The case for the hydrosplit heat pump


15 January 2024

Viessmann’s Product Manager Hugh Jones takes a look at why hydrosplit heat pumps are less common in the UK than parts of Europe, and if that’s about to change.

Installers have a crucial role to play in guiding clients to the best solution to suit their brief and budget. A heat pump is a long-term investment and it’s essential to get it right. This is particularly important given how new heat pump technology is to most UK consumers. As a nation, we have yet to make the mental shift away from gas-fired combi boilers to renewable heating systems. Understanding how heat pumps work is like learning a new language; until you’re proficient, you need an interpreter.


To split or not to split?

One of the important decisions to make early on is whether to go for a monobloc, with an enclosed refrigerant circuit in the outdoor unit (ODU) and only water pipes connecting the ODU to an indoor water cylinder and the rest of the heating system, or a split heat pump, with two refrigerant pipes connecting the ODU to an indoor unit (IDU).

As monobloc heat pumps don’t require refrigerant piping, they are quicker and easier to install, while also removing the risk of refrigerant leaking into the house. On the other hand, with a separate indoor unit, split heat pumps need less room outside, which can be handy for properties with small gardens. Furthermore, as only refrigerant is piped between the outdoor and indoor units, there is no risk of freezing water pipes in winter.


Best of both worlds

Hydrosplit is a third option which, although popular in Germany and Belgium, is still less common in the UK, despite offering some compelling additional advantages.

Hydrosplit heat pumps are a form of monobloc, as the refrigerant circuit is contained within the ODU. However, like a split heat pump, they also incorporate an IDU which manages the building’s hydraulic heating system.

As all the components such as pump, three-way valve, expansion vessel, control unit and electric flow and back-up heaters are contained within the indoor unit, there is no need to install these separately with a hydrosplit. Even a defrost buffer and/or water cylinder can sometimes be integrated.

This makes for a much quicker and simpler installation, saving around 90 minutes compared to heat pumps with separate components. Furthermore, it’s more reliable as the components are perfectly matched, assembled and tested on a production line and the entire system is covered by the manufacturer’s warranty. If an engineer does need to visit, everything is easy to find and accessible in one place, so it’s quicker and more convenient for both the installer and the customer.

Having everything inside a sleek and well-designed box is also more aesthetically pleasing than the alternative, which is usually a water cylinder with all the necessary equipment and peripheries attached to it, looking unsightly. For these reasons and as a German heating systems manufacturer, the majority of Viessmann’s domestic heat pump range is hydrosplit.

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Question of space

Given all these benefits, plus the fact that hydrosplit heat pumps cost around the same as other types – or even slightly less when the installation savings are factored in one could be forgiven for asking why they aren’t more popular in the UK?

It usually comes down to internal space. Unlike many continental properties, UK homes don’t normally have the luxury of a basement. In smaller houses, where every inch of space is at a premium, it can be simply impossible to find a suitable location for a floor-standing appliance or wall-mounted unit and separate water cylinder. Adding components individually in different locations, as with a split heat pump, provides more flexibility for installers to fit them into irregular spaces, albeit at the cost of a neat appearance.

This situation may change over time, especially after 2025 when new build properties are designed with heat pumps in mind and if the benefits of the hydrosplit approach are more widely understood. If space allows, a hydrosplit heat pump is almost always the better solution, which is why manufacturers are increasingly introducing them into their UK product ranges.

Whenever HVACR professionals have influence over building design, we believe they should steer architects down the hydrosplit route. This is particularly true in the case of larger housing developments, where ease of installation becomes a highly significant advantage when multiplied hundreds of times. But even in the case of single retrofits, a hydrosplit will offer significant advantages for homeowners.


Don’t skip the (defrost) buffer

Whether you’re opting for a monobloc, split or hydrosplit, you should include a defrost buffer vessel in most cases. Some manufacturers are not explicit in specifying this, however, meaning people don’t always put them in. This can lead to major headaches.

Buffer vessels serve two purposes which is why they are important. Firstly, in cold conditions water from the (defrost) buffer, which is at the same temperature as the water in the heating system, is used to keep the ODU from freezing. To avoid the need for a defrost buffer vessel, there is a minimum water volume requirement within the system itself, which depends on factors like the heat pump's capacity, the outdoor temperature, and the system design. While a system with a high water volume, such as in the case of underfloor heating, may not need a separate buffer vessel, it will almost certainly be needed with a radiator system.

The second function of the buffer is to prevent the heat pump turning itself on/off too quickly when the weather is slightly warmer around the beginning and end of the heating season. In the spring and autumn, there is a risk that the heat pump will be continuously stopping and starting in response to changes in temperature. The buffer vessel stops this from happening because the water in it absorbs heat from the system, thereby extending the system’s run time by around 15-40 minutes, depending on the size of the vessel. This is important not only because cycling through frequent stops and starts is less efficient, but also because the life span of a compressor is determined by the number of times it starts, not by the length of time it runs. This means that a buffer vessel can significantly extend the life of your heat pump.

Lack of indoor space can be a factor in deciding to omit a buffer vessel from a heat pump system. But if you opt for a hydrosplit system with an integrated buffer such as Viessmann’s Vitocal 150-A or 151-A, the problem of finding that extra installation space is avoided.