How does an air source heat pump work? There are three different cycles for an air-source pump. They are:
- Heating Cycle – It forms the source of thermal energy needed in a building
- Cooling Cycle – It gets rid of unneeded thermal energy in a building
- Defrost Cycle – It is responsible for getting rid of frost that collects on outdoor coils
During the heat cycle, heat is gathered from the air outside the building and directed indoors:
- The first step involves the refrigerant liquid pushing through the expansion device and converting it to a low-pressure liquid and vapour mixture. Next, it passes to the outdoor coil, which is the evaporator coil. The work of the liquid refrigerant is to take in heat from outside air, boil, and transform it to vapour at a low temperature.
- The vapour moves via the reversing valve towards the accumulator; the accumulator gathers any leftover liquid before the vapour goes into the compressor. The vapour is compressed, causing it to reduce in volume and gain more heat.
- In the final step, the reversing valves pushed the hot gas to the indoor coil – the condenser. One can find the indoor coil in the ductwork near the furnace. The heat in the hot gas moves to the indoor air, which converts the refrigerant from a gas into a liquid state.
The liquid returns to the expansion in readiness for another heating cycle to begin again.
Outside Temperature Matters
Since the pump gets heat from outdoor air, the temperate of the air outside the building affects the heating pump’s capability to gather heat and take it inside the building. When the temperature of the outdoor air decreases, the heat pump’s capacity to take in heat also falls.
At this point is where the air-source heat pump’s thermal balance comes to the fore. The thermal equilibrium is the temperature when the pump’s heating ability is the same as the heat loss in the building. When the outdoor air temperatures fall below the thermal balance point, the pump can supply only a part of the heat needed for comfortable living space, not to mention the required supplementary heat.
Also, please note that many types of air-source have a minimum operating temperature; they cannot operate if temperatures fall below this threshold point. The more recent air-source models have a minimum operating temperature between -15°C and -25°C. If temperatures drop beyond this point, you have to go for the supplementary heating system to the building.
The work of the cooling cycle is to take the heat from the house, making the indoor air cool during the hot seasons. In essence, it reverses the heating process described in the previous cycle by ejecting heat from inside the house.
The liquid refrigerant is pushed via the expansion device inside the heating cycle, converting the liquid into a low-pressure mixture of liquid and vapour. Subsequently, the low-pressure fluid moves to the indoor coil, the evaporator. The liquid refrigerant takes in the heat from the indoor air and changes into a low-temperature vapour.
The vapour is passed via the reversing valve and taken to the accumulator; the accumulator gathers any leftover liquid and takes it to the compressor. The heat pump will compress the vapour, causing it to reduce in volume and increase its heat levels.
In the final step, the hot gas is passed via the reversing valve and taken to the outdoor coil, which functions as the condenser. The heat absorbed from the hot gas moves to the outdoor air, which allows the refrigerant to change into a liquid. This refrigerant liquid transfers back to the expansion drive to enable the cycle to occur again.
The heat pump removes humidity from the indoor air during the cooling cycle. The humidity removal happens because the moisture in the air condenses as it passes through the indoor coil, collecting in a pan situated at the lower end of the coil. The condensed fluid flows to the house drain via the condensate drain.
Frost forms when air moisture condenses and freezes on top of the outside coil as the air passes over the coil. This phenomenon happens the temperature of outdoor air falls below or close to the freezing point when the heat pump is running in the heating cycle. The amount of frost may be high depending on the temperature of outdoor air and the amount of moisture in the air.
The accumulated frost reduces the coil’s ability to transmit heat to the refrigerant. The heat pump needs to switch to defrost mode sometime later to remove the frost.
The most familiar approach for defrosting is:
At the start, the reversing valve changes the mode of the device to the cooling cycle. The heat pump transfers hot gas to the outdoor coil to melt frost during the cooling stage. Simultaneously, the outdoor fan goes off to minimize the heat required to melt the frost. Typically, the outdoor fan blows cold air on top of the coil.
While these processes occur, the heat pump cools the air inside the ductwork. Usually, the heating system warms this air as it moves throughout the building premises.
The unit will use either of these two methods to define when it begins the defrosting mode:
- Demand-frost controls monitor the buildup of frost, controls for demand frost monitor the airflow, refrigerant pressure, coil temperature, and pressure difference along the outdoor coil.
- Time-temperature frost starts and stops the time-temperature defrost mode. The defrosting cycle can begin after every half hour, one hour, or one and half hours, subject to the device’s design or the area’s climate.
Excessive defrost cycles minimize the periodic performance of the heat pump. For this reason, the demand-frost method is better and more effective as it begins the defrost cycle only when the need arises.
Supplementary Heat Sources
One should consider setting up a supplemental heating source for air-source heat pump operation. A backup heat source is essential since manufacturers design air-source heat pumps with an outdoor operating temperature falling between -15°C to -25°C. Additionally, you can expect air-source heat pumps to experience lower heating capacity during freezing temperatures.
Additionally, supplementary heating may be necessary for periods when the building’s heat pump goes through the defrost cycle.
Some of the available options for supplementary heating are:
The Fully Electric System: In this option, the electrical resistance components located in the ductwork or electrical baseboards provide the required supplementary heating solutions. These resistance components have low efficiency compared to the heat pump. However, outdoor temperatures do not influence their capacity to provide heat.
The Hybrid System: In this system, the air-source heat pump depends on a supplemental system like a furnace or boiler set up in a hybrid configuration. It is a viable option when including a heat pump in an already existing system, such as replacing a central air conditioner using a heat pump.