This article is based on a paper published in the 62nd volume of the Research Group for cost-effective Passive House buildings on the topic of “ Cost-Effective Refurbishments”. The original article by Jürgen Schnieders is availabe in German in the proceedings, which can be accessed here.

This article primarily focuses on the best way to implement a heating system for a cost-effective building renovation. Related topics such as the increasingly important issue of air conditioning in summer and domestic hot water supply are also addressed.

However, the discussion should not be limited to heat pumps or to supplying heating and cooling. The building's heat demand also plays a role. In most cases, the question of whether to insulate first or replace the heating system first can be answered as follows: do whatever is currently due for renewal, and then do it right, because that is the most economical option. There are some exceptions: if the heating system in an existing building that effectively requires a flow temperature of 90 °C breaks down, it can't simply be replaced with a heat pump. Then, if district heating is not available, one will have to look at the specific case more comprehensively and decide whether improving the building envelope, replacing the radiators or investing in a hybrid heating system is preferable in this specific case.

Thermal protection to the Passive House standard should be aimed for regardless of the type of heat supply, as it is cost-effective and saves energy. The latter makes sense because for heating, even renewable energy will always be particularly expensive: to a large extent it will be needed when hardly any solar power is available. Another reason for a good level of thermal protection is: the lower the heating load, the easier it is to cover and the more options there are for doing so.

The PER demand provides information on how much a building will burden the future fully renewable energy system. It specifies how much renewable primary energy, e.g. PV or wind power, must be generated for the amount of final energy supplied at the property boundary. In the case of electricity, which can only be indirectly stored, the period of utilisation is also important; electricity for heating is particularly valuable, while electricity for cooling is relatively abundant. The PER demand quantifies these relationships and is therefore used below to evaluate alternatives.

In principle, a number of options are conceivable for providing space heating.

• A district heating connection is particularly suitable if the district heating pipeline already runs close to the building. This option is discussed in the Section about District heating

• A heat pump is certainly a future-proof solution. Various options are conceivable and are discussed starting from Section Air-to-water heat pump and in Wolfgang Feist's article about split units. With regard to cost-effective building renovation, we will concentrate almost exclusively on air as a heat source. The use of geothermal energy or groundwater usually results in at least double the already high investment costs of an air-to-water heat pump.

• Direct electric heating, whether with fan heaters/convectors, underfloor heating or “infrared heating”, can be unrivalled in terms of purchase costs, but has consumption costs that are 2 to 4 times higher than those of heat pumps. It is therefore only useful as supplementary heating with a small share of the total heat demand.
• Solar radiation as a basis for solar heating is practically unavailable in the depths of winter in central Europe. Heating solely using solar energy is only possible if heat or electricity from the summer is stored for use in the winter. Technically, this is possible, but the cost of a kilowatt hour from a storage facility that is only used once a year is very high (see also the next section on hydrogen). Large hot water storage tanks with a capacity of many thousands of cubic metres as part of a heating network constitute the best option for this. Such concepts have often failed in the past, but have currently become the subject of research again.

Without seasonal storage, supplementary solar heating can only cover a relevant proportion of the heat demand in poorly insulated existing buildings that still require significant heating in April and October. In such cases, a PV system to supplement the heat pump has an economic advantage over thermal solar collectors.  

• Burning natural gas for space heating is contrary to the objectives of climate protection. Installing new gas boilers despite this in the hope that the natural gas networks and boilers will be converted to renewable hydrogen in the future is not compatible with the goal of cost-effective supply. The reason for this is poor efficiency: while a heat pump requires around one-third of a kilowatt hour of electricity to generate one kilowatt hour of useful heat, the detour via hydrogen requires at least 1.5 kilowatt hours of electricity.

According to [Ariadne 2025], green hydrogen is expected to cost 16 cents/kWh in 2030 and 11 cents/kWh in 2045. According to the authors, hydrogen will therefore remain expensive in the long term and more expensive than electricity in particular. The advantage of storability can compensate for this shortcoming only during few hours of the year.

