New approach to addressing HP inefficiency in hot water supply


04 August 2023

How do you reconcile the need for a low flow temperature for efficient heat pump operation with the requirement for hot water for DHW? Viessmann Technical Director, Christian Engelke proposes an innovative European solution.

As heat pumps gain popularity thanks to their energy-efficiency and environmental credentials, there remain concerns and challenges that hinder their widespread adoption in commercial settings.

One of the major hurdles involves finding a balance between maintaining a low flow temperature for optimal heat pump efficiency while also ensuring that water remains hot enough for effective legionella management and to meet end-users' demands for domestic hot water (DHW).

In many cases, energy managers resort to running heat pumps constantly at 65°C to prevent the growth of legionella bacteria. While significantly lower than the 7080°C required for conventional gas-boiler systems, this is far above the ideal heat pump flow temperature range of 35-55°C for space heating or up to 35°C for domestic hot water production. The latest UK Building Regulations mandate a flow temperature of 45°C for heat pumps to achieve maximum efficiency, while all new wet space heating systems should now be under 55°C.

It's true that heat pumps have the potential to yield CO2 savings of up to 70% compared to electric boilers and up to 65% compared to A-rated gas boilers. However, these savings can only be achieved if the heat pumps are operated as intended. Running a heat pump at twice its optimal flow temperature can compromise its efficiency by around 200250%. This is likely to lower its Seasonal Coefficient of Performance (SCOP) to below the required 2.8 to be classified as a renewable technology. Moreover, it can cause maintenance issues and reduce the system's lifespan. It’s akin to trying to inflate an already full bicycle tire, straining the pump and consuming excessive energy for minimal results.

Despite the potential advantages, the cost and disruption associated with adapting or replacing existing radiator networks to accommodate heat pumps deters many organisations, especially those with limited funding for plant room upgrades. While it may be tempting to opt for a like-forlike replacement of an old gas boiler with a heat pump, trying to operate it in the same manner results in unacceptably high electricity bills and other potential problems.

We’re aware of several schools in England that were incentivised to install heat pumps but are now forced to switch them off because local government cannot afford the electricity to run them when faced with uncapped tariffs ranging from 60-100p/kWh. Electricity demand is high because of the need to deliver high flow temperatures around the school, and to meet the constantly required 60°C hot water requirements.

As a result, many of these establishments have resorted to switching off their heat pumps and reverting to gas heating. This is bad news both for the individual institutions, and public perceptions of heat pumps in general. It’s an unfortunate misunderstanding that much of the public connects energy-friendly or zero-energy solutions with lower energy bills. 


Using a centralised DHW booster

To address this predicament, Viessmann has developed a technology bundle that allows heat pumps to operate at low temperatures for maximum efficiency, while a centralised hot-water supply system delivers DHW at the desired temperature with zero risk of legionella. Optimising each energy requirement individually in this way, rather than raising the space-heating temperature unnecessarily to meet hot water needs, makes good sense, especially when you consider space heating typically accounts for around 80% of the total energy requirement.

Viessmann’s new compact water source booster heat pump can be equipped with a buffer tank and combined with a Viessmann Vitotrans 353 freshwater module. The water in the low-temperature buffer cylinder is heated by a heat pump, for example a Vitocal 300-A, Vitocal 300/350-G PRO or the forthcoming Vitocal 200-A Pro air/ water heat pump to between 25°C and 50°C – typically 35°C – for space heating. (A lowtemperature heat network could also serve as the heat source.)

The buffer tank then serves as an energy source for further heat generation by a downstream plate heat exchanger in the form of a PEWO Titan HT booster heat pump, which can also be cascaded if necessary. The booster pump uses the heat from the tank, cooling it to a minimum of 20°C and raising the temperature on the heating water side to approximately 65°C for DHW. In this way, an instant supply of hygienic DHW is provided, resembling the functionality of a combi boiler, but on a larger scale. Since no potable hot water is stored, the risk of legionella is eliminated.

The water/water heat pumps, which are compact in design and range in output from 12 to 29 kilowatts, operate with one passage valve and one common circulation pump each. This optimises performance and enables a small spread to be used in circulation mode.

Although this method is relatively new to the UK, where high-temperature heating systems encompass both heating and hot water supply, it’s widely and cost-effectively used in Europe. It’s compatible with any type of heat generator and heat networks, making it particularly effective for public buildings like schools and hotels.


Alternative to ambient loop set-up for apartment buildings

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As we have outlined, in almost all cases it’s inefficient for a commercial heat pump to adequately serve both heating and DHW requirements in commercial buildings, and this includes apartment blocks. However, there are a number of options available for those developing apartment buildings who do want to specify heat pumps for both functions.

One potential solution that allows for the separate boosting of DHW temperature into individual apartments is the ambient loop model. With this approach, lower (medium) temperature water is distributed into the network from a central plant room-based heat generator to then feed individual heat pumps in apartments. An ambient, open or shared loop system can be adopted with a boiler, CHP or heat pump supplying the building’s base flow temperature.

There are problems with this approach though. While the heat pump unit may be suitable for the individual apartments and ticks the right boxes, there is still the challenge of fitting all the necessary associated equipment – hot water cylinder, heat pump and circulating pumps etc. – within the property. Most apartments are not designed to facilitate this provision.

The approach presented above allows the DHW booster heat pump(s) to be situated in the plant room, freeing up space in the apartments, while performing the same function. While they could be installed in each apartment, accompanied by a cylinder, the booster heat pumps discussed here, are primarily designed for plant rooms where they can be cascaded as per the requirement for the whole building.

There are other issues with open loop systems too, particularly if selected for environmental reasons. Using a commercial heat pump to heat up water to a certain temperature requires electricity, and then more electricity is needed to power the smaller heat pump in each individual apartment. If you need 100 kW of energy overall, the first heat pump needs to provide around 70% of this and the other heat pumps needs to provide the heat for the individual apartments. At apartment level, the heat pump is very efficient in its delivery of high supply temperatures, and the COP of the heat pump feeding the ambient loop may also be efficient but when you add the efficiencies of the two units together, they rarely come out as that efficient as a system.

There is also a noise level issue of small heat pumps in apartments.


Cost Analysis

Solutions like the DHW booster heat pump need to take hold if the apartment-block development and heat network sectors are not to become disillusioned with heat pumps, favouring easier but less cost- and carbon-efficient options like electric heating panels and storage heaters or electric showers for DHW. An analysis of the figures involved quickly demonstrates the significant impact of this new proposed approach on energy costs and carbon emissions.

Here is an overview example of that, for apartment block that requires 150 kW of space and DHW heating.


Educational efforts and future prospects

By combining heat pumps with centralised hot water systems, it is possible to optimise energy efficiency, mitigate the risk of legionella, and achieve substantial cost savings.

However, the UK commercial heating sector may require increased education and awareness before it’s ready to embrace a transition from hot water storage to a centralised hot water supply system, as commonly practiced in Europe. Nevertheless, the stakes are high. Insufficient understanding of heat pump operation serves as a genuine barrier to their adoption and proper utilisation, resulting in financial losses for organisations and impeding the country's progress toward meeting emissions targets. We’d like to see more organisations overcoming their hesitations and embracing this innovative solution, propelling the UK toward a greener and more sustainable future.