An air/water heat pump is able to extract heat from the outside air for heating and hot water preparation, even at temperatures below 20° Celsius. This is possible because the refrigerant used in the heat pump evaporates even at this low temperature.
HOW AN AIR/WATER HEAT PUMP WORKS
The operation of the refrigerant circuit, with which the air/water heat pump uses ambient air to produce heat, can be clearly explained in four steps. In the first phase, a fan sucks in external air and transmits it to a heat exchanger, the so-called evaporator. A refrigerant circulates in this evaporator which changes its physical state even at the lowest temperatures. When the initially liquid refrigerant comes into contact with the warmer outside air supplied, it heats up until it evaporates.
In the second phase the steam flows into an electric compressor. The compressor compresses the refrigerant vapor, further increasing its temperature. It’s basically the same as an air pump that heats up when you use it to pump air through the narrow valve in a bicycle tire.
Once the vapor in the compressor has reached the required temperature, in the third stage it flows to another heat exchanger, the condenser. Here the refrigerant releases its heat to the water in the heating system, where it cools and gradually returns to its liquid state (condensation).
In the fourth and final stage, the refrigerant flows through an expansion valve, whereby the temperature and pressure drop to the initial level and the cycle can start again.
COMPACT OR SPLIT DESIGN
Air/water heat pumps are available as compact devices or as split devices. Compact devices consist of a single device that contains all the components described above. It can be placed indoors or outdoors. When installed indoors, the compact appliance draws in outside air through a duct running through the outside wall and directs it back outside through another duct on the outside wall.
Split version air/water heat pumps consist of two devices, an internal and an external unit. The outdoor unit houses the fan with the evaporator, while the other components are located in the indoor unit. The split device is usually cheaper than a compact device and does not require large openings in the walls.
However, the noise generated by the fan can become a problem when setting up the outdoor unit. Therefore, when choosing a split heat pump, pay attention to its noise emission. Silent devices have a sound power level of less than 50 dB(A). When installing the outdoor unit, you must also take into account the distance from the neighboring house and possible sound absorption, for example through hedges or walls.
THE ANNUAL EMPLOYMENT NUMBER (JAZ)
Electricity is usually needed to run a heat pump’s compressor. The less electricity needed to generate heat, the more efficient the heat pump will operate. Efficiency is measured using the annual performance factor (JAZ). This number indicates the ratio of electrical energy used to thermal energy produced, i.e. the higher the JAZ, the better. A very good value, for example, is if a heat pump produces 6,750 kWh of thermal energy in a year using 1,500 kWh of electricity and therefore has a JAZ of 4.5.
The measured efficiency of a heat pump depends not only on the efficiency of the device itself but also on the operating conditions of the heating system. If more heat is needed in the house than the heat pump can currently draw from the room air, an electric heating element is usually turned on. In this way, the water in the heating circuit is heated 1:1 electrically. This is not very efficient and increases electricity costs.
INSTALLATION OF AN AIR/WATER HEAT PUMP
The air/water heat pump works more efficiently the smaller the temperature difference between the ambient air and the flow temperature of the heating system. This means that the compressor has to work less and energy consumption remains at a low level. For this reason, the air/water heat pump is particularly suitable in combination with underfloor heating, which itself requires a lower flow temperature than conventional radiators.
Furthermore, the house should be well insulated so that the overall energy requirement for heating is lower. This is always the case with new buildings nowadays. In an existing building, good thermal insulation is a necessary prerequisite for it to make sense to subsequently install an air/water heat pump as a heat generator.
When combining an air/water heat pump with underfloor heating, it is absolutely not necessary to install a buffer tank for the heating function. The buffer storage is used to accumulate heat, i.e. to store it temporarily, so that the heat pump can work constantly, even if the heating circuit does not require heat. With underfloor heating, however, the entire screed becomes a heat accumulation, so buffer storage is not necessary. Something different is the hot water tank, which is always needed to maintain a supply of heated drinking water for showering, bathing or washing hands.
Correct sizing is important for the long life of the air/water heat pump. During planning it is necessary to ensure that the power of the heat pump corresponds to the heat needs of the house and is not chosen too high. If the minimum power is too high, during the transition period the machine will not work efficiently and continuously, but will constantly turn off and on. This increases wear. On the other hand, the heat pump must not be so small that its power in continuous operation is rarely sufficient to heat the house and that the electrical resistance must intervene constantly. This is inefficient.
ADVANTAGES AND LIMITATIONS OF AN AIR/WATER HEAT PUMP
Consequently, the air/water heat pump, provided it is correctly sized, is a good choice for heating and hot water preparation in your home in combination with underfloor heating. The system is relatively cheap to purchase compared to other heat pumps and takes up little space in your home. Furthermore, the air/water heat pump can also be used to cool the house in summer by reversing the refrigerant cycle. And their energy source, ambient air, is available in unlimited quantities and is guaranteed to be renewable.
However, an air/water heat pump is only actually climate-friendly to the extent that the electricity used comes from renewable energies and not, for example, from coal-fired power plants. Especially because the electricity consumption is inherently higher than that of a geothermal or groundwater heat pump. For this reason the air/water heat pump reaches its limits in terms of profitability in uninsulated or poorly insulated homes. A limitation is also the installation of an air/water heat pump in very cold areas with constantly low external temperatures, where the high power requirement leads to high energy consumption for the compressor and the electrical resistance.
Apart from that, it makes a lot of sense to combine an air/water heat pump with a photovoltaic system. Because in any case the electricity you need costs less from your roof than from the grid. And where photovoltaic systems are still required by law, as in Baden-Württemberg, this combination occurs almost automatically.
However, particularly careful planning is required to properly coordinate the components and subsequently control the power flow. Your nearest construction partner will be happy to help you. Because at the air/water heat pump, correctly sized, with or without a photovoltaic system, is the first choice for heating and hot water preparation of your dream home.
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