• Pellet boilers are currently (2025) subsidised in Germany, in the same way as heat pumps. However, whether they can contribute to climate protection is increasingly disputed. The limited availability of biomass for energy use has long been known (see, for example, [Nitsch 2004]), and this is taken into account in the PER system via the bioenergy budget. The issues with nature conservation and food production are also well known. In addition, although wood is a renewable raw material, the carbon emitted from combustion was previously bound due to the absorption of CO₂ from the air. However, depending on the period under consideration, the CO₂ balance looks different: if a tree is felled and burned, the CO₂ bound in it is released into the atmosphere, and it may take centuries for a tree of the same size to grow back in the same place. Therefore, based on [UBA 2024], the German Federal Environment Agency estimates the emissions resulting from combustion of wood pellets with 374 g/kWh. This makes the combustion of pellets far less favourable than that of natural gas (257 g/kWh). More favourable emission factors (19 g/kWh) are calculated only for waste wood from garden maintenance/forest thinning.

In any case, scarce and by nature storable bioenergy is not suitable for coverage of the base load for space heating on a broad scale.

• Oil boilers are scarcely being installed currently, and it is - rightly - expected that their market share will continue to decline. Supposed substitute fuels such as HVO100 or bio-based heating oil are not viable alternatives due to their limited availability and high prices.
• Hard coal and lignite are no longer an option for space heating anyway.

This leaves district heating and various types of heat pumps as the relevant, sustainable options. In conjunction with a heat pump, an additional peak load heat generator may be useful in larger buildings. For more information, see Section Insert: Control options and thermal protection.

District heating is a mastered and proven technology that is simple and long-lived from the utilisation point of view. Heat generation and maintenance, as well as finding solutions for decarbonisation are the responsibility of the provider. Prices are subject to government supervision since district heating constitutes a natural monopoly – usually there isn't any other district heating provider in the same location.

Economic operation of a district heating network requires that the high initial investments pay off; long-term supply contracts are therefore often common. A certain minimum heat demand density in the supply region is also necessary. Connection and usage of the district heating system that has been installed is therefore compulsory in some locations, in order to ensure that the costs of installing and operating the network are spread across a sufficient number of customers or a sufficient amount of supplied heat. However, this often leads to conflicts and even legal proceedings, which is why the PHI also advises against compulsory connection and usage. Some consider it inadmissible to demand the removal of an already installed heat pump or to prohibit switching from district heating to a heat pump ([BWP 2024]). District heating must therefore be attractive in its own right and competitive with individual heat pumps.

The prices and characteristics of district heating vary greatly from region to region, due to factors such as different heat generators, customer structure and the expenditure for heat distribution. Since May 2024, it is possible to compare many district heating suppliers in Germany at the price transparency platform for district heating www.waermepreise.info created at the suggestion of the federal government. A random selection of some suppliers as shown in Figure 1 indicates that the average heat price is around 17 cents/kWh, with values between 11 and 30 cents/kWh found in the database. Connection costs also vary considerably depending on the specific situation, with typical costs for a district heating connection ranging between €5,000 and €10,000.

Figure 1: Heat prices and network losses of a few randomly selected district heating suppliers

Network losses are also highly variable, typically reported at 10%, but can exceed 30% in small networks in particular. These losses must be compensated for through correspondingly more efficient and economical heat generation in relation to building-specific solutions.

Most district heating networks today must still be decarbonised. It is easier to access different heat sources in a large network than in individual systems. In this case, the low annual full-load hours of the individual heat generators must be compensated for by favourable energy prices at the relevant point in time. Mixed supply from large heat pumps, solar thermal systems, electrode boilers for surplus electricity, waste heat in connection with seasonal storage of renewable energies, e.g. from electrolysis or the reconversion of hydrogen, renewable methane or ammonia, is conceivable. The use of unavoidable waste heat is also an option here, although in many cases it should be questioned whether the waste heat is actually unavoidable. Waste incineration, for example, means releasing the carbon from the waste into the atmosphere, which is precisely what is supposed to be avoided; waste heat from industry may also indicate inefficient processes.

Overall, there are a number of challenges to be addressed by operators. If these are overcome, connection to the district heating network can be an attractive solution.

