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The Status Of Heat Pumps For DHW

Adveco considers the status of heat pumps for domestic hot water applications in commercial buildings…

Set to achieve the ambition of demanding zero emission buildings, and with the consultation period now complete, under the 2025 Future Building Standard the expectation is that new buildings will no longer receive gas connections. In a move from the current five building types there will also be simplification to just two, sheds with direct electric heating and point of use, whilst all the remainder are set to be heat pump only. This is not only set to usher in an all-electric future by 2026 but also cements the important role that heat pumps are to play in making it a reality.   

Despite this, there remains a challenge within the HVAC industry to provide better guidance on the use of heat pumps, especially for water heating, when achieving low carbo and ultimately net zero targets. There continues to be a discrepancy, often at the highest levels, when communicating requirements and obligations between residential (domestic) and commercial installations. Obligations that, amongst others, demand balances in legionella risks and other water quality issues.

Air Source Heat Pumps (ASHPs), offering significant carbon emission reduction over traditional gas boilers, have emerged as a compelling option for generating hot water in UK commercial buildings. But the technology also presents unique considerations. ASHPs can be a good fit for buildings with moderate or low hot water demand, but those with higher hot water requirements, typically seen in many commercial scenarios require either a larger ASHP unit or a hybrid approach that blends other technologies, such as electric boilers or solar thermal to meet peak demands. The efficiency of an ASHP is also affected by ambient air temperature and the water temperature an application must achieve. Efficiency can drop during colder periods as well as when a unit is pushed to deliver higher temperatures, requiring the unit to consume more electrical energy to achieve desired working flows.

High temperatures in domestic hot water (DHW) systems for commercial buildings must maintain a minimum temperature (60°C) to prevent Legionella bacteria growth. For this reason, and to optimise the efficiency of the heat pump, the preferred application of ASHP has been to exclusively supply the pre-heat for water. This has notably driven a resurgence in undersized system storage, especially in like-for-like replacement of gas water heating, as larger thermal storage is now required to offset slower reheating after periods of peak demand.

Greater system complexity with external units and larger thermal storage demands also means ASHP-based systems will require more space and capital expenditure than traditional gas systems. They will also cost considerably more to operate. The overarching advantage is the technology reduces carbon emissions, but many smaller businesses will continue to struggle to balance this against higher replacement and operational costs.

The challenge then is to efficiently deliver high-temperature water with a heat pump without necessitating complex system designs and major structural alterations. Standard heat pump components are generally unsuitable for very high temperatures, demanding larger specialised compressors and heat exchangers which add to the size and weight of a unit. High temperatures also require a change in refrigerant to achieve suitable efficiencies.

The current default refrigerant in most ASHPs on the market is R-32 (Difluoromethane) which has been popular due to its efficiency and lower global warming potential (GWP) compared to prior R-410A refrigerant. Its ability to deliver DHW temperatures is realistically limited to 50-50°C before loss of efficiency is too great.  For this reason, the next generation of ASHP is embracing R-290 (propane) which offers greater efficiency at moderate DHW temperatures and has the capability to reach DHW temperatures of 70°C with the bonus of an ultra-low GWP for an extra tick in the box for being better for the environment. R-290 is however highly flammable, and more strictly regulated, so expect more low-GWP hydrocarbon blends with R-290, such as R-1270 (Propene) or R-161e (C3H8) which can achieve higher DHW temperatures while maintaining reasonable safety profiles.

There is a growing demand amongst specifiers for a singular heat pump application that can meet high-temperature demands creating a ready market for the next generation of ASHP. Our research with R-290 propane units indicates that whilst the refrigerant enables large 40-50 kW units to achieve a promising coefficient of performance, 2.4 at 65°C working flow/7°C ambient, there remains a propensity for output to fall steeply when the exterior temperature drops, lowering COP to as little as 1.8 at 70°C working flow and -5°C ambient. This brings the units back in line with smaller, lighter R32 models. First-generation R-290 units are large and heavier, as much as ½ tonne for 40 and 50 kW models, which can introduce new limitations in terms of where and how the ASHP can be safely installed. They also tend to be noisier which is a particular consideration should the building in question have a residential aspect such as a hotel, hospital, care home or school/university dormitory.  

Looking further into the development cycle, R513 is a relatively new refrigerant which, potentially offers a safer, more balanced performance at higher temperatures with a relatively low GWP (570) making it well-suited for DHW. R513 ASHPs are set to be compact and lightweight due to a smaller compressor and fan assembly, making them easier to install. Critically they are likely to be cheaper to specify.  

So where do we stand today? In truth, the current generation of R32 ASHPs offers an extremely viable way to introduce low-carbon, renewable technology into buildings and the technology meets all current BREEAMM requirements. With carefully designed, balanced DHW applications R32 heat pumps lend themselves well to hybrid approaches, reducing emissions from existing gas-fired applications or helping offset direct electric demands from topping up heat with electric boilers in indirect systems. They can also work in close conjunction with solar thermal as a mid-heat, which maximises the efficiency of the heat pump as it works with the cold in. The advantages in carbon reduction and energy offsetting mean even complex systems can be cost-effective despite higher capital investment. In terms of DHW demand in commercial buildings, we need to accept and embrace hybrid approaches now and well into the future as the optimal path to net zero commercial buildings.

Learn more about Adveco’s range of air source heat pumps for commercial hot water projects…