Heat pumps are a type of heating and cooling system that are popular due to their versatility and efficiency. They differ from standard air conditioning units, and the term “heat pump” is sometimes used to refer to several types of HVAC systems. Therefore, the goal of this article is to answer the following questions:
What is a heat pump?
How do heat pump systems differ from standard air conditioning?
What are the different types of heat pumps?
What is a Heat Pump?
In simple terms, a heat pump is an HVAC system that uses a refrigerant cycle to provide both heating and cooling functions. These systems pump heat into or out of a space depending on the mode.
Please check out our blog on standard refrigeration cycle components for more background information.
However, whether used in residential, commercial, or large industrial applications, their function is the same—to “pump” heat into or out of a space. Additionally, one reason heat pumps are popular is that once heat is moved out of space, it can be used for functions such as heating domestic hot water (DHW) or a hydronic floor heating system. Heat pumps are also used in industrial applications to reduce heat energy waste and contribute to overall efficiency improvements.
How do Heat Pump Systems Differ from Standard Air Conditioning?
While the two can sometimes be used interchangeably, a heat pump and an air conditioner are not necessarily the same thing. Both utilize refrigeration cycles, but in a single-mode air conditioning unit, the refrigerant flows in only one direction. In a heat pump system, this flow is reversible, allowing the system to alternate between heating and cooling functions as needed. To achieve this, heat pumps require some specialized components, two examples of which are:
Reversing Valve
Check Valves
Reversing Valve
To enable a heat pump to switch between heating and cooling modes, the flow of the system’s refrigerant must be reversible. This is where the reversing valve comes into play. Also known as a four-way valve, it directs the refrigerant to the next component in the cycle based on its configuration.
In heating mode, the fan blows outdoor air over the outdoor coil, and the heat energy in the air is absorbed by the refrigerant in the coil. The refrigerant is then directed to the compressor via the reversing valve since it is in heating mode.
The refrigerant leaves the compressor as a high-pressure, high-temperature vapor. It then enters the indoor coil, where cooler indoor air passes over it, heating the space and condensing the refrigerant into a liquid. The liquid refrigerant then flows to the system’s thermal expansion valve (TXV), where it expands into a gas and then flows over the outdoor coil, restarting the cycle.
In cooling mode, the gaseous, high-pressure refrigerant leaves the compressor and enters the reversing valve, which is positioned to direct the refrigerant flow to the outdoor coil. There, heat is released from the refrigerant into the surrounding environment, condensing it into a high-pressure, low-temperature liquid.
Next, the refrigerant travels to the TXV, where its pressure and temperature decrease, entering the indoor coil as a two-phase mixture. When energy from the warmer indoor space is absorbed by the refrigerant, and the area is cooled, the refrigerant completes its phase change from liquid to gas.
The refrigerant then returns through the reversing valve and enters the compressor, repeating the process. In a heat pump‘s heating cycle, the indoor coil essentially functions as the system’s condenser, while the outdoor coil serves as the evaporator.
Check Valves
Sometimes heat pump systems have two TXVs—one for heating mode and one for cooling mode. To ensure that the refrigerant flows through only one of them, check valves function similarly to railroad switches, directing the refrigerant to either flow through the TXV or bypass it, depending on the mode. Below are two examples of check valves: the rotary check valve (top) and the spring check valve.
What are the Different Types of Heat Pumps?
Now that we’ve covered what a heat pump is and some of its unique components, let’s discuss some common types. Heat pump systems come in various types, all performing the same function but in different ways. Some more common configurations include:
Air-to-Air Heat Pumps
Air Source Heat Pumps
Ground Source Heat Pumps (GSHP)
Air-to-Air Heat Pumps
Air-to-air heat pump systems, also known as air-source heat pumps, are designed with simplicity in mind. They draw in outdoor air and use a standard refrigeration cycle to condition the air for cooling or heating a space.
Due to their efficiency and versatility, air-to-air heat pumps are very popular in residential and small commercial applications in moderate climates. When you hear “heat pump“, you might think of these systems, and there’s a good chance you have a similar system in your home. The image above shows a typical residential heat pump system, and the one below is a commercial heat pump in an apartment building.
Air-to-Water Heat Pumps
Air-to-water heat pump systems operate similarly to air-to-air heat pump systems. However, instead of absorbing heat from the external air to heat the air stream, they use the refrigeration cycle to control the temperature of a fluid flow.
In heating mode, heat is extracted from the cold outside air into a working fluid (typically water or a glycol/water mixture) through a standard compression/expansion cycle. Then, the heated water is distributed throughout the building for space heating, domestic hot water, or other useful functions. In cooling mode, the process is reversed, with heat being transferred from the working fluid to the outside air through the refrigeration cycle, and the absorbed heat is expelled to the external environment.
Ground Source Heat Pump (GSHP)
The Earth’s crust maintains a relatively stable temperature throughout the year at depths greater than 15-20 feet. Ground source heat pumps (GSHP) – also known as ground-coupled heat pumps or geothermal heat pumps – harness this thermal energy in a closed-loop system.
Ground source heat pumps feature buried heat exchangers, or ground loops, as shown below, connected to the rest of the heat pump components located within the structure. A propylene glycol solution is often used as the working fluid due to its lower environmental impact compared to ethylene glycol.
In heating mode, thermal energy from the Earth is absorbed by the fluid within the ground loop. This raises the temperature of the working fluid, but not enough to meet the load requirements. The semi-heated fluid mixture is then sent to the evaporator of the system, initiating a vapor compression/expansion cycle that heats the fluid to a level suitable for space heating or domestic hot water.
In cooling mode, hot air from the building is circulated through the geothermal loop, where the cooler temperature of the Earth regulates the fluid within it. The heat is then returned to the indoor heat pump. There, the refrigeration cycle is used to extract the remaining energy required to meet the system’s demands. The geothermal loop can be buried underground, submerged in groundwater, or connected to wells or other water sources. Geothermal heat pumps involving water are often referred to as water-source heat pumps, but the system functions and components are the same.
While the upfront costs of ground source heat pumps can be quite high, once installed, they are highly efficient. It’s not uncommon for such systems to output significantly more energy than they consume, with performance coefficients (COP) as high as 4:1 being quite typical.
In summary, heat pumps are versatile heating and cooling systems that utilize a reversible refrigerant cycle to provide both heating and cooling functions. They differ from standard air conditioning units in that they can switch between these modes as needed, thanks to specialized components like reversing valves and check valves. There are various types of heat pumps, including air-to-air, air-to-water, and ground-source heat pumps, each with its own advantages and ideal applications.
Air-to-air heat pumps, also known as air source heat pumps, are simple and commonly used in residential and small commercial settings in moderate climates. They draw in outdoor air and condition it for heating or cooling indoor spaces.
Air-to-water heat pumps operate similarly but use the refrigeration cycle to control the temperature of a fluid flow, making them suitable for space heating, domestic hot water, and other uses.
Ground source heat pumps, or geothermal heat pumps, tap into the Earth’s stable temperature at greater depths to provide efficient heating and cooling. They use buried heat exchangers and are highly efficient once installed.
The choice of heat pump type depends on factors such as climate, available space, and specific heating and cooling needs. Heat pumps are known for their efficiency, making them an environmentally friendly option for many applications.