Heat Pump Hot Water Cylinders: Choosing and Installing the Right Cylinder for Air Source Heat Pump Systems

The UK's rapid uptake of air source heat pumps (ASHPs) under the Boiler Upgrade Scheme has created a surge in demand for heat pump-compatible hot water systems. A standard indirect cylinder — designed for boiler flow temperatures of 65–80°C — performs poorly with an ASHP operating at 45–55°C flow temperature. Installers who don't understand the difference between a standard indirect cylinder and a dedicated heat pump cylinder are creating systems that fail to meet domestic hot water demand, run inefficiently, or carry Legionella risk. This guide covers why ASHPs need different hot water storage, how heat pump cylinders are designed, how to size them correctly, and how to install and commission them effectively.

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Why Standard Cylinders Don't Work Well with ASHPs

The fundamental difference between a boiler and an ASHP when it comes to hot water is temperature. A gas boiler heats water to 65–80°C for primary flow. At these temperatures, a small coil — often 1–2 m² of surface area in a standard indirect cylinder — can transfer enough heat to the cylinder water to raise it to 60°C within a reasonable time (30–60 minutes for a 180L cylinder).

An ASHP operates at much lower flow temperatures: typically 45–55°C for space heating, and 55–60°C for domestic hot water (DHW). At 55°C flow, the temperature differential between the coil and the cylinder water is much smaller — especially as the cylinder approaches its target temperature. Heat transfer rate is proportional to the log mean temperature difference (LMTD): a smaller temperature difference means slower heat transfer from the coil to the cylinder water.

The result with a standard cylinder on an ASHP:

• Very slow heat-up times — a 180L cylinder may take 3–5 hours to heat from cold
• The ASHP runs continuously at high capacity, reducing efficiency (lower COP)
• The cylinder may never fully reach the target temperature under heavy demand conditions
• Legionella risk if the cylinder cannot reliably reach 60°C

Heat Pump Cylinder Design: Larger Coils

A dedicated heat pump cylinder compensates for the lower flow temperature by using a significantly larger heat transfer coil — typically 2.5–4 m² of surface area, compared to 0.8–1.5 m² in a standard cylinder. The larger coil maintains adequate heat transfer even at the smaller LMTD between ASHP flow temperature (55°C) and cylinder water temperature (which starts cold and rises towards 55°C).

Key features of heat pump cylinders:

• Oversized lower coil: Large stainless steel or copper coil in the lower third of the cylinder, designed to extract maximum heat from the ASHP flow.
• Coil surface area: Typically 2.5–4 m²; some premium models (e.g. Joule Cyclone, OSO Ecoline, Mixergy heat pump variant) use 3.5–4 m² coils for maximum efficiency.
• Cylinder height: Heat pump cylinders are often taller and slimmer than standard cylinders — this promotes thermal stratification, with hot water at the top available for use while the lower section continues to be heated. Better stratification means the first draw of hot water is available sooner.
• Dual coil option: Some models include a second upper coil for a backup immersion heater or solar thermal connection, allowing the ASHP to handle the primary heat load while the immersion covers peak demand or provides Legionella pasteurisation.
• Immersion heater backup: An electric immersion heater (typically 3 kW) is standard in most heat pump cylinders. This is used for Legionella pasteurisation cycles (weekly heating to 60°C+) and as emergency backup if the ASHP is offline.
• Unvented or vented: Heat pump cylinders are available in both configurations, though unvented is standard for most modern ASHP installations to ensure adequate flow rate at hot water outlets.

Buffer Tanks: Do You Need One?

A buffer tank is a hydraulic separator — a thermal store that decouples the ASHP flow circuit from the heating distribution circuit. Whether a buffer tank is required depends on the system design:

When a Buffer Tank Is Needed

• Small system with a large ASHP: If the minimum flow rate of the ASHP exceeds the flow rate through the heating circuit at part load (when only a few zones are calling), the ASHP will short-cycle — starting and stopping frequently. A buffer tank prevents this by providing a volume of water that the ASHP can heat between cycles.
• Manufacturer's minimum water volume requirement: Most ASHP manufacturers specify a minimum system water volume. For a Daikin Altherma or Mitsubishi Ecodan, this may be 25–50 litres. If the pipework and emitter volume doesn't reach this, a buffer tank makes up the difference.
• Two-pipe zoned systems with multiple TRVs: When many zones can close simultaneously, a buffer prevents high head pressure on the pump and ASHP short-cycling.

