There are 3 different bottoms that are all contacted purple wire that looks like they all go to the AUX according to the diagram and there are 7 wires there.

Expert:  ComfortZone replied 7 years ago.
This is a Goodman furnace?
Customer: replied 7 years ago.

This is a Goodman Furance, Model number GMP075-3. I have the serial number, the input and output BTU information, plus all the information on the manufactures sticker if needed.

Expert:  ComfortZone replied 7 years ago.

You will need to power the furnace off, then back on. I want you to watch the start up procedure. This is what should happen:

1. The inducer fan should start, this is the little black fan at the top of the furnace

2. The ignitor will glow bright red, this may take 10 - 15 seconds

3. The gas valve will open

4. The burners will ignite

5. The blower will come on.

How far does your furnace get?

Customer: replied 7 years ago.

The fan comes on and blows cold air, it will not go off even if you turn the thermostat off. I have to turn off the emergency switch. The igniter does not glow at all and the burners do not come on. Is this the flame sensor or the igniter switch?

Expert:  ComfortZone replied 7 years ago.

Okay. You need to turn the power at the furnace off.

You need a multimeter so you can check continuity. Do you see anything that looks like this:

graphic

This is a flame rollout switch or a limit switch. Try pressing the red button in the center.

You may also have one of these right below the inducer fan housing. This is a high limit switch and you will only see the cardboard and the terminals, the rest of it sits in the furnace. If you have one, take the wires off of the end and see if you have continuity between the terminals.

graphic

Customer: replied 7 years ago.
I do not see anything that looks like either of these on my unit
Expert:  ComfortZone replied 7 years ago.
The 3 buttons you mention with the purple wires, are they the size of a quarter?
Customer: replied 7 years ago.

No, they are smaller than a dime. Maybe the size of a pea.

Expert:  ComfortZone replied 7 years ago.

You need to test all those for continuity. Take the wire loose and use an OHM meter to see if you have continuity between the connections. If you do not the limit is bad. If you do move to the next one down the line until you have checked them all.

Customer: replied 7 years ago.

What should the meter say for the continuity between the connections. Should there be a specific number on the meter?

Expert:  ComfortZone replied 7 years ago.
It varies. But if you get continuity the limit is good.
Customer: replied 7 years ago.

What should the limit amount be that I need to look for. And do I need to check every wire on the panel?

Expert:  ComfortZone replied 7 years ago.

No, just check the terminals on the limits. I have a meter that beeps when I have continuity. You need to make sure your limits do not have infinite resistance, which indicates an open circuit.

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Customer: replied 7 years ago.

I had a technician look at it tonight, and there was a red switch under the blow that need to be jumped for the furnace to start working again the way it should.

Expert:  ComfortZone replied 7 years ago.

Ok