Currently, the most common type of heat pumps are those that use outside air as a heat source and transfer the heat by means of a conventional hydraulic heating system with radiators or underfloor heating and integrated hot water preparation.

As a rule, these systems operate with satisfactory efficiency. Numerous factors contribute to this:

• The generous subsidies granted in Germany by the BAFA are subject to a number of conditions. These include calculations proving the minimum efficiency level under the given boundary conditions, hydraulic balancing of the heating system according to a room-by-room heating load calculation and pipe network analysis and metrological monitoring of the coefficients of performance that are actually achieved. Even though the planning effort is considerable, this lays the foundation for efficient operation.
• The flow temperature generated by the heat pump should be as low as possible. In contrast, existing heating systems with condensing boilers must be set to the lowest possible return temperature to allow the utilisation of condensing technology in the first place (often enough this wasn't the case in practice). An existing heating system should therefore be adapted as far as possible to enable high volume flows and a small difference between the flow and return temperatures.
• Underfloor heating systems that are operated at low flow temperatures from the outset are advantageous. Sometimes these are coupled with individual radiators in the same heating circuit, whereby the temperature for the underfloor heating was reduced by a mixer till now. This significantly reduces the efficiency of the heat pump. In such systems, replacement of the usually few radiators with heat pump radiators or fan coils with lower temperatures should be considered, or they should be converted so that the heat for the underfloor heating is generated at a low temperature level by the heat pump directly.
• Improved thermal protection always allows the flow temperature to be lowered in an unchanged heating system (see Section Insert: Control options and thermal protection for the interplay of thermal protection and seasonal performance factor).
• The heat pump should preferably be dimensioned appropriately. Unlike fossil fuel-based heat generators, larger heat pumps are not only significantly more expensive, they often also have cyclic operation, which reduces both efficiency and service life. The usual design heating load methods include considerable reserves, so that the heat pump can be dimensioned to be slightly smaller.
• To avoid frequent cycling and provide sufficient heat for defrosting, a buffer storage tank is required for systems with radiators. In underfloor heating systems, the thermal capacity of the screed is sufficient for storage, but it must be available at all times.
• In addition to mixers for reducing the temperature, hydraulic separators in heat pump systems should also be avoided if possible. The latter are storage tanks with four ports that are connected to the heat pump on one side and to the heating system on the other. The hydraulic separator compensates for differences between the volume flows of the heat pump circuit (high minimum volume flow) and the heating system (very low volume flow under partial load), but also leads to some mixing of the water flows and thus to temperature losses in a typical range of 5 K.

A buffer tank also performs the same function; this has only one connection at the top so water therefore always flows through in one direction only, without mixing of the flows of both circuits.

• The heat pump manufacturer's control system offers the best chance of good functionality. External controllers can, for example, prevent utilisation of the inverter range (the output range in which the heat pump can run without cycling).
• Unfortunately, for efficient hot water generation, the existing hot water storage tanks of fossil fuel-powered systems often have to be replaced, preferably with a storage system from the heat pump manufacturer. The VDI 4645 recommends a heat transfer area of 0.25 m² per kW (minimum) heat pump output.

If several installation errors are combined, this can reduce the efficiency of the system by a factor of 2.

In Switzerland, a certification scheme was developed for smaller systems up to 15 kW, which specifies established hydraulic diagrams, among other things (https://www.wp-systemmodul.ch/de/).

As always, the simplest possible systems should be preferred. For example, if a wood-fired boiler and/or a solar thermal system are to be coupled, a suitable stratified storage tank for a single-family home alone can cost over 5,000 euros. In addition, significantly more careful planning and commissioning is required. If this is not done – as is unfortunately often the case – it can easily happen that the additional complexity means that no savings will be generated in the end.

It should be noted that fossil fuel heat generators also have varying degrees of efficiency depending on the installation. In an older field study [Wolff 2004] involving 60 condensing boilers, the average annual efficiency was 86% (based on calorific value), while the manufacturers gave values of around 99%. The individual values fluctuated between approximately 77 and 97%, meaning that the fluctuation range was approximately ±12%.

