Heat pumps: The right solution, in the wrong place
Offering a more energy efficient alternative to gas boilers, heat pumps are rapidly becoming popular with developers, specifiers and architects keen to achieve the government’s Net Zero targets. However, there is some confusion in regards to the most suitable heat pumps and how and where they should be installed. Dean Loizou, Design Director at Whitecode Consulting, highlights the importance of correct specification, detailing common mistakes that lead to costly reworks in the future.
While heat pumps are rapidly gaining prominence in building design, knowledge on the specification and installation of these innovative systems remains limited. As a result, there are some common installation mistakes made that can have critical consequences for future repairs and rework.
Heat pumps are often incorrectly sized. Undersized systems can lead to reduced outputs or the systems cutting out and not providing any heat. Oversized heat pumps risk poor performance, increased wear, significant inefficiencies and high operating costs.
A properly sized heat pump will reduce the strain on the compressor and extend its lifespan while operating as efficiently as possible. Accurate sizing that accounts for room-by-room heat loss calculations will ensure the property remains warm in winter and cool in summer.
Units can also be placed where airflow is restricted, such as near dense shrubs or in tight corners. This causes poor airflow and low efficiency. They can also be placed too close to neighbours; while they’re fairly quiet, they can still cause a noise disturbance if installed near reflective walls or bedroom windows.
Heat pump flow temperatures can often be set too high to ensure older, smaller radiators emit effective heating levels. However, the higher the temperature, the harder the heat pump has to work – meaning reduced efficiency.
What heat pumps are available and where should they be fitted? There are four types of heat pump: air source, exhaust air, ground source and water source.
Air source
Air source heat pumps (ASHPs) gather heat from the air and transfer it to a fluid refrigerant before it’s passed through a compressor, raising the fluid’s temperature before transferring heat to the building’s central heating system.
Monobloc ASHP systems transfer the heat to water and then either replace a boiler or be used on new low temperature district heating systems. Split systems, however, continue the refrigeration circuit into the building and use VRF pipework that’s distributed to internal units to provide cooling and heating.
Utilising both indoor and outdoor units, split systems see some heat transfer occurring inside the building where it’s warmer – resulting in less heat being lost and more efficiency than monobloc systems.
With ASHPs requiring efficient airflow, they must not be installed in enclosed spaces with insufficient ventilation, such as basements. Furthermore, to avoid any risk of noise pollution, they must be installed away from bedrooms.
They typically produce more heat energy than electricity consumed, leading to lower overall operating costs over time. Average installation cost is approximately £11,000.
Residents in England and Wales can secure £7,500 in funding towards the cost via the government’s Boiler Upgrade Scheme (BUS).
Exhaust air
Ducted to the building façade and each room within a dwelling, exhaust air heat pumps (EAHPs) utilise heat from outside to heat or cool the space via a wet heating/cooling system or the building’s ventilation system.
With only water and electrical connections required to the dwelling, and heating and cooling provided via the internal unit, EAHPs remove the need for additional communal heating plantrooms. There are multiple EAHPs, providing hot water only, utilising an integral MVHR system to provide cooling and heating, or providing heating and cooling via a wet based system, such as radiators, fan coil units or underfloor heating.
Boasting significant Part O cooling and Part F ventilation capabilities, EAHPs don’t require the additional venting needed for more conventional MVHR systems. Unlike ASHPs that are too large for low energy buildings, EAHPs can deal with smaller heating demands for Passivhaus and low energy properties.
While installation typically costs between £3,000 and £9,000, EAHPs are also eligible for the BUS.
Ground source
Collecting heat from the ground and transferring that into energy, ground source heat pumps (GSHPs) are separated into horizontal and vertical systems.
Laid in a shallow trench over a large surface area, horizontal systems are ideal for large-scale developments with plenty of space. More suitable for developers with little space to work with, vertical systems utilise a borehole to ensure pipes are securely buried.
Regardless of the system, pipe interiors feature a mixture of antifreeze and water, known as a thermal transfer fluid (TTF). This absorbs heat from the ground, which is passed through a heat exchanger into a refrigerant and compressed to raise its temperature. Subsequent heat is transferred into the central heating system.
Utilising the consistent, stable ground temperature, they operate at 300% to 400% efficiency and offer more effective heating and lower running costs than conventional systems.
With horizontal trenches requiring large amounts of land, they can be unsuitable for smaller properties. Additionally, the Environmental Agency may need to be consulted. GSHPs require placement in utility rooms, basements or garages and may need additional space for hot water cylinders and buffer tanks.
Applications to the BUS can be made to help funding towards the typical £29,000 installation cost.
Water source
Water source heat pumps (WSHPs) extract energy from water and turn it into heat.
There are closed-loop and open-loop WSHPs. Used in lochs, lakes, lochs or cooling towers, closed-loop systems feature sealed pipes that are submerged under water. They are filled with an antifreeze liquid that’s pumped through the pipework, gathering heat energy from the water before being circulated back to the heat pump.
Open-loop systems are used with boreholes near rivers or areas with suitable geological conditions. Water is taken from the borehole and lifted to the surface, with heat energy extracted and cooled water returned to a separate borehole.
To avoid the risk of the systems freezing, they must be correctly sized and have appropriate water flow. Producing a low-level humming noise and fairly large in size, WSHPs require installation in basements, utility spaces or plant rooms and must be situated away from bedrooms.
While installation costs range between £6,000 and £11,000, developers and specifiers can apply to receive funding via the BUS.
Maximising heat pump efficiency to reduce energy consumption and expenditure should be a key aspect of building design, emphasising the need for a holistic approach that involves developers, architects and specifiers at the earliest possible opportunity.
