WS-DUO by STIEBEL ELTRON: space-saving delivery set for central heat pump

The WS-Duo from STIEBEL ELTRON is a delivery set for apartment buildings with a central and thus collective heat pump. By providing each apartment with its own delivery set, both heating and domestic hot water are controlled completely individually.

The concept of a central heat pump with individual delivery sets naturally affects energy consumption. In addition, the consumption costs must be transparent to the occupants. How does the system compare to other solutions? Pim Snoek specialist collective installations at STIEBEL ELTRON explains the choices made in developing the concept.

Central heat pump

Why choose a central heat pump at all instead of giving each apartment its own heat pump? Pim Snoek explains in great detail. "It makes quite a difference whether you install, say, thirty heat pumps or just one central heat pump. Not only in terms of installation work, required materials and periodic maintenance efforts, but especially in terms of space consumption. An individual heat pump, like a boiler tank, takes up a lot of space in each apartment. While you simply hang the compact and flat WS-DUO delivery set on the wall, it doesn't even require a room. All in all, that adds up to a lot of extra saleable square footage."

Low-maintenance manifold

'WS' stands for home station and 'duo' refers to the combination of heating and domestic hot water. This delivery set is low-maintenance, limiting maintenance to the collective part. Snoek: "Access to apartments is therefore not necessary. Residents therefore never have to stay home especially for maintenance. This means that maintenance work can be carried out efficiently and an installer or maintenance technician is never at a closed door." Another advantage for residents is that there is no need to install any installation in the apartment that makes noise.

Heating and cooling

The collective part of the system consists of a heat pump, supplemented by a number of buffer tanks in a central technical room. That heat pump may be either an air-to-water heat pump or a ground source heat pump.

From there, the apartments are supplied with heat "We calculated for this system that a constant temperature of 43 °C is most efficient. A waterborne system pumps this through the building to the apartments. The WS-DUO delivery set contains a mixing group that allows colder water to be added to give the underfloor heating or LTV radiators exactly the desired temperature. This pump is controlled via the room thermostat."

Cooling has also been considered. "Heating and cooling can be provided simultaneously within the same building if a 4-pipe system is chosen. Cooling is via an integrated bypass in the delivery set."

Legionella-free tap water

Because of legionella, 43°C is obviously not enough for tap water. The WS-DUO has an integrated flow-through valve. This ensures that tap water is heated neatly to 60 °C, keeping it legionella-free."

This is done via a kind of two-stage rocket, Snoek explains. "Fresh cold tap water is first heated to about 40 °C via a heat exchanger. Then the flow-through unit electrically raises it to the required 60 °C. The collective part of the system is completely closed and is never used directly for tap water. Nor is hot tap water stored anywhere. Not in the building nor in apartments."

No heating without demand

The fact that electrical after-heating is required to safely prepare hot tap water does raise the question of whether a high-temperature heat pump would not be a better and more efficient solution Snoek: "A large part of the energy required for tap water is still provided by the heat pump. As a result, a high COP is achieved. Moreover, the last electrical push of the flow-through heat pump is only needed at the moment when hot tap water is actually requested. Nothing needs to be kept warm around the clock, which makes a huge difference. A high-temperature heat pump already immediately pumps water of, say, 70 °C to all corners of a building. But the higher the temperature, the less efficient a heat pump works. Furthermore, you have to deal with heat loss during transport and also because you have to continuously have a considerable amount of hot water in stock."

Reduced risk of grid congestion

But suppose a large number of residents decide more or less simultaneously to take a nice hot shower, can't that create a peak that is a problem for the grid operator. "The WS-DUO immediately brings tap water to the desired temperature and only as long as there is demand, so after showering this stops immediately. Also, it only involves post-heating from 40 to 60 °C. With a heat pump or boiler, on the contrary, it takes several hours after each shower session for the system to return to the desired temperature. If people shower right after each other, there is a huge overlap here. A throughput per apartment therefore actually reduces the risk of grid congestion. Our calculation model shows that the peak load can be up to four times lower in favor of a flow-through shower."

Both new construction and renovation projects

The delivery set is a godsend not only for new buildings, says Snoek. It can often be used in renovation projects as well. "Think of apartment complexes from the 1980s that often still have a gas-fired collective installation. In short, in this type of renovation you replace the gas boiler with a heat pump and install a delivery set per dwelling. Many VvEs are now showing interest in this type of solution."

A major advantage is that many buildings are immediately fitted with solar panels during renovations. Snoek: "This allows you to cover a large part of the power demand of the central system. Smart energy management systems hereby take into account the weather forecast and dynamic energy tariffs for the highest possible efficiency."

Insight into total energy consumption

Suppose you give each apartment its own heat pump. It sits behind the electricity meter, giving each resident a good view of the total electricity consumption. What about this collective solution? Snoek: "The WS-DUO has a recess in which you can place a heat meter, so that individual consumption can be easily tracked. Then we are talking purely about heating or cooling the home, plus sixty percent of heating tap water. With tap water, the remaining forty percent is controlled by the delivery system, so that goes through your own power meter. A heat meter from a supplier of your choice can be placed in the recess. Often those come with an online dashboard."

Cascade arrangement

We keep talking about the central heat pump. But that can be several, as it turns out. "We often work with a cascade setup. Instead of one heat pump with a high output of 100 kW, this is distributed over, say, six smaller heat pumps. This allows them to run more quietly and therefore quieter, and each heat pump can be automatically switched on and off as needed. You also immediately have a backup this way, should a heat pump break down or need maintenance and you don't suddenly find yourself without a heat pump."

"Unique is our optional cascade module that you place right next to the heat pumps on the roof," adds Pim Snoek. "This is a collector box with exactly the same look and color scheme as our heat pumps and serves as a collection unit for all the piping. As a result, only one supply and return to the building is needed, instead of six. So two pipes instead of twelve."

Performance in practice

Finally, Snoek emphasizes that everything from STIEBEL ELTRON is produced in its own factories within the European Union. "That is truly unique. And to ensure that everything works perfectly, new equipment is always tested in the field for four years first. Including under extreme weather conditions in special climate studios. All equipment is then tested and can easily be included in the required MPG and BENG calculations."

Ready for the future of multi-storey construction

With the WS-DUO, STIEBEL ELTRON combines the advantages of collective generation with individual comfort and insight. In doing so, the concept meets today's challenges: lack of space, grid congestion, stricter regulations and the growing demand for future-proof residential construction