An appropriate hydraulic system is even more important for heat pumps. Boilers generally convert the fuel completely, and are therefore bound by the law of conservation of energy and lose heat primarily through differing exhaust gas temperatures. Energy conservation naturally also applies to heat pumps, but their efficiency varies independently of this depending on the flow temperature and start-up losses. This is also evident in field measurements. In [ISE 2020], seasonal performance factors were determined for 29 air-to-water heat pumps for space heating and domestic hot water heating in existing buildings. Some of the systems had a peak boiler. The average annual performance factor was 3.1, with results varying between 2.5 and 4.6, i.e. by ±30%.

In the case of heat pumps, efficiency depends much more on the flow temperature compared to other heat generators. Building owners report that even in existing buildings with moderate thermal protection, they achieve excellent coefficients of performance when all thermostatic valves are fully open and the heat pump is continuously operated.

This raises the question of whether this improved efficiency has a greater impact than the temperature reduction (hereafter referred to as “night setback”) in rooms that are not being used. With night setback operation, the average temperature of the building is lower (see Figure 3), and the transmission and ventilation losses and thus the heating demand are accordingly lower. On the other hand, heating up again after the night setback requires a lot of heat output, i.e. a high flow temperature for most of the output supplied during the day. If the whole building is continuously heated, more heat is required overall, but on the other hand, the heating curve can be lowered because the amount of heat required is spread over 24 hours of the day and across all radiators.

Whether the “night setback” or “continuous heating” option is more cost-effective can be determined with the help of a building simulation. For this purpose, a dynamic multi-zone model of a small end-of-terrace house corresponding to Figure 2 with different usage profiles was used.

Figure 2: Terraced house for building simulation with zoning

The calculation model has been deliberately kept simple in order to better highlight the key influencing factors. The building simulation provides the time curve of the required heat output for each room in the case of “night setback” or “continuous heating”, depending on the insulation standard. Figure 3 shows typical temperature curves for both cases.

Figure 3: Temperature curve in a poorly insulated building in the case of continuous heating (left) and temperature reduction outside of usage (right).

It is assumed that the heating system in the uninsulated old building with single glazing was formerly dimensioned for 90/70 °C. 120 W/m² of heating power is available, with a specific additional amount for heating up. The transferable output decreases disproportionately at lower flow temperatures, with a pessimistic assumption of 1.3 for the radiator exponent (Figure 4, left). The COP of the air-to-water heat pump depends proportionally on the Carnot efficiency of the flow temperature and outside temperature, but it is capped for small temperature differences (Figure 4, right).

Figure 4: Relative heating output of the heating system and assumed coefficient of performance of the heat pump as a function of the flow temperature, at an outside temperature of 5 °C

The simulation will then provide the required heat output for each hour, from which the flow temperature necessary for introducing this heat quantity into the individual rooms can be determined. In Figure 5, this flow temperature is shown as a function of the outside temperature for the hour in question. From this, an appropriate heating curve can be derived for the respective building and the respective operating mode.

Figure 5: Required flow temperatures in the case of continuous heating (left) and temperature reduction outside of use (right).

In the example in Figure 5, with steady continuous heating, the temperatures never exceed 40 °C, whereas after the night-time setback they can reach up to 70 °C. There are individual hours when a few hundred watts of power would be needed in small rooms even in summer, for example if a damp towel is drying in a washroom above the basement ceiling. The heating curve does not take this into account; instead, it is assumed that the missing output is covered by direct electricity. In actual operation, one would instead let the room temperature fall by a few tenths of a degree below the setpoint.

The heating curve is now set as a simple straight line and in each variant selected so that the total electricity consumption (heat pump plus direct electricity) is minimal. The flow temperature is limited to 65 °C, which is typical for heat pumps.

The result is shown in Figure 6: in the end, the type of operation proves to be largely irrelevant. Although the night-time setback reduces the heating demand especially in (energy inefficient) existing buildings that cool down quickly, it only brings minimal benefits in terms of electricity consumption, and even then, only in the inefficient existing buildings.

