Sunday, May 31, 2009


A thermostat is a switching device. Therefore it does not create a voltage drop across any terminal device except W1 when the heating contacts are closed and on Y1 when the cooling contacts are open. So you can use a volt meter to check the operation of a thermostat and sub base that have their wires properly connected.

1. You need to verify that the transformer is operating correctly with your voltmeter and an easy way to check that is to set the fan selector switch to fan on in the heating or cooling mode. It the fan comes on then you know you have transformer power.

2. You need to begin by identifying the load that is not operating correctly such as the gas valve, control relay, damper actuator, etc.

3. Remove the thermostat from the sub base to gain access to R,W,Y,G and other terminal screws.

4. Verify that the wire of the affected circuit is properly connected under its terminal screw by pulling on it to see if it is loose.

5. Verify that the wires insulation is not caught under the screw creating an open circuit condition.

6. Place the fan and mode selector switch in their proper positions for the testing to be done.(heat or cooling).

7. Begin your testing by removing the wire of the affected circuit from the sub base.

8. Place one test lead of the voltmeter on the R terminal of the sub base and place the other test lead on the wire that was disconnected from the sub base terminal(G,W,Y,B,O).

9. If the voltmeter displays 24 volts, the circuit from the common terminal of the transformer through the connecting wires and the load (relay coil) is closed and should operate because there is 24 volts across the coil.

10. If the voltmeter reads 0 volts, the circuit between the R terminal and the load is open.

11. If you place a jumper wire from the R terminal and the wire being tested and go to the furnace or the condensing unit and you get 24 volts, the open has been located. To confirm the open, measure the coil with an ohm meter.

12 To confirm if the sub base is ok, place a jumper wire from R to any other control wire (W,Y G,etc.) and if they all start the sub base is operating correctly. To verify this assumption, reinstall the thermostat to the sub base.

13 Make sure all the terminal screws are tight, adjust the set point to close the cooling or heating circuit. If the unit does not start, the thermostat must be replaced. In conclusion, you must understand that an electrical circuit is a closed loop formed by a power source, wires, a fuse, a load, and a switch.

By Roger J. Desrosiers

About the Author:
Roger has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter, United Association and is 608 Universal Certified. Roger is a contributing faculty member of
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