A heat pump is an energy-efficient cooling and heating system that is installed outside of your home and connected to an indoor mini-split unit. These HVAC systems cycle hot and cold air where you want it depending on the season and provide personalized comfort throughout your home year-round.
Gaining early popularity in the 1970s, previous generations of these systems were mainly considered for milder climates. Modern heat pumps, especially variable speed mini-split heat pumps, are reliable and sustainable, providing high-performance heating and air conditioning to homes anywhere, even extremely cold climates. The system consists of an indoor air handler, which can be ducted or ductless, and an outdoor condenser unit.
Heat pumps do not create heat. They redistribute heat from the air or ground and use a refrigerant that circulates between the indoor fan coil (air handler) unit and the outdoor compressor to transfer the heat.
In cooling mode, a heat pump absorbs heat inside your home and releases it outdoors. In heating mode, the heat pump absorbs heat from the ground or outside air (even cold air) and releases it indoors.
One of the most important things to understand about heat pump operation and the process of transferring heat is that heat energy naturally wants to move to areas with lower temperatures and less pressure. Heat pumps rely on this physical property, putting heat in contact with cooler, lower pressure environments so that the heat can naturally transfer. This is how a heat pump works.
STEP 1
Liquid refrigerant is pumped through an expansion device at the indoor coil, which is functioning as the evaporator. Air from inside the house is blown across the coils, where heat energy is absorbed by the refrigerant. The resulting cool air is blown throughout the home’s ducts. The process of absorbing the heat energy has caused the liquid refrigerant to heat up and evaporate into gas form.
STEP 2
The gaseous refrigerant now passes through a compressor, which pressurizes the gas. The process of pressurizing the gas causes it to heat up (a physical property of compressed gases). The hot, pressurized refrigerant moves through the system to the coil in the outdoor unit.
STEP 3
A fan in the outdoor unit moves outside air across the coils, which are serving as condenser coils in cooling mode. Because the air outside the home is cooler than the hot compressed gas refrigerant in the coil, heat is transferred from the refrigerant to the outside air. During this process, the refrigerant condenses back to a liquid state as it cools. The warm liquid refrigerant is pumped through the system to the expansion valve at the indoor units.
STEP 4
The expansion valve reduces the pressure of the warm liquid refrigerant, which cools it significantly. At this point, the refrigerant is in a cool, liquid state and ready to be pumped back to the evaporator coil in the indoor unit to begin the cycle again.
A Heat pump in heating mode operates just like cooling mode, except that the flow of refrigerant is reversed by the aptly named reversing valve. The flow reversal means that the heating source becomes the outside air (even when outdoor temperatures are low) and the heat energy is released inside the home. The outside coil now has the function of an evaporator, and the indoor coil now has the role of the condenser.
The physics of the process are the same. Heat energy is absorbed in the outdoor unit by cool liquid refrigerant, turning it into cold gas. Pressure is then applied to the cold gas, turning it to hot gas. The hot gas is cooled in the indoor unit by passing air, heating the air and condensing the the gas to warm liquid. The warm liquid is relieved of pressure as it enters the outdoor unit, turning it to cool liquid and renewing the cycle.
There are several different types of heat pumps, each with its own unique features and capabilities. The three most common types of heat pump systems are air-source, water source, and geothermal.
Air-source heat pumps work by extracting heat from the air outside and transferring it indoors during the winter months to heat a home or building and reverses the process during the summer months to cool a home or building.
Air-source heat pumps can be ducted or ductless. Ductless systems transfer heat via a refrigerant line, while ducted systems use a home’s existing ductwork to move heat throughout the space.
Mitsubishi Electric Trane heat pumps can be single-zone or multi-zone. Single-zone systems are designed to control the temperature of a single area or room. On the other hand, multi-zone systems are designed to control the temperature of multiple areas or rooms. This allows for different temperatures to be set in each zone, based on the specific needs of each area. Each zone has its own thermostat, which allows for independent temperature control and greater flexibility in managing comfort levels throughout the home or building.
Water source heat pumps use a nearby body of water to heat in when heating mode and as a heat sink when in cooling mode.
They can extract heat from water even if the water temperature is lower than the desired temperature in the home. The two primary types of water source heat pumps are closed loop and open loop systems. Closed loop systems are used with lakes and large ponds by submerging sealed pipes with anti-freeze into the water, and those pipes then circulate the heat it gathers back to the heat pump. Rather than using sealed pipes and anti-freeze, open loop systems take water from the source, extract heat energy, and then return the cooled water back to the source.
Geothermal heat pumps (also known as ground source heat pumps) use the ground as the source of heat in heating mode and as a heat sink in the cooling mode.
Closed loop systems circulate anti-freeze or refrigerant through pipes or tubing that is buried in the ground. A heat exchanger transfers heat between refrigerant in the heat pump and the antifreeze. Open loop systems use water from a well or surface body of water and a heat exchanger to extract heat from the water before returning it to the source.
Homeowners in need of a new heating or cooling system, may consider the type of climate they live in before purchasing a heat pump system. Heat pumps are more common in milder climates, where the temperature does not typically drop below freezing. In colder regions, they can also be combined with furnaces for energy-efficient heating on all but the coldest days. When the temperature outside drops too low for the heat pump to operate effectively, the system will instead use the furnace to generate heat. This kind of system is often called a dual fuel system – it is very energy efficient and cost effective.