Improved thermal protection, on the other hand, has a double benefit: not only is the heating demand reduced, but at the same time a higher seasonal performance factor can be achieved with the same heat distribution system, because the improved thermal protection reduces the temperature difference that the heat pump has to overcome.

Figure 6: Heating and electricity demands for different insulation standards with and without night-time setback

In principle, even better control systems are conceivable. If a thermostatic valve is installed on each radiator that reports its opening status to a central control system via radio, this control system can adjust the flow temperature depending on the required output so that all radiators are supplied with just enough heat. Such valves that are equipped with LoRaWAN are available for just under 100 €.

This kind of approach was applied in a simulation in [Huchtemann 2013]. Initially, it became apparent that the competing regulation of flow temperature and thermostatic valve requires careful adjustment of the parameters even in this seemingly simple system. Subsequently, the annual COP of the heat pump was increased from 3.05 to 3.2 – the system achieves an improvement, but it is so small that the effort is only worthwhile if the heat demand is very high.

Currently (2025), air-to-water heat pumps are extremely expensive, at least in Germany (see also Section System cost comparison). A significantly cheaper option is an air-to-air heat pump based on split units that were originally developed for cooling. Here, the refrigerant circulates between an outdoor unit, which also contains the compressor, and one or more indoor units. The first split units using climate-friendly propane as a refrigerant are now also available. Split units adapted for space heating achieve a high level of efficiency even at low outdoor temperatures. The indoor units can be operated at sound levels below 20 dB(A) so that they are inaudible, but this does reduce their efficiency.

In Passive House buildings and retrofits to the EnerPHit standard, very few indoor units per residential unit are sufficient for heating the building. In Passive House terrace houses, it has been demonstrated on several occasions that a single mini-split unit costing around €3,000 can provide comfortable indoor conditions in winter, in which case sometimes it may be necessary to leave the internal doors open for a few weeks of the year.

Multi-split units have proved effective even in unrenovated apartment buildings. For example, the LEG has equipped several social housing apartment buildings with multi-split units in each apartment. Installation is usually unproblematic, even in inhabited buildings. An indoor unit is installed above the doors in living areas, and the refrigerant lines can be routed through the hallways. Electric radiators are installed in the bathrooms due to the high humidity. Hot water is provided by instantaneous water heaters or electric boilers. The investment costs for this type of electrification are estimated at €11,000 per residential unit. Billing for heating costs is generally very simple. However, if the social welfare office or job centre will be paying for the heating costs, the cooling function must be activated separately and the electricity for it must be billed separately. Cooling also requires a condensate drain for each indoor unit, which may incur additional costs.

More details about heating with split units can be found here.

Another option for heating and cooling an entire flat with just one indoor unit can be achieved with ducted split systems. In this case, the heat exchanger of the indoor unit and the connected fan are located in a larger ventilation duct, which can distribute a recirculated air volume flow of 200 to 500 m³/h to the living rooms and bedrooms. At the same time, this pipe network – which is usually located in the hallway above a suspended ceiling – can be used to transport the supply air from the ventilation unit. The recirculated air enters the space above the suspended ceiling through sufficiently large openings above the interior doors and returns to the heating and cooling heat coils. The supply air flows into the bathroom and kitchen as usual and is extracted from there. Such systems are often used in Passive House apartment buildings in Spain. They allow the distribution of slightly higher heating and cooling outputs (20–30 W/m²) in the home at a moderate temperature difference.

As an alternative to split units with directly condensing or evaporating refrigerants, water-based heating and where necessary cooling coils may also be used.

The main advantage is the low investment costs for both heating and cooling, because a suspended ceiling in the hallway and a supply air distribution network would already be necessary for a complete EnerPHit renovation in any case. Sound insulation between rooms may be somewhat limited due to the larger air transfer openings. It is also important to ensure that the system itself does not cause any noise in spite of the larger air volumes.

The cost of heat pumps increases relatively sharply with the output. At the same time, the efficiency of outdoor air heat pumps in particular drops noticeably at low temperatures. If we also take into account the fact that the lowest outdoor temperatures only occur for a few hours in the year, it makes sense to combine relatively small heat pumps with a separate peak load heat generator. In this way the heat pump will cover the base load. Below a certain temperature, it will either be superseded (“bivalent-alternative”) or supplemented (“bivalent-parallel”) by the peak load heat generator.

