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How a Heat Pump Reversing Valve Works
If you don’t have a gas furnace or fireplace in your home, your unit’s reversing valve is probably your best friend during the winter months.
As their name suggests, reversing valves reverse the refrigerant flow to send the hot, compressed vapor to the indoor coil instead of the outdoor coil. The system releases heat into your home, which keeps you comfortable in the winter.
This article will go over the form and function of reversing valves on air-source heat pumps. We’ll cover the differences between air-source heat pumps and ordinary air conditioners, and we’ll discuss the forces and components involved in reversing valve operation.
What the heat pump reversing valve does
In a typical air conditioner, the refrigerant absorbs heat inside the home in the evaporator coils. Its temperature and pressure rise in the compressor, and it moves to the condenser coils to reject its heat outside.
A heat pump reversing valve simply reverses the process by cycling the refrigerant the opposite way. It makes the condenser function as the evaporator and vice versa. You can see the heating cycle in the image below.
Even though it may feel cold outside in the winter, the refrigerant can still absorb outdoor heat. The outdoor coil functions as the evaporator coil and absorbs that heat. As usual, the refrigerant proceeds to the compressor before it condenses within the indoor coil. When the refrigerant condenses on the indoor coil, the heat gets rejected to the home. The toasty air that comes out of the grilles is really just heat taken from the outdoors (no matter how scarce it may seem).
Differences between air-source heat pumps and A/C units
Air-source heat pumps and standard air conditioners have the same fundamental parts: compressor, condenser, evaporator, and metering device. Below, you can see a picture of an air-source heat pump in standard cooling mode. Take note of some differences between air-source heat pumps and air conditioners.
An air-source heat pump can run in two separate modes: heating mode and cooling mode. As a result, it has two metering devices: one indoors and one outdoors. An air-source heat pump uses the outdoor metering device for heating mode, and it uses the indoor one for cooling mode. As you can see in the image above, a check valve on each metering device determines which expansion device to use and which one to bypass.
Then there’s the obvious answer: air-source heat pumps have reversing valves while basic A/C units do not. The reversing valve does its job by diverting the refrigerant flow in the suction and discharge lines. The reversal process isn’t as simple as turning the flow backward. The process involves rerouting the suction and discharge lines with moving parts.
“Reversing” parts of a reversing valve
A reversing valve primarily uses a sliding mechanism to divert the refrigerant flow, but a few different parts make this process possible.
The actual part that slides to redirect the refrigerant is simply called the slide. As you can see in the image below, the slide is a mini cylinder that moves back and forth inside the reversing valve. Its location determines if the system is in heating or cooling mode. Below, you will see a reversing valve in heating mode. The slide is the silver part inside the tube where the compressor discharge feeds into the reversing valve.
That reddish part within the slide looks kind of like an upside-down canoe. So, we tend to call it a “canoe” informally. This part fits over two of the bottom lines and redirects the suction, as you can see by the big blue arrow.
The electromagnetic solenoid
An electromagnetic solenoid valve allows the slide to move and switch operation modes. This solenoid is usually connected to the thermostat control by two wires: a blue (common) wire going to one side and an orange wire going to the other side. With that wire configuration, the system can use the orange wire to energize the reversing valve in cooling mode. You can see where the wires connect in the image below.
However, it’s worth noting that manufacturers Ruud and Rheem don’t use an orange wire to energize their systems’ reversing valves in cooling mode. Instead, they use a B-terminal or blue wire to energize the solenoid in heating mode.
Redirecting the flow (cooling mode)
The electromagnetic solenoid does not directly cause the slide to move and reverse the refrigerant flow. It merely acts as the first falling domino in a short series of actions.
The solenoid connects to a pilot valve, which acts like a mini reversing valve that causes the slide to move. When the solenoid is energized in cooling mode, it slides the pilot valve. The reoriented pilot valve can then redirect the refrigerant flow and create a pressure differential on the opposite side of the reversing valve. The pressure differential forces the slide to move to the other side, which sets the system back into cooling mode. You can see this in action by looking at the sequence of smaller red arrows in the image below. (The system is in cooling mode, as the pressure differential is on the right side and has pushed the slide over from its position in the previous cutaway image.)
However, the pilot valve itself does not create a pressure differential. The compressor creates the pressure differential when it pumps vapor. If you have a weak or faulty compressor, it might not create a strong enough pressure differential to switch between heating and cooling mode effectively.
Look at the common discharge line in the image above. The red arrows feed into it, but they also travel to the pilot valve. They flow through the pilot valve and create the pressure differential that pushes the slide to the right. If the vapor pressure is too low in the discharge line, it may not be strong enough to push the slide in either direction.
Since the reversing valve requires a pressure differential to switch operating modes, the reversing valve cannot work when the system hasn’t had power for a little while. It may still slide over and change modes when you’ve just turned the unit off, though.
Suction and discharge lines
Keeping track of the suction and discharge lines can get a bit tricky. Four lines meet at the reversing valve: one on top and three on the bottom. (That’s why the reversing valve is also called a four-way valve.)
It gets easier to keep track of them when you can recognize the common discharge line and the common suction line. The common discharge and suction lines will always be used for either discharge or suction exclusively. They don’t change their function regardless of the operation mode.
The common discharge line will always route from the compressor to the side of the valve all by itself. This is usually on the top but not always.
The common suction line is the middle line on the bottom of the reversing valve. No matter what the operating mode is, the suction line will always lead to the compressor.
The two lines beside the common suction line may switch between discharge and suction. Their function depends on how the reversing valve routes the refrigerant. The canoe loops the suction line back to either of the surrounding lines. Look at the lower line of blue arrows in the image below to see this in action. (This system is in cooling mode, as the pressure differential is on the right side.)
The outer line that has not been trapped by the canoe will serve as the discharge line.
In the end, just remember that reversing valves work by shifting the path of the refrigerant. The solenoid doesn’t directly cause the pressure differential that makes the shift possible, but it sets the process in motion. The activated solenoid moves the pilot valve, which opens the paths for the high-pressure vapor to flow to one side of the slide or the other.
Where do the 2 black wires go from the reversing valve solenoid? One to the defrost board (terminal O) and the other black wire to the contactor 24 volt terminal?
They go to terminals on the defrost board dedicated for the reversing valve. But yes, it is a 24 volt signal so it is essentially “O/B” and common