Hot Water Supply in Multi-Family Residential Buildings, Both New and Existing

Multifamily buildings play a central role in the future of housing. Because they make more efficient use of resources, they represent a particularly environmentally friendly and sustainable housing option. Driven by policy initiatives, intelligent energy concepts that meet legal requirements regarding the share of renewable energy are more important than ever—both in new construction projects and in the renovation of existing buildings. In particular, the focus is shifting to hot water production in multi-family buildings through centralized or decentralized solutions using heat pumps.

In Germany alone, more than half of all apartments—namely, around 22 million—are located in multi-family buildings. This trend is on the rise, as experts estimate an annual need for over 400,000 new apartments. In regions with particularly scarce housing, priority will be given to building larger multi-family housing projects. There is a significant need for renovation in the existing housing stock: About 60 percent of the apartments in multi-family buildings require energy-efficiency upgrades. Around four million apartments are still heated with gas-fired individual heating systems, and simply replacing these systems does not represent a sustainable solution for heat supply.

Energy Use and Drinking Water Quality Regulated by Law
To meet legal requirements for achieving climate goals, we therefore need smart new energy systems in which heat pumps play a key role. These naturally also include water heating, which accounts for a significant portion of a household’s total energy consumption.
The quality requirements for drinking water are regulated by law in Germany. The Drinking Water Ordinance (TrinkwV) 2023 sets limit values for microbiological, chemical, and radiological substances that must not be exceeded in drinking water. Key aspects in this context include temperature controls and sufficient flow rates to prevent the spread of Legionella, the selection of safe materials that do not compromise water quality, and an annual Legionella test for large-scale systems.
Hot water can be supplied in an apartment building in various ways—either centrally or decentralized. If a heat pump is used as an environmentally friendly heat source, the requirement that, in the future, at least 65 percent of heating and hot water must be provided from renewable energy sources can be reliably met in both cases.

Centralized Systems with Heat Pumps
In a central system, the heating system heats the drinking water. To do this, the heating energy is directed into a hot-water storage tank, which comes in various sizes depending on hot-water demand. From there, the heated drinking water is distributed through pipes to all floors of the house. Connected to the main distribution system are the pipes that carry the hot water to each faucet—such as the sink, shower, or bathtub.
In this system, the heat pump serves as the central heat generator and is used for hot water production in combination with a buffer tank and a hot water storage tank, which offers many advantages. This makes it possible to replace or supplement a fossil-fuel heating system with a heat pump during a renovation with minimal effort. The existing plumbing and any existing components, such as a buffer tank, can continue to be used. As usual, the equipment is located centrally in the basement or utility room and is not visible in the individual apartments. Additionally, no additional devices such as boilers or storage tanks are required at the faucets.

Easy to combine with a solar system
A central system can supply multiple faucets in a home with large quantities of hot water simultaneously, making it available (almost) immediately as soon as the faucet is turned on. Another advantage: The technology can be combined with renewable heat sources, such as a solar water heating system, to further reduce operating costs and minimize CO2 emissions.

Flow Meters to Improve System Efficiency
However, it is important to note that central systems with circulation lines place high demands on the heat pump’s flow temperature in order to reliably ensure drinking water quality—especially at low draw-off rates—which can have a negative impact on efficiency. To avoid this and enable the system to operate at lower temperatures, Dimplex has developed an innovative tool for monitoring water turnover: The DFM 1988 monitors water flow rate and hot water temperature and only raises the hot water temperature above 60 °C if the required water turnover in the storage tank has not occurred within the last 72 hours. DIN 1988-200 permits setting operating temperatures of ≥ 50 °C provided that the entire contents of the hot water storage tank are replaced within three days. This results in a significant gain in efficiency for hot water production—particularly with high draw-off volumes and in combination with a heat pump—without compromising hygiene. With this optimized system, which is compatible exclusively with Dimplex hot water storage tanks, significant cost savings can be achieved in central hot water supply, particularly in apartment buildings and office complexes.