VDI 4650 specifies values for the respective coverage ratio for such hybrid systems. As can be seen from Table 1, considerable under-dimensioning is possible, especially for outdoor air heat pumps with bivalent-parallel operation, without the energy demand of e.g. a peak boiler accounting for a significant proportion.

**Table 1: Coverage percentage of the base load heat generator heat pump in hybrid systems. Example: If an outdoor air heat pump with bivalent parallel operation can cover 60% of the required output at standard external temperatures, it will still supply 98% of the required total heating energy. **

The reason for this is that the output of a typical outdoor air heat pump increases relatively quickly as the outdoor temperature rises, while at the same time the heat load and the required flow temperature decrease. Even if a lot of output is lacking at the design outdoor temperature, the heat pump can cover the load again at just a few degrees above this.

Manufacturers have recognised that in hybrid systems they can sell both a heat pump and a gas boiler. They are happy to offer such solutions, especially for existing buildings. The reasoning that the peak boiler could be decommissioned when the building envelope is renovated later on is fundamentally understandable.

However, for single-family homes, the expenditure for two separate heat generators, together with a gas connection, maintenance for both, and chimney sweeping nevertheless seems very high. In contrast, in larger buildings – even with district heating systems – such combinations may make sense especially since electricity generation will continue to rely on fossil fuels, particularly during cold periods. If the heat pump and peak boiler are connected to the same heat distribution system, it is advisable to use a coordinated system with a boiler, heat pump and control unit from the same manufacturer.

As Figure 7 shows, electricity prices on the stock exchange (as a measure of the required non-renewable energy) are already particularly high today when outside temperatures are low, a correlation that would be expected with an increasing proportion of heat pumps in poorly insulated buildings. This highlights the obvious benefits of very good thermal protection, which reduces energy consumption in winter and also allows periods of very high electricity prices to be weathered without heating.

Figure 7: “Heat map” of stock exchange prices for electricity in Germany over the course of 2024. The daily average outdoor temperatures in Potsdam are shown above for comparison. Sources: www.energy-charts.info, German Weather Service.

As long as non-dynamic electricity tariffs with fixed prices are still available, an undersized heat pump in combination with direct electric peak load coverage via heating cartridges may also be appropriate in microeconomic terms. This is a particularly good way of bridging the gap until the renovation of parts of the building envelope that are due in the next 5 to 10 years.

Regardless of this, when purchasing an outdoor air heat pump, it is advisable to choose a model that still delivers full performance even at the design outdoor temperature of the respective location. This only requires relatively minor modifications to the refrigeration circuit, so the additional costs are kept within reasonable limits.

As part of the Klimafreundliches Riedstadt (climate-friendly Riedstadt) project [Riedstadt 2025], the PHI was commissioned by the municipality of Riedstadt to compare various insulation standards and heat supply systems in terms of the PER demand and life cycle costs based on an example building. For this, the geometry of a small end-of-terrace house was assumed, which was already used in Section - Insert: Control options and thermal protection. In the initial state, the building has a gas boiler, windows with insulated glazing and uninsulated exterior walls with a U-value of 1.4 W/(m²K); the roof and basement ceiling are of a similar quality. For a room temperature of 21 °C in the Mannheim region in Germany, this results in a very high heating demand of 249 kWh/(m²a).

The starting building condition is so bad that an energy-efficient retrofit is worthwhile even without a specific reason, whereas usually the prerequisite for the economic efficiency of an energy retrofit, regardless of whether it concerns the building envelope or the heating system, is that the components in question should already be in need of renewal.

Four different insulation standards were compared: the minimum requirements according to the German legislation at the time (GEG), the requirements for funding according to the German subsidy scheme BEG, a renovation in accordance with EnerPHit criteria, and an even more extensive renovation based on EnerPHit criteria for “cold” climate zones (e.g. Sweden, Norway, Iceland).

