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The role of heat pumps in the energy transition is important. But the question arises as to whether they also work in old buildings and whether extensive renovation measures are required. It is also relevant what the situation is with existing apartment buildings. You will find answers below.

Use of heat pumps in old buildings

Using heat pumps in old buildings – does it make sense?

Heat pumps play a crucial role in the goal of supplying our buildings entirely with CO2-free heat in the next 20 to 30 years. Old buildings will also have to be taken into account sooner or later. Nevertheless, there is still a persistent prejudice that heat pumps are only profitable in new buildings. But is that really the case? Answers can be found in the research results of the Fraunhofer Institute for Solar Energy Systems (ISE).

Over the past 20 years, Fraunhofer ISE has measured and analyzed around 300 heat pump systems in the field. These included systems in both new buildings and old buildings with varying degrees of renovation. In this article, we would like to present the results to you. In addition, Fraunhofer ISE has also focused on existing multi-family houses with a separate project. You can also find out the results of this research work here.

Considerations for using heat pumps in existing buildings

In general, only two questions need to be answered to determine whether a heat pump in an old building is worthwhile:

Is it technically possible? Can the heat pump reach the required flow temperatures?

Does it make sense from an ecological and economic point of view?

Fraunhofer ISE answers both questions with a clear yes. A heat pump in an old building definitely makes sense. In some cases, the challenge of finding a suitable technical solution is greater than in others. Nevertheless, it is feasible in most cases, even if the old building has poor energy properties and no renovation measures are planned at first.

Anyone who is faced with the question of whether to install a gas heating system or a heat pump can choose the second option with a clear conscience. Although the initial costs are somewhat higher, you save a considerable amount on heating costs every month. Even with the current electricity generation mix of around 50 percent renewable energy, the heat pump is already more environmentally friendly than a gas heating system. In the future, this ecological aspect will continue to improve.

In the following section, we will address important questions that often arise when choosing a heat pump for an old building. We will start with the question of whether this technology also works with high flow temperatures and with radiators of an older design. You often hear that heat pumps require large heating surfaces and low operating temperatures, such as those offered by underfloor heating.

Do heat pumps deliver sufficiently high heating circuit temperatures? Heating systems with a flow temperature of 60 degrees Celsius or 70 degrees Celsius are often used in old buildings. Is it technically possible to operate heat pumps at such temperatures? Absolutely, although this also depends on the type of coolant or compressor. Standard heat pumps can easily achieve flow temperatures of 55 to 60 degrees Celsius. However, there are also high-performance heat pumps on the market that can deliver 65 to 70 degrees Celsius, especially those with propane as a coolant.

Measurements by Fraunhofer ISE have shown that heat pumps can deliver pleasant temperatures even on very cold days. The systems often even work without the additional use of heating rods that heat with electricity. The heat pumps still work efficiently, although not quite as efficiently as on warmer days. Nevertheless, they are in the green zone. This means that heat pumps are worthwhile compared to gas heating systems even in the most adverse temperatures.

Fraunhofer ISE measured this in the first two weeks of February 2021, when the average temperature was -3.6 degrees Celsius. In the last 50 years, there have only been 5 months with average temperatures below this value. During this time, the average efficiency of heat pumps in old buildings was 2.3. This means that more than twice as much heat could be extracted from the ambient air from each kilowatt hour of electricity, even in this cold weather.

Renovate first, then heat pump, or vice versa? This question has already been answered elsewhere: you can also install the heat pump first and then renovate the house later if necessary. In general, however, it is advantageous if a room requires little energy to get up to temperature. However, this applies to all heating systems, not just heat pumps.

As learned in the previous chapter, heat pumps can also be operated with higher flow temperatures. However, since many of the old gas or oil heating systems are oversized, switching to a heat pump can often lower the flow temperature. It can also be helpful to replace individual radiators. Modern radiators can give off the same amount of heat with significantly lower heating circuit temperatures. Replacing the windows can also improve the efficiency of the heat pump.

Now the question arises whether a heat pump is oversized if it is first installed in an unrenovated building that is then gradually upgraded in terms of energy. Here, too, the Fraunhofer ISE gives the all-clear. Technically speaking, heat pumps can respond flexibly to different requirements. Many modern heat pumps are also equipped with inverter technology, which enables easy power control.

A simple test provides clarity. Heat pumps achieve the best results at low flow temperatures between 35 and 55 degrees Celsius. Therefore, if you are thinking about (or are forced by the new Building Energy Act) equipping your old building with a heat pump, you should first test whether your existing heating system can cope with a flow temperature of 55 degrees Celsius, even on cold winter days with frost. Observe whether your comfortable temperature is still reached. If this is the case, converting to a heat pump should be no problem. If the rooms are not warm enough, other measures such as replacing the windows, insulating the facade or installing larger radiators will probably be necessary.

How well do heat pumps work in old buildings? Here too, Fraunhofer ISE has carried out extensive field research, with both air and ground heat pumps. In general, it can be said that ground systems work more efficiently and have an average efficiency of 4.1. For air heat pumps, however, the average value is 3.1. This is mainly because air-water heat pumps often have to work with colder temperatures than those found in the ground.

