In HVAC, "O" typically refers to the reversing valve terminal or wire that is energized when a heat pump system is operating in its cooling mode.
Understanding the "O" Terminal in Heat Pumps
The "O" terminal on a thermostat, often connected to an orange wire, is crucial for controlling the reversing valve in a heat pump. This valve is a key component that allows a heat pump to provide both heating and cooling from a single outdoor unit by changing the direction of the refrigerant flow.
Here's what "O" signifies in this context:
- Cooling Mode Activation: For the vast majority of heat pump systems, the reversing valve coil is energized (receives power) when the thermostat calls for cooling. This signal to the "O" terminal tells the heat pump to direct the refrigerant in a way that absorbs heat from inside your home and releases it outdoors, thus cooling your indoor space.
- Common Configuration: The "O" type reversing valve is the most prevalent configuration found in residential and light commercial heat pump installations.
"O" vs. "B" Reversing Valve Types
While "O" is the most common, it's important to be aware of the less frequent "B" terminal, which controls the reversing valve differently:
| Feature | "O" Type Reversing Valve (Most Common) | "B" Type Reversing Valve (Less Common) |
|---|---|---|
| Activation | Energized when the system is operating in cooling mode. | Energized when the system is operating in heating mode. |
| Deactivation | De-energized when the system is in heating mode. | De-energized when the system is in cooling mode. |
| Typical Brands | Most HVAC manufacturers use this configuration. | Rheem and Ruud are notable brands that typically use this type. |
Understanding this distinction is vital for proper thermostat wiring and installation, especially when replacing a thermostat on an existing heat pump system. Incorrectly setting the reversing valve operation can cause the heat pump to provide heating when cooling is desired, or vice versa.
For more in-depth information on how reversing valves work in heat pump systems, you can explore resources on heat pump components and operation.
