Domestic hot water heat pumps are used exclusively for the provision of hot water, but not as a heating system. This allows the production of simple and inexpensive models because, on the one hand, the energy requirement is lower than for a heating system and, on the other hand, no connection to the buffer tank of the heating system is required. Domestic hot water heat pumps have their own water storage tank, which typically has a capacity of about 300 liters.
How does a domestic hot water heat pump work?
The way it works is no different from that of other heat pumps. However, the ground or groundwater are extremely rarely tapped as heat sources because the financial outlay for deep drilling is too high. Such systems only pay off for private households if they reduce heating costs noticeably. Instead, the air is used as a heat source. Ideally, preheated air is used for this purpose if the possibility exists. For example, the exhaust air from ventilation systems or basement rooms heated by a boiler that is not optimally insulated can be considered. However, it is also possible to simply use the outside air. In winter, however, this method is not very effective because the efficiency of a heat pump drops significantly if too large a temperature difference has to be bridged. A difference of perhaps 70 degrees between the air temperature and the desired water temperature is simply too much.
A domestic hot water heat pump can supplement other methods of water heating or replace them. Heating systems with a combined hot water and heating tank can be turned off in the summer when the heat pump takes over the hot water supply. This can noticeably reduce energy costs because heating systems operate at very poor efficiency when they have a low load in the summer. Heat pumps that use outside air as a heat source is suitable for this application since the heating system takes over the hot water preparation in winter. It is also possible to replace an electric instantaneous water heater with a domestic hot water heat pump. To make this possible, heat pumps contain an electric heating element that is automatically switched on if the desired temperature cannot be reached in winter.
The complicated question of life cycle assessment
The question sounds simple: does a domestic hot water heat pump reduce CO2 emissions? The answer is that the life cycle assessment has improved significantly in recent years, but this is not due to heat pumps. Air-source heat pumps typically deliver two to three times the electrical energy used as heat, so they have an annual performance factor between two and three. Compared to the considerably more expensive geothermal heat pumps, this is little. Nevertheless, the ecological comparison with an electric instantaneous water heater is clear, because more than half of the electricity is saved. The comparison with a conventional gas heater with an efficiency of around 90 percent is more complicated. An annual performance factor of three is often cited as the lower threshold above which a heat pump produces fewer emissions than a gas heater. However, this calculation dates from 2008, when the electricity mix in the power grid has changed significantly. So today, a heat pump causes fewer CO2 emissions because more green electricity is flowing in the power grid. As a result, the threshold for a positive eco-balance has currently dropped to an annual performance factor of around 2.1.
Acquisition costs and operating costs
The initial costs of a domestic hot water heat pump are significantly lower than the costs of a solar thermal system, for example. There is already a wide range of models in the price segment of around 2,000 euros. On the other hand, the operating costs are significantly higher. These are determined almost exclusively by the power consumption, the maintenance costs fall compared with it hard into the weight. The power consumption of a hot water heat pump depends not only on water consumption but also on whether outside air or the preheated air of the boiler room is used as a heat source. As a rough guide, 400-kilowatt hours per year and per person can be used, but in individual cases, there may be significant deviations.
The simple entry into renewable energies
In summary, domestic hot water heat pumps can be described as an easy entry into renewable energies. Compared to an instantaneous water heater, they reduce electricity consumption by about 60 to 70 percent. Their impact on the total energy costs of a household is limited, as they do not affect heating costs. On the other hand, the initial cost is low and installation is simple.