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Robert
Robert, HVACR Educator
Category: HVAC
Satisfied Customers: 161
Experience:  Licensed Mechanical Contractor, 30 years experience, 20 years teaching experience
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Why is my furnace not turning on? There is power supply

Resolved Question:

My furnace is not turning on. It has power, and the indicator light has the correct "heartbeat". I can turn the fan on manually with the switch on the thermostat, and it comes on and seems fine???

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

Hi,
I will need you to try some things when you are ready.

I need you to go to the furnace and find the main circuit board. You will find some small gauge wiring connected to a terminal strip that has terminals labeled R, W, C, Y, G. I need you to jump the R terminal and the W terminal together without touching any other terminal. A small wire is appropriate for this. You will need to tape the blower door switch closed to make the unit run, also if the door is off.

Customer: replied 7 years ago.

I taped the switch, pulled the fuse, and put the jumper wire in. As soon as I put the fuse back in the blower came on.

Expert:  Robert replied 7 years ago.

That is probably the fan cycle power-up. Let it run through the cycle. You jumpered R to W correct?

Customer: replied 7 years ago.

Yes R to W. My diagnostic light is flashing three times now. The book says 3 = pressure switch open when it should be closed.

Expert:  Robert replied 7 years ago.

Is the small inducer fan operating?

Customer: replied 7 years ago.

Is that on top? It is not running. The blower is still running.

Expert:  Robert replied 7 years ago.

Ok, was the blower running before?

Customer: replied 7 years ago.

No. I could turn it on with the switch, but it wasn't running on "auto".

Expert:  Robert replied 7 years ago.

Jumpering between R and W bypasses the thermostat and should bring in the inducer fan. It is the small fan with the vent pipe connected to it. The sequence of operation is as follows: thermostat calls for heat (between R and W) Inducer fan starts, Pressure switch closes, Igniter energized and then trial for ignition.
Shut the unit off and remove the rubber tubes from the pressure switch (round, disc shaped device with small tubes connected to it). Blow through the tubes and reconnect them. Turn unit on and see what happens.

Customer: replied 7 years ago.

No changes. This unit is less than a year old.

Expert:  Robert replied 7 years ago.

Ok so the small inducer fan is not running right?

Customer: replied 7 years ago.

No it is not. I put a better wire to jump the R and W. The blower fan is not running anymore, and the indicator light is flashing 3 times.

Expert:  Robert replied 7 years ago.

Ok, that indicates a problem with combustion air-flow. The inducer fan needs to be running, Again, is it running?
Thanks

Customer: replied 7 years ago.

No, it is not running.

Expert:  Robert replied 7 years ago.
Can you check voltage?
Customer: replied 7 years ago.

No

Expert:  Robert replied 7 years ago.

Ok, That would be the next step. We need to determine if there is power going to the inducer motor. I need to know if the inducer fan motor has failed or something else.

There should be a molex connector that you can disconnect and check voltage when you try to start the furnace. Remember, the inducer fan has to run before anything else can happen.

Customer: replied 7 years ago.

I have my multi meter now.

Ok, what and where is a molex connector? How many volts should it be?

Expert:  Robert replied 7 years ago.

It is a plastic connector where the motor wires connect to the wires feeding it.

Customer: replied 7 years ago.
Is it a 2 wire connector?
Expert:  Robert replied 7 years ago.

Yes. Do the wires lead to the inducer fan? If so disconnect the plug and check to see if power is going to the two terminals in the plu (not the motor end connections). You will need to shut off the power and get your meter leads on the connections and then turn the power back on. You should see 120 volts (give or take a few).

Customer: replied 7 years ago.

I could not get any volts from the two pins. I did get 121 v from another source on the panel. I jumped two wires to the connector, thinking the inducer motor would come on, but instead I got an orange glow from somewhere (ignitor?).
Should the motor come on right away?
If my control panel was bad would everything not work, or could this connector be bad by itself?

Expert:  Robert replied 7 years ago.

The first thing that has to happen is the inducer fan starts up. Nothing else can happen until the inducer fan starts. The orange glow is the igniter. You have already jumpered between R and W on the terminal strip and you have 120 volts to the board plus you have a diagnostic light. Tells me you probably have a bad board.
I am not sure what wires you are jumpering to get the igniter to come on but it doesn't sound like you are on the wires going to the inducer fan.

Robert and 2 other HVAC Specialists are ready to help you
Customer: replied 7 years ago.
Got a new board today and installed it. It worked! Thanks.
Expert:  Robert replied 7 years ago.
That's great news!...Thanks!