1. Check terminals R to C at the furnace terminal board for 24 V AC.a. If you have 24 volt...

1. Check terminals R to C at the furnace terminal board for 24 V AC.

a. If you have 24 volts at terminals R-C, move to step 2; if not, move to step lb.

b. If you do not have 24 volts at R-C, check for voltage at the primary terminals of the transformer and move to step c.

c. If you have voltage at the primary terminals, you should suspect the transformer is bad and turn off power, remove it, and test its primary and secondary windings for continuity. If either of the windings has an open, you will need to install a new transformer.

d. If the primary terminals do not have 120 volts, you should suspect the high limit switch is open and find the reset button on it. If the furnace starts after you depress the limit reset button, you should be sure to determine what made the overtemperature condition occur.

2. Check from terminal C to terminal W on the terminal board of the furnace con-trol for 24 V AC to test whether the thermostat is calling for heat._

a. If you measure 24 volts at terminals W-C, the thermostat is calling for heat and you should move to step 3.

b. If you do not measure 24 volts from W-C, you should suspect that the ther-mostat is not set high enough, that it is not working correctly, or that a wire between the thermostat and the furnace is open.

3. Check directly across the terminals on the gas valve for 24 V AC to test whether the gas valve is getting full voltage__

a. If you measure 24 volts directly across the terminals of the gas valve, it should open, the burners should ignite, the furnace should start to warm up, and you can move to step 4. If you do not measure 24 volts, you should move to step 3b.

b. If you do not measure 24 volts across the terminals of the gas valve, you should suspect the wiring between the terminal board where your last test was made in step 2 and where the wires connect to the gas valve. Replace any bad wires and continue with trying to make the furnace run.

4. If the gas valve opens and the burners ignite, the temperature should begin to rise. If the furnace fan comes on, the furnace is now working correctly. If the furnace fan does not turn on, you should move to step 5. If the furnace does come on correctly, you can turn the thermostat down and ensure that the furnace turns off. You should cycle the furnace one more time before you leave the service call.

5. If the furnace fan does not cycle on when the temperature increases, you should check the voltage through the fan switch. You can check for 120 volts at the ter-minals of the furnace fan motor. If you have 120 volts and the motor does not run, you should suspect the motor has a problem. If you do not have 120 volts at the motor terminals, you should suspect that the fan switch is not closed. Move to step 5a._

a. If you do not have voltage at the furnace fan motor terminals, you should check for voltage where voltage is supplied to the fan switch. If you have voltage at the input of the switch, but not at the output, you should turn off power, remove the fan switch, and check it for correct operation. If the fan switch is faulty, replace it and cycle the furnace several times to ensure that it is operating correctly.