Heat Pumps & Ventilation

How a Heat Pump Works

A heat pump is a refrigeration system that can reverse the direction of refrigerant flow, allowing it to provide both cooling and heating from a single piece of equipment. In cooling mode, a heat pump works identically to a standard air conditioner: it absorbs heat from indoor air through the evaporator and rejects that heat outdoors through the condenser. In heating mode, the process reverses - the outdoor coil becomes the evaporator (absorbing heat from outdoor air), and the indoor coil becomes the condenser (releasing heat into the building).

This reversal is accomplished by a component called the reversing valve (also known as a four-way valve). Understanding how the reversing valve works and when it is energized is essential for both the EPA 608 exam and field service.

The Reversing Valve (Four-Way Valve)

The reversing valve is a cylindrical valve body with four refrigerant connections:

1. Compressor discharge (hot gas) - always connected to the compressor output
2. Compressor suction - always connected to the compressor input
3. Indoor coil connection
4. Outdoor coil connection

Inside the valve body is a sliding mechanism (a slider or piston) that redirects the flow of hot discharge gas from the compressor to either the indoor coil or the outdoor coil. The slider is moved by differential pressure created by a small solenoid-operated pilot valve mounted on top of the main valve body.

The solenoid pilot valve is an electromagnetic coil. When the solenoid is energized (powered), it shifts the pilot valve, which redirects compressor discharge pressure to one end of the main valve's slider, pushing it to the opposite position. When the solenoid is de-energized (unpowered), the pilot valve returns to its default position, and the slider moves back.

Energized = Cooling Mode

In most heat pump systems, when the reversing valve solenoid is energized, the system operates in cooling mode. This is an important exam point. The reasoning behind this design convention is practical: if the reversing valve solenoid coil fails (burns out, loses power, wire breaks), the system defaults to heating mode. Since the most critical need in most climates is heat during winter, this fail-safe ensures that a solenoid failure does not leave a home without heat during cold weather.

The memory aid many technicians use is: "The valve is energized in cooling because you want the system to default to heating if something fails."

Note: Some manufacturers, particularly Rheem, use the opposite convention where the reversing valve is energized in heating mode. However, the industry standard - and what the EPA 608 exam tests - is energized in cooling.

When you encounter a system where you are unsure of the convention, check the wiring diagram. The reversing valve solenoid wire is typically labeled "O" (for cooling-energized systems, which is the majority) or "B" (for heating-energized systems, primarily Rheem/Ruud).

Demand Defrost

When a heat pump operates in heating mode, the outdoor coil functions as the evaporator, absorbing heat from outdoor air. When outdoor temperatures approach freezing or below, moisture in the air condenses on the cold outdoor coil surface and freezes, forming frost and eventually ice. This ice buildup insulates the coil, blocking airflow and severely reducing the system's ability to absorb heat.

To remove this ice, heat pumps have a defrost cycle. During defrost, the system temporarily switches to cooling mode (using the reversing valve), which sends hot discharge gas through the outdoor coil, melting the ice. The outdoor fan is typically turned off during defrost to prevent cold air from blowing across the coil and slowing the melting process. Indoor electric heat strips usually energize during defrost to prevent cold air from being blown into the occupied space while the system is technically in cooling mode.

Most modern heat pumps use demand defrost rather than time-temperature defrost. Here is the difference:

• Time-temperature defrost (older method): The system initiates defrost at fixed intervals (every 30, 60, or 90 minutes of compressor run time) if the outdoor coil temperature sensor detects that the coil is below a threshold (typically 32 degrees Fahrenheit). This method is simple but wasteful - the system may initiate defrost when little or no ice has formed, wasting energy and reducing comfort.

• Demand defrost (modern method): The system monitors actual conditions to determine when defrost is truly needed. Demand defrost is based on temperature differential or pressure differential across the outdoor coil. Sensors measure the difference between the outdoor coil temperature and either the outdoor ambient temperature or the refrigerant saturation temperature. When this differential exceeds a threshold (indicating ice is insulating the coil and reducing heat transfer), the system initiates defrost. Some systems use pressure sensors to detect the increase in air resistance caused by ice buildup. Demand defrost is more efficient because the system only defrosts when there is actual ice to remove.

A typical defrost cycle lasts 5-10 minutes or until the outdoor coil temperature rises above approximately 55-65 degrees Fahrenheit, whichever comes first. A defrost termination thermostat or sensor on the outdoor coil signals when the ice has melted and defrost can end.

Check Valves in Heat Pump Systems

A check valve is a valve that allows refrigerant to flow in one direction only. In a heat pump system, check valves play a critical role in managing metering device operation across both heating and cooling modes.

Heat pump systems have two metering devices: one near the indoor coil and one near the outdoor coil. However, a metering device is only effective when refrigerant flows through it in the correct direction. When the system reverses, the metering device that was restricting flow now needs to be bypassed because refrigerant is flowing through it in the wrong direction.

This is where check valves come in. A check valve is installed in parallel with each metering device (piped around it as a bypass). When refrigerant flows in the normal direction through the metering device, the check valve is forced closed by the pressure differential, and all refrigerant passes through the metering device as intended. When the system reverses and refrigerant flows in the opposite direction, the metering device presents high resistance to reverse flow, so the refrigerant instead flows through the check valve, bypassing the metering device.

For example:

• In cooling mode, refrigerant flows from the outdoor condenser through the liquid line to the indoor metering device (TXV or orifice), which meters it into the indoor evaporator coil. The check valve near the indoor metering device is closed. Near the outdoor coil, the check valve is open, bypassing that metering device.
• In heating mode, the flow reverses. Refrigerant flows from the indoor condenser through the liquid line to the outdoor metering device, which meters it into the outdoor evaporator coil. The check valve near the outdoor metering device is closed. Near the indoor coil, the check valve is open, bypassing that metering device.

Some modern heat pump systems use bi-directional TXVs instead of check valves, which can meter flow in both directions. However, the check valve arrangement remains common and is the concept tested on the exam.