The Complete Guide to Monoblock, Split, and High-Temperature ASHP Systems for optimal and efficient Hot Water and Space Heating.
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Air source heat pumps (ASHPs), also known as air-to-water heat pumps, are a highly energy-efficient solution for providing both space heating and domestic hot water.
These systems work by extracting latent heat from the ambient air to generate heat energy. The heat energy is transferred via heat exchangers to the heat pump water system, which is then used to provide space heating and hot water heating. This process makes them an ideal choice for properties looking to reduce energy consumption and carbon emissions.
A key metric for ASHPs is their Coefficient of Performance (COP), which is typically between 3 and 4. This means for every single unit of electrical energy consumed, the heat pump delivers three to four units of thermal energy. This high efficiency translates directly into lower energy bills and a significant reduction in a property’s carbon footprint, offering long-term financial and environmental returns.
ASHPs can operate as standalone systems capable of satisfying all hot water demands, or they can be seamlessly integrated with other heating systems, such as solar thermal panels or existing backup boilers, to form a hybrid heating solution. Because they typically operate as low-temperature hot water systems (around 55-60°C), they are especially effective when paired with underfloor heating where lower flow and return temperatures are required. However, they are equally capable of delivering reliable hot water and radiator heating by incorporating high-efficiency or oversized radiators designed for low-temperature operation.
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Monoblock air source heat pumps are self-contained units designed for quiet, external installation. All essential heat pump technology—including the compressor, heat exchanger, and controls—are housed within a single, highly durable, and weatherproof casing.
While the monoblock unit itself is external, it is important to note that dedicated space within the property is still required to house supporting equipment, such as the hot water cylinder and any necessary buffer tank. Additionally, the system’s controls can be ‘mirrored’ and installed into the property for greater user convenience, to simplify commissioning, as well as allow straightforward connection and integration to proprietary components and other systems.
The core advantage of the monoblock design is its simplified installation process. Because the unit uses water-based pipework that connects directly to your building’s heating system (flow and return), the installation does not require an installer to handle or commission the self-contained refrigeration system. Crucially, due to their external placement, these units require either integrated frost protection or the addition of an anti-freeze agent (such as non-corrosive antifreezes) in the primary circuit to prevent freezing in extreme cold.
These systems offer excellent operational flexibility. They are particularly well-suited for seamless integration with other renewable technologies like solar thermal systems, maximising the use of green energy. By adding a hydraulic separator or buffer tank, the system can efficiently support low temperature emitters like underfloor heating, while a correctly sized stainless steel hot water cylinder enables effective storage and solar-assisted heating of Domestic Hot Water (DHW).
For added versatility and security, monoblocks can be connected to a conventional gas boiler or electric immersion heater. This integration allows you to benefit from high-efficiency renewable energy as the primary heat source, while the conventional system acts as a reliable backup or top-up for DHW during peak demand or extremely cold conditions.
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Split system air source heat pumps live up to their name by comprising two distinct parts: an outdoor unit and an indoor unit. These two components are linked by interconnecting refrigeration pipework and communication cabling, allowing the system’s heat exchange to occur safely inside the property.
The outdoor unit houses the evaporator coil and the compressor, responsible for extracting latent heat from the ambient air. The indoor unit, often referred to as a Hydrobox or Hydronic Module, is typically compact-similar in size and layout to a conventional wall-hung boiler. The Hydrobox contains a refrigerant gas-to-water plate heat exchanger. This component facilitates the heat transfer process efficiently rejecting the latent heat into the circulating water system.
As with all refrigerant containing systems, the installation and commissioning of a split system air source heat pump must be carried out by an F-Gas certified installer.
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Hydraulic module air source heat pumps represent the most advanced configuration compared to standard Monoblock and Split systems. Like split systems, they feature an outdoor unit and a separate indoor module. This indoor unit can often be upgraded to include an integral hot water cylinder for DHW storage and efficiency, depending on the manufacturer and model.
What fundamentally sets hydraulic modules apart is their use of a dual-circuit design with two independent refrigerant loops:
This dual-circuit design enables hydraulic modules to efficiently deliver flow and return water temperatures up to 80°C. This is a significant operational leap compared to the 55–60°C typically achieved by single-circuit Monoblock and Split systems. This capability makes them ideal for applications where low-temperature heating is needed most of the time (such as underfloor heating), but a reliable supply of much higher temperature domestic hot water (DHW) is required either intermittently or continuously.
Crucially, unlike other systems that rely on less efficient electric immersion heaters to achieve temperatures over 60°C for legionella safety or hot water top-ups, hydraulic modules achieve these high temperatures via the heat pump cycle alone. This significantly reduces overall energy consumption and minimizes running costs.
The Hitachi Yutaki S is a leading example of a hydraulic module ASHP, known for its high capacity and wide operating range.
Technical Highlights:
Use & Features:
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