In general, it can be said that the efficiency of heat pumps increases as the flow temperatures fall. This means that heating systems with underfloor heating tend to have higher efficiency values ​​than systems with higher flow temperatures. However, this does not mean that heat pumps with radiators cannot be operated efficiently.

The measurements have shown that the heating circuit temperatures are not always decisive for the efficiency of the systems. High efficiency values ​​can also be achieved with high flow temperatures. Much depends on the planning, installation and settings of the heat pump. It is helpful if the installer is familiar with the challenges of old buildings. Fraunhofer ISE already rates the current average values ​​for the efficiency of heat pumps in old buildings as good. However, there is still room for improvement through improved models and further innovations.

Here are two practical examples:

With regard to energy-renovated old buildings, it is easy to check the efficiency of heat pumps – the results are just as good as for new buildings. But what about unrenovated buildings? Can heat pumps still contribute to energy savings there? Fraunhofer ISE has examined two such cases in more detail and obtained surprising results:

The first house examined is 84 years old and was in very poor energy condition, with hardly any renovation work carried out. Heating energy consumption was a high 210 kWh/(m²a). When the air heat pump was installed, the radiators were also replaced. Despite the high heating demand, the heat pump achieved a good efficiency of 3.0. The heating element hardly had to be switched on and accounted for less than one percent of the energy demand.

In the second, 48-year-old house, a ground source heat pump was installed. The building was in a better energy condition than the first and had a heating energy consumption of around 100 kWh/(m²a). Here the system achieved an efficiency of 3.7, although the same panel radiators as before were used for the oil heating.

Another aspect concerns the efficiency of the heating rod as additional heating. Heat pumps are often equipped with a heating rod that can generate heat from electricity alone or in parallel with the heat pump operation. Compared to the heat pump, the efficiency of the heating rod is much lower. The measurements by Fraunhofer ISE showed that the proportion of energy required by the heating rods was on average only 2.8 percent for air-water heat pumps and only 1.2 percent for ground source heat pumps. In many cases, the use of the heating rods for heat production was not necessary at all, even in extremely cold temperatures.

These practical examples illustrate that heat pumps can also make a contribution to energy savings in unrenovated old buildings. However, efficiency depends on various factors, such as the condition of the building, the type of heat pump chosen and the correct dimensioning of the system.

It is important to consider both the ecological and economic aspects of heat pumps in existing buildings. From an ecological point of view, the use of heat pumps in combination with renewable energies is almost always worthwhile and leads to significant CO2 savings. When it comes to economic aspects, investment plays a key role.

The economic perspective is not easy to consider, as energy prices are currently fluctuating greatly and it is uncertain how the situation will ultimately develop. It is known that the investment costs for heat pumps are higher compared to gas or oil heating. However, these costs could decrease if more heat pump systems are sold.

As far as operating costs are concerned, it is difficult to make precise statements. The costs calculated by Fraunhofer ISE for heating with heat pumps or gas are already outdated. It is assumed that heating with gas will be more expensive in the future than heating with a heat pump. Currently, due to current gas prices, heat pumps are in most cases more cost-effective than gas heating, regardless of the efficiency of the systems.

The figures and research results from Fraunhofer ISE used in this article come from an interesting blog series by Dr. Marek Miara about heat pumps in existing buildings. In this context, the conversation between the scientist from Fraunhofer ISE and the YouTube channel “erneuerbare tv” about heat pumps in old buildings is also worth mentioning.

Heat pumps can also work efficiently in unrenovated old buildings if the heating demand does not exceed 150 kilowatt hours per square meter. If the demand is higher, insulation measures such as facade or roof insulation should be considered. Insulating the top floor ceiling is sufficient if the attic is not occupied. It is also advisable to insulate the basement ceiling to minimize energy losses.

Heat pumps are not yet widely used in existing multi-family houses, especially if the buildings are already older. As part of the joint project “LowEX in Existing Buildings”, Fraunhofer ISE, together with partners, analyzed, developed and demonstrated solutions for the use of heat pumps in energy-renovated multi-family houses. New technologies such as a multi-source heat pump system, a hybrid system, facade-integrated ventilation units and high-temperature heat pumps were tested.

The results show that the multi-source heat pump system can achieve high source temperatures and has a good annual performance factor of 3.2. CO2 emissions were reduced by 42 percent compared to the initial state and by an impressive 73 percent compared to the uninsulated state of the building in 1963.

Experts rate the results of the project positively and emphasize the importance of an individual analysis of each situation. It is recommended to carry out a hydraulic adjustment of the heating system and to check whether the flow temperatures can be further reduced by replacing individual radiators. There is still a need for research in areas such as the replacement of gas-fired central heating systems, high-temperature provision and drinking water heating using heat pumps. Nevertheless, the project has shown that heat pumps are certainly feasible in existing multi-family houses.