As can be seen in Figure 8 and Figure 9 based on the example of heat supply using split units, a substantial improvement to the building envelope is the key to low energy consumption. Renovation to the EnerPHit standard proves to be the economic optimum. Although the cost minimum is very flat, the electricity consumption of the EnerPHit variant is significantly lower than that of the GEG and BEG variants.

Based on the EnerPHit variant, various heat supply systems can now be compared. The costs are documented in Table 2. Above all, it should be noted here that the price of heat pumps increases much more rapidly with higher output than the rpice of other heat generators.

Table 2: Cost estimates for building services systems. Subsidies were not taken into account. Data basis: Municipality of Riedstadt.

^a dependent on heating load
^b inc. CO₂ price, average 200 €/t
^c incl. CO₂ price, average 200 €/t, and rising grid fees, average 3 cents/kWh

The results on the right-hand side in Figure 8 and Figure 9 initially show that split units are indeed the most cost-effective option. However, despite the currently high investment costs, air-to-water heat pumps can also compete with the other systems. Heat pumps are also the leaders in terms of the burden on the future energy system. District heating also has a low PER demand, although this depends heavily on the local district heating system.

Figure 8: Life cycle costs of the various cases

Figure 9: PER demand for the various cases

Active cooling will become the norm in residential buildings over the coming decades, especially in the warmer regions of Germany. Rising average temperatures, more frequent extreme weather conditions, increasing comfort requirements and an ageing population make this development seem inevitable.

From an environmental and operating cost perspective, this is not a problem. Prices for PV modules have fallen by a factor of 1000 since the 1970s, when the first large-scale systems were installed in Germany for research purposes. In fact, electricity prices during the day in the summer months are already extremely low, as can be seen in Figure 7. Anyone operating their own PV system will also generate surplus electricity in the summer, which can be used for cooling.

This is obviously another advantage of heat pump solutions. Air-to-air heat pumps were originally developed for cooling; they can also cope with high indoor humidity and have high COPs for cooling, because the temperature difference to be overcome in summer is not high. If split units are used for space heating anyway, the additional effort required for cooling is limited to the condensate drain on the indoor unit. However, it should be noted that split units that supply rooms on several floors via an open stairwell must be installed at the bottom for heating and at the top in the corresponding part of the building for cooling. More details can be found in Wolfgang Feist's article - Heating and cooling with split units in EnerPHit renovations

Air-to-water heat pumps are also often reversible; with geothermal heat pumps, cooling without an intermediate heat pump is even possible directly using the cold brine from the geothermal probes, which also regenerates the soil for the following winter. Removal of heat from a room can take place via underfloor heating or separate fan coils (fan-assisted cooling coils). With cooling via underfloor heating, the cooling capacity is limited by the fact that the floor must not be cooled below the dew point. A typical limit value here is a surface temperature of 19 °C, which is just about acceptable for thermal comfort ([Zhou 2019]). This results in a fairly high available cooling capacity of 40 W/m², while radiators would only make a very small contribution to summer cooling due to the small temperature difference available down to the dew point. If this potential is to be exploited, or if a fan coil with a typical temperature pairing of 6/12 °C is planned, the distribution pipes must be insulated against condensation, which is relatively expensive. The control system must also be suitable for both heating and cooling. The additional investment costs for cooling with an air-to-water heat pump are thus not negligible.

The following applies for all systems not based on heat pumps: if a cooling function is desired, this must be implemented by means of an additional system, which will usually be a split unit.

In residential buildings renovated to the EnerPHit standard, hot water supply accounts for a significant proportion of the total heat demand, and its energy demand can be more significant than that for space heating. It is therefore worth comparing the various options for hot water supply. Based on [Schnieders 2021], here we will limit ourselves to a few variants for electric hot water supply in multi-family houses, where distribution losses are usually particularly high. If a building is supplied with district heating, this will usually also provide domestic hot water; the options mentioned below for reducing distribution losses in centralised systems can also be implemented there.

Conventional hot water supply with a central heat generator that provides heating and hot water and a circulation line to the homes is technically relatively simple and inexpensive to manufacture, but has significant disadvantages:

• High hot water temperatures are required; in Germany, a return temperature of 55 °C is stipulated for legionella prevention.
• Storage and distribution losses are usually on a similar scale to the quantity of useful heat for hot water.
• With hot water generation with a heat pump, a poor coefficient of performance results due to the high temperatures.