When a Buffer Tank Is NOT Needed

• Systems with large underfloor heating manifolds — the UFH pipework itself provides substantial water volume.
• Systems sized correctly with adequate emitter surface area — larger emitters mean more water volume in the circuit.
• Some modern inverter-driven ASHPs (e.g. Samsung EHS, Mitsubishi Ecodan) can modulate down to very low capacities, reducing minimum flow rate requirements and eliminating the need for a buffer in many cases. Check the manufacturer's technical documentation.

DHW Priority vs Simultaneous Heating

One of the key system design decisions in an ASHP + DHW installation is how to manage the interaction between space heating and hot water. Two approaches:

DHW Priority

The ASHP switches to DHW heating mode when the cylinder calls for heat, temporarily stopping space heating. This is simpler to control and allows the ASHP to run at the higher flow temperature needed for DHW (55–60°C) without affecting the lower flow temperature used for space heating (35–50°C for UFH or radiators). A 3-port motorised valve typically directs ASHP flow to either the heating circuit or the DHW coil.

Disadvantage: space heating is interrupted during DHW cycles. In a well-insulated building with good thermal mass (UFH), this is unnoticeable. In a less well-insulated building or with radiators, occupants may notice cooling during DHW cycles.

Simultaneous Heating

The ASHP heats both the space heating circuit and the DHW cylinder simultaneously. This requires either a single high-capacity ASHP running at a higher flow temperature, or a cascade arrangement. The Mitsubishi Ecodan and some other systems can run simultaneous space heating and DHW at different temperatures using a heat exchanger arrangement. This is more complex but avoids space heating interruption.

Sizing the Heat Pump Cylinder

Heat pump cylinders should be sized generously because:

• ASHPs take longer to recover than boilers — a depleted cylinder on an ASHP may take 2–3 hours to reheat, so a larger store reduces recovery events
• Economy 7 and other time-of-use tariffs (Octopus Go, Agile) make overnight reheating attractive — a larger cylinder stores more cheap-rate electricity
• DHW demand spikes (morning, evening) are better served by a larger stored volume

Legionella Management

Legionella bacteria proliferate at temperatures between 20–45°C. An ASHP heating the cylinder to only 55°C is close to the upper edge of the danger zone — and if the cylinder doesn't reliably reach 55°C throughout its volume (particularly at the bottom), Legionella risk increases.

The standard approach for ASHP DHW systems:

• Normal operating temperature: ASHP heats the cylinder to 55°C (adequate for Legionella control at this temperature if pasteurisation is maintained at distribution level)
• Weekly Legionella pasteurisation cycle: the immersion heater heats the full cylinder to 60°C+ for at least 1 hour. This is typically set up as an automatic timer or via the heat pump controller. A 3 kW immersion in a 250L cylinder takes approximately 2.5–3 hours to raise from 55°C to 60°C
• Some heat pump controllers can instruct the ASHP to boost to 60°C for the pasteurisation cycle — this is less efficient but avoids the need for the immersion heater

Installation Arrangement

A typical ASHP + unvented heat pump cylinder installation:

1. ASHP outdoor unit → flow and return pipework (typically 28mm or 35mm copper, or pre-insulated plastic)
2. Buffer tank (if required) → connected in a hydraulic separator arrangement between ASHP and heating circuits
3. 3-port motorised valve for DHW priority: one port to space heating circuit, one port to DHW cylinder coil; valve actuated by cylinder thermostat
4. Heat pump cylinder: lower coil connected to ASHP flow/return; upper coil (if dual coil) for immersion or solar; immersion heater connected to dedicated 25 A circuit with timer for pasteurisation
5. Unvented cylinder safety train: PRV, expansion vessel, check valve, T&P relief valve, tundish, D1 and D2 discharge pipes (same as any unvented cylinder — G3 qualified installer required)
6. Cylinder thermostat and overheat stat — many dedicated heat pump cylinders have these pre-fitted or specify compatible thermostats from the heat pump controller system

Commissioning and System Optimisation

• Set DHW target temperature on the heat pump controller to 55°C (not higher — running at 60°C constantly degrades COP significantly)
• Set weekly pasteurisation cycle via immersion timer or heat pump controller
• Verify cylinder recovery time: from cold, the ASHP should bring a correctly sized cylinder to 55°C within 2–3 hours under normal conditions
• Check system operating COP data via the heat pump manufacturer's app or controller — a COP below 2.0 during DHW heating may indicate an undersized cylinder coil, excessive DHW temperature setpoