Thermostat Function Quiz

Ten questions on thermostat terminals and what each one controls. Pick an answer to see whether you are right and why. Related reading: The Code: controls.

1. The R terminal on a thermostat is:

   1. The reversing valve
   2. The 24-volt power from the transformer
   3. The return air sensor
   4. The common

   R is the hot side of the 24-volt control transformer. The thermostat switches R to the other terminals to make a call.

2. Which terminal turns on the indoor blower?

   1. W
   2. Y
   3. G
   4. C

   G energizes the indoor fan relay or tells the furnace or air handler board to run the blower.

3. A call for cooling energizes which terminal?

   1. Y
   2. W
   3. O/B
   4. E

   Y energizes the compressor contactor in the outdoor unit. The indoor blower runs too, through G or through the equipment's own control.

4. The C terminal provides:

   1. Cooling
   2. A common return path to the transformer, so the thermostat has continuous power
   3. A connection to the compressor
   4. Constant fan

   C completes the 24-volt circuit back to the transformer. Smart and Wi-Fi thermostats usually need it for steady power.

5. On a heat pump, the O terminal typically energizes the reversing valve in:

   1. Heating
   2. Cooling
   3. Defrost only
   4. Emergency heat

   With O, the reversing valve is energized in cooling. Some manufacturers use B, which energizes it in heating. Set the thermostat to match the equipment.

6. W on a furnace thermostat calls for:

   1. Heat
   2. Cooling
   3. Fan only
   4. A humidifier

   W starts the heating sequence on a furnace, or auxiliary heat on some heat pump setups.

7. Emergency heat on a heat pump thermostat:

   1. Runs the compressor at full speed
   2. Locks out the compressor and heats with the backup heat source
   3. Turns off all heat
   4. Starts a defrost cycle

   Emergency heat is for when the heat pump is out of service. It heats with electric strips or a furnace only, which usually costs more to run.

8. Y2 or W2 on a thermostat is used for:

   1. A second thermostat
   2. A second stage of cooling or heating
   3. A second transformer
   4. A Wi-Fi module

   Two-stage equipment uses Y1 and Y2 for cooling stages and W1 and W2 for heating stages. The second stage runs when the first cannot keep up.

9. A thermostat is best mounted:

   1. On an outside wall in direct sun
   2. On an inside wall away from drafts, supply registers and heat sources
   3. Above the kitchen stove
   4. Inside the return duct

   The thermostat should sense the temperature of the space. Sun, drafts, appliances and supply air make it read wrong and cycle the system badly.

10. Wire colors at the thermostat:

    1. Are fixed by code and always correct
    2. Are a convention; the terminal letter is what matters
    3. Determine the voltage
    4. Must match the colors in the outdoor unit

    Installers often use red for R, white for W, green for G and yellow for Y, but nothing guarantees it. Label wires by terminal before you disconnect them.