Air Source Heat Pump Accessories

A complete air source heat pump installation requires several accessories beyond the outdoor unit itself to function correctly and efficiently. A hot water cylinder is needed to store domestic hot water, as heat pumps cannot produce instantaneous hot water in the way a combi boiler does. A buffer vessel or hydraulic separator is often specified to protect the compressor from short cycling and to decouple the heat pump circuit from the heating distribution circuit. Expansion vessels, pressure relief valves, and correctly sized circulating pumps are also required as part of the primary circuit. Thermostatic controls and a weather compensation controller allow the system to modulate output based on outdoor temperature, which is one of the most effective ways to maximise efficiency. Pipework insulation, mounting brackets, anti-vibration feet, and condensate drainage provisions complete the installation package. Your MCS-certified installer will specify the exact accessories required based on the heat pump model, property heat demand, and existing system configuration.

A buffer vessel is a small insulated tank, typically between 20 and 100 litres, installed in the primary heating circuit to provide additional water volume between the heat pump and the heating distribution system. Its main purpose is to prevent short cycling, which occurs when the heat pump compressor switches on and off too rapidly due to insufficient water volume in the system. Short cycling causes wear on the compressor and reduces efficiency over time, so a buffer vessel protects the unit and extends its working life. Not every installation requires a dedicated buffer vessel, as some heat pumps have sufficient internal volume and some heating systems with large radiators or underfloor heating provide enough circuit volume naturally. A correctly sized expansion vessel should also be included in the primary circuit alongside the buffer vessel to absorb pressure fluctuations safely. Whether a buffer vessel is needed will depend on the heat pump model, system design, and the total volume of water in the heating circuit — your installer will carry out system calculations and may also specify a central heating pump to maintain adequate flow rates throughout the system.

Weather compensation is a control strategy that automatically adjusts the flow temperature of the heating system based on the outdoor air temperature, using a sensor fitted to the exterior of the building. On milder days, when less heat is needed, the system lowers the flow temperature, which allows the heat pump to operate at a higher coefficient of performance and consume less electricity. On colder days, the flow temperature is raised to meet the higher heat demand of the building. Without weather compensation, a heat pump may default to a fixed flow temperature that is higher than necessary for most of the year, reducing overall efficiency. Installing a weather compensation controller and outdoor sensor is widely considered one of the most cost-effective upgrades for any heat pump system — you can browse compatible options in our thermostats and controls range. Most modern heat pumps have weather compensation built in or available as a factory accessory, but the outdoor sensor must be correctly positioned away from direct sunlight and wind shelter to give accurate readings.

Most hot water cylinders specified for heat pump installations include an electric immersion heater as a secondary heat source, and this is considered best practice rather than an optional extra. The immersion heater serves two important functions: it provides a backup heat source if the heat pump requires servicing or is temporarily out of operation, and it enables a weekly legionella prevention cycle that heats the cylinder contents to 60°C to eliminate any risk of bacterial growth. Heat pumps are most efficient producing hot water at 45 to 55°C, which is below the 60°C threshold recommended for legionella control, so relying on the heat pump alone for this periodic boost would require it to operate at reduced efficiency. The immersion heater carries out the 60°C boost quickly and infrequently, minimising the additional electricity cost. When specifying a replacement cylinder for a heat pump retrofit, always confirm it includes an immersion heater boss and is rated for the higher temperatures the immersion will reach during the legionella cycle.

A heat pump requires an unvented or vented indirect cylinder specifically designed for use with a heat pump, rather than a standard cylinder sized for a boiler system. The key difference is the size of the coil heat exchanger inside the cylinder: heat pump cylinders use a larger coil surface area to allow effective heat transfer at the lower flow temperatures a heat pump produces, typically between 45 and 55°C. A standard boiler cylinder with a small coil will not transfer heat efficiently at these temperatures and will result in long reheat times and poor performance. Cylinder capacity should be sized to meet the household's daily hot water demand without relying on frequent reheating cycles; a family of four will typically need 200 to 250 litres. The cylinder should also be well insulated to retain heat between heating cycles, reducing the frequency with which the heat pump needs to run. Always confirm cylinder compatibility with the specific heat pump model being installed, and check our range of immersion heaters to ensure the cylinder can be fitted with a legionella boost element as part of the completed installation.