A multi-family house renovated according to the EnerPHit component method with an outside air heat pump was considered subject to the condition that hot water generation takes place electrically. The heating demand is 33 kWh/(m²a), with heat transferred via radiators with a design flow temperature of 40 °C. The following domestic hot water systems were compared:

A) Standard 4-pipe system: 60 °C hot water flow temperature. The annual COP of the heat pump for hot water generation is 2.5; storage and distribution losses are just as high as the useful heat of the hot water.

B) Central heat pump and ultrafiltration: legionella and other microbes in the hot water circulation pipe are eliminated by a very fine filter. Installation at the house connection is also sometimes recommended. The flow temperature can be reduced to 50 °C and the return temperature to 45 °C. This reduces circulation losses by about a third. At present, increased sampling effort may be necessary for verification of satisfactory hygienic conditions.

C) Central heat pump and heat interface units in each flat: the flow temperature can be reduced to 55 °C and the return temperature can be reduced to an average of 30 °C. This reduces circulation losses by almost 50%.

D) Electronically controlled instantaneous water heaters: losses are almost zero due to the absence of storage tanks and circulation, but the actual hot water generation is not very efficient. Costs are also incurred for the three-phase current connection.

E) When the instantaneous water heater is combined with a shower water heat recovery system, which reduces the useful heat demand for hot water by about a third, the energy consumption is similar to that of central heat pump solutions.

F) Low-temperature heating with two-pipe system: the central heat supply system provides a flow temperature of 40 °C for the heating system throughout the year. The water is also heated to 35 °C by heat interface units. An electronically controlled instantaneous water heater brings the hot water to the target temperature of 45 °C. Costs and complexity are high, as a central heat pump, storage tank and circulation system, heat interface unit and instantaneous water heater are required. The overall efficiency is high, although the heat pump for space heating has to generate unnecessarily high temperatures even during the transitional periods.

Additional decentralised options for hot water supply are discussed in Wolfgang Feist's article about split units.

Figure 10 shows the results for electricity consumption and the relevant life cycle costs over a period of 20 years.

Figure 10: Electricity consumption for heating and hot water, and life cycle costs

All systems cost about the same within the achievable accuracy here; higher expenditure for better efficiency generally pays for itself. On the other hand, the electricity consumption of the systems differs measurably. The classic 4-pipe system A) with 60 °C circulation and the simple instantaneous water heater D) are particularly inefficient.

If the electricity consumption is weighted with the respective PER factor for electricity for heating and hot water, the result is as shown in Figure 11. Here, the 4-pipe system A) and the instantaneous water heater without shower water heat recovery D) again stand out negatively.

Figure 11: PER demand

Renovations are always particularly cost-effective when they are linked to the usual renewal cycles. However, if construction does take place, then sustainable products and qualities should be used. For heating systems, this means either connecting the building to district heating – especially in densely built areas – or installing a heat pump. For cost-benefit reasons, this will usually be an outside air heat pump.

With appropriately competent planning and installation, good results will be achieved with air-to-water heat pumps, which are also more likely to be economical than conventional heat generators in the future. This will usually take care of hot water generation at the same time.

Split units, the use of which for space heating has been less widespread until now, are proving to be particularly cost-effective and flexible. If a good quality of the building envelope permits doing away with radiators under the windows, split units are also suitable as a stand-alone heating system with very low investment costs. However, they can also be operated in parallel with other heating systems in buildings that have not been completely renovated. If required, they can also be used for cooling in summer.

There are many options for efficient hot water generation, all of which have similar life cycle costs. However, central hot water generation with circulation pipes at high temperature levels is not recommended, especially in combination with heat pumps.

For the renovation of existing buildings, planning must always be based on the situation in the respective building. In the context described, this applies to the building envelope as well as to the building services.

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Research Group 62 - Cost-Effective Refurbishments

Refurbishment with Passive House components

Building Services

Primary Energy Renewable PER