Heat Pumps have become increasingly prevalent in the HVAC industry, and they’re not going anywhere. I remember learning about the Reverse Refrigeration Cycle, and wanting it to go away until I was more confident with the “Forward Refrigeration Cycle”. With most everyone working with Heat Pumps, being comfortable with their operating premise and their unique component, the Reversing Valve is of paramount importance.
If you’re looking to deepen your understanding of heat pump systems, check out our General Guide to HVAC Troubleshooting where we cover fundamental diagnostic approaches that apply to heat pump systems.
Heat Pump Terminology
Instead of saying “Evaporator” and “Condenser“, a Heat Pump’s Coils are referred to as Indoor, and Outdoor. The Indoor Coil is made cool in the summer to provide air conditioning, and it is made warm in the winter to provide heating. The Outdoor Coil is opposite to this.
This function is obtained simply by redirecting the refrigerant flow to be “opposite” of normal air conditioning, when the unit runs in heating mode. This is possible by the use of a Reversing Valve. There are some specialized components, such as Bi-Directional Driers, which allow this to work, but will not be described in this writing for simplicity.
The Basic Refrigeration Cycle gets some bells and whistles for a Heat Pump with a Reversing Valve.
The left side represents cooling (normal), and the right side represents heating, where the cycle is reversed. The Compressor and other components continue to run during a changeover, while the Reversing Valve changes position.
For example, if the system is running in Cooling, and a call for Heating is required, the Reversing Valves’ Solenoid Coil is energized. This causes the Reversing Valve’s Solenoid Valve to change positions, allowing discharge gas to be sent to the indoor coil to heat the space. In the meantime, the Outdoor Coil extracts the Enthalpy available from the outdoors.
Note: in the heating cycle, a defrost must occur to free the Outdoor Coil of frost. This is done by simply again “Reversing” the system flow so that Discharge Gas temporarily provides its heat to the Outdoor Coil. For proper heat pump installation in cold climates, consider adding a drain pan heater as demonstrated in our How To Install A Drain Pan Heater On A Cold Weather Heat Pump video.
How the Reversing Valve Works
The Reversing Valve utilizes differential pressure to get the “Valve” to move. This is achieved through utilizing High Pressure Discharge gas to flow through the valve’s “Pilot Lines“, to influence the movement of the Valve.
On the left side of the above image, Discharge gas is shown routing through the Pilot Line to push the Reversing Valves’ cylinder towards the left. This orientation allows for Discharge Gas (red) and Suction Gas (blue) through the Valve in the shown path. This state could realize the Solenoid Coil being deenergized.
On the right side of the above image, think of the Solenoid Coil being energized. The causes the Solenoid Valve to change positions, and provide a new Discharge Gas Path within the Pilot Lines. The new path pushes the cylinder towards the right side of the Reversing Valve. This allows the second orientation of Discharge and Suction Gas through the valve.
In cooling, the Discharge gas goes through the Reversing Valve, and to the Condenser. When the solenoid is energized, the reversing valve pushes Discharge Gas to the indoor coil for heating.
Control Designation and Regional Considerations
Different manufacturers use different control strategies for their reversing valves. As explained in our article on Heat Pump Reversing Valves and Their Control Designation, most manufacturers default to heat (O terminal is energized for cooling), though some still default to cooling (B terminal is energized for heating).
Note: Different areas (Toronto vs. Miami) have different failure modes for the Heat Pump/Reversing Valve. In a market with cold winters such as Toronto, the unit will fail to Heating. In a warmer market (Miami), the unit will fail to provide Cooling. The common failure is the Solenoid Coil burning out, so failure occurs with the Solenoid Coil deenergized.
Some manufacturers that use B terminal designation (energize for heating) include:
Rheem
Ruud
Weathermaker
Ameristar
Bosch Air Source
Always consult the manufacturer’s documentation for specific wiring information, as incorrect terminal connections can cause the system to operate in the opposite mode than intended.
Heat Pumps are everywhere, and understanding their operating principle is very important. Reversing Valves are an integral part of a Heat Pump, and they are important to understand. Many Heat Pump operational, troubleshooting, and repair scenarios relate directly to it.
The Reverse Refrigeration Cycle is demystified when its operation and the Reversing Valves’ function are understood. Being comfortable with the operating principle of the Reversing Valve allows a technician to be successful when diagnosing issues with Heat Pump Systems.