Decentralized Systems with Heat Pumps
Depending on the requirements and circumstances, a decentralized solution may also be suitable for supplying drinking water in a multi-family residence. In this system, drinking water is heated in apartment stations located in close proximity to the point of use. The centrally generated thermal energy is distributed throughout the building as heating water via a pipe network consisting of supply and return lines. In the stations, cold drinking water is heated to the desired hot water temperature in a plate heat exchanger using the flow-through principle. This system can be efficiently implemented using a heat pump as the heat generator, along with a buffer tank and corresponding fresh water stations for hot water production.
Fresh water stations heat drinking water on a flow-through basis, always providing it as needed, energy-efficiently, and hygienically fresh. Depending on their configuration and requirements, they can be used in single-family homes, multi-family dwellings, restaurants, hotels, and commercial settings—in other words, for virtually all applications. The flow rate is up to 25 l/min—depending on the model. This system is ideal for use in residential buildings where compliance with the Drinking Water Ordinance is only achieved at high hot water temperatures or where circulation losses are disproportionately high.
In addition to decentralized domestic hot water heating, a building distribution station also distributes the heating water to one or more heating circuits. During the planning phase, it is important to take into account the number and type of heating circuits, as well as the entire distribution network within the building.

Customizable settings and maximum hygiene
Such a decentralized system allows for individual control of heating and hot water demand per apartment and simplifies heating cost billing through pre-installed mounting points for cold water and heat meters in each apartment. Hygienic drinking water preparation in accordance with the Drinking Water Ordinance (TrinkwV) is possible without an annual inspection due to the flow-through principle.
Because hot water is always produced on demand, there are no standby or pipeline losses, and the system operates at a lower overall temperature, the energy efficiency of the entire heating system can be increased, while limescale buildup—which occurs particularly at temperatures above 60 °C—is simultaneously reduced.
Maximum comfort and safety in terms of drinking water hygiene are achieved with only minimal installation effort—offering potential savings of up to 30 percent, as the need for supply lines, ceiling penetrations, thermal insulation, and fire-resistant barriers is significantly reduced or, in some cases, eliminated entirely.

Conclusion: Whether for centralized or decentralized applications—a heat pump makes both possible, but the choice depends on the specific application
Sustainable solutions that use renewable energy for hot water production are available for both centralized and decentralized applications. Given the expected rise in energy prices due to CO2 emission surcharges, electricity is becoming an increasingly cost-effective energy source.
If, for example, hot water is needed regularly in your home for washing dishes, showering, or bathing, a central water heater is a worthwhile investment—especially if a central heating distribution system is already in place and the heat generator is due for replacement or needs to be supplemented. This is because the drinking water is heated cost-effectively and efficiently via the heating system. The costs of hot water production can be reduced to a minimum with a storage system and a solar thermal system.
A decentralized solution may also be a good option, particularly in residential construction and for apartment buildings, or when supplying hot water to distant and infrequently used faucets. Since decentralized solutions do not require high hot water temperatures, they are particularly energy-efficient and, because heating is tailored to demand, pose no hygiene risks. In many cases, this could be not only a sustainable but also an economical solution.

Infobox: How a Home Station Works
The primary supply circuit is opened via the changeover valve only when the station detects a demand for hot water. Once the draw cycle is complete, the valve closes immediately. The heating circuit is fully open during normal operation and closes 100% when hot water is drawn. An integrated hot-water retention module in the supply line ensures that hot water is available particularly quickly. The hybrid models with a downstream instantaneous water heater feature an integrated preheating function. This serves to heat the hot water as needed for the upcoming draw. To activate this function, a hot water draw must occur. The draw temperature can be individually adjusted via an integrated control unit.
The built-in flow sensor measures the volume of hot water being drawn. In addition, flush, fill, and drain valves are already integrated into the unit for quick installation and maintenance. Temperature sensors for cold and hot water, as well as for the supply and return lines, are installed to ensure precise control.
All drinking water heating modules are designed as 2-pipe systems. The TWES module (without an instantaneous water heater) can be expanded to a 4-pipe system (with accessories) if needed. With an installation depth of 110 mm, including the housing cover, it can be mounted in a surface-mounted or flush-mounted cabinet, e.g., in interior walls or drywall. The connection rail module with ball valves allows for the pre-installation of the drinking water network within the building as well as straightforward pressure testing during the construction phase. The various versions of the TWES module meet customer requirements for fresh water stations tailored to different local conditions and hot water comfort needs.