Yes, insulating all pipework between the external heat pump unit and the internal plant room is essential, particularly any sections that run externally or through unheated spaces such as roof voids, garages, or underfloor voids. Heat pump systems run at lower flow temperatures than boilers, which means uninsulated pipework loses a greater proportion of its useful heat to the surrounding environment, directly reducing system efficiency and increasing running costs. External pipework is also vulnerable to freezing in cold weather if it is not adequately insulated and trace-heated where necessary. Browse our pipe insulation range for suitable closed-cell elastomeric foam options — the most commonly specified materials for heat pump pipework due to their low thermal conductivity and resistance to condensation. All insulation used externally should be UV-resistant or protected with a suitable outer jacket to prevent degradation over time. Correctly insulated pipework is a straightforward measure that protects system performance, and our chemicals and additives range includes inhibitor and antifreeze products to further protect the system from the inside throughout the year.

Air source heat pump systems that include external pipework runs, or where the system design requires frost protection, are typically filled with a glycol-based antifreeze solution to prevent freezing during cold weather. Monoblock heat pump installations, where only water pipework runs between the external unit and the building, are particularly susceptible to freezing in the external section of pipework if the system loses power or is shut down during a cold spell. The glycol concentration required depends on the minimum expected ambient temperature at the installation location, with most UK installations using a 25 to 30% glycol mix providing protection to approximately minus 15°C. In addition to antifreeze, a corrosion and scale inhibitor should be added to the system water to protect internal components including the heat exchanger, circulating pumps, and manifolds — you can find suitable products in our chemicals and additives range. Inhibitor concentration should be checked annually as part of the service visit and topped up if necessary. Always use products specified or approved by the heat pump manufacturer, and pair them with correctly sized expansion vessels to maintain safe system pressure as fluid volume changes with temperature.

The external unit must be mounted on a stable, level base that raises it clear of the ground to allow condensate and defrost water to drain freely and to prevent the unit sitting in standing water during wet weather. Purpose-made anti-vibration mounting feet or pads are used between the unit and its base to reduce structure-borne noise transmission into the building, which is particularly important for installations where the unit is close to a wall or on a roof. Wall-mounted bracket kits are available for installations where a ground-level concrete pad is not practical, and these should be rated for the weight and wind loading of the specific unit. The base itself should be a concrete pad of sufficient thickness, or a purpose-made plastic or composite mounting frame designed for heat pump installations. Condensate drainage must be directed away from the unit base and building foundations to prevent waterlogging or frost heave. A correctly specified central heating pump should also be included in the internal circuit to maintain adequate flow rates, and all primary circuit components should be sourced from our broader heat pumps category to ensure full compatibility.

Most modern air source heat pumps come with a proprietary controller or are compatible with a manufacturer-approved smart thermostat, and using the correct control system is important for achieving the efficiency the heat pump is designed to deliver. Unlike a gas boiler, which responds well to simple on/off thermostatic control, a heat pump benefits from controls that allow it to modulate output gradually and run for longer periods at lower temperatures rather than switching on and off repeatedly. Many manufacturers, including Vaillant with their sensoAPP system, provide smart controls with app connectivity that allow homeowners to monitor performance, set heating schedules, and view energy consumption data. You can browse our full range of thermostats and controls, including wireless thermostats compatible with a range of heating systems. Weather compensation, load compensation, or a combination of both should be enabled through the control system to maximise seasonal efficiency. Always confirm control compatibility with your installer before purchasing third-party controllers separately.

Yes, most heat pump accessories including cylinders, buffer vessels, controls, mounting hardware, pipework insulation, and inhibitors can be purchased separately from the main unit and sourced through a trade builders merchant. Buying accessories through a merchant rather than a manufacturer-direct package can offer more flexibility on specification and better pricing, particularly for installers managing their own material procurement. It is important to confirm compatibility between accessories and the specific heat pump model being installed, as some manufacturers specify approved accessories to protect warranty conditions or to meet MCS installation requirements. Cylinders in particular must match the heat pump output and be correctly sized for the property's hot water demand. When procuring materials for a heat pump installation, a bill of materials from the designing engineer should list all required accessories with specifications, making it straightforward to source the correct items. MP Moran stocks a range of heating system accessories across its branches in London and Hertfordshire to support trade contractors working on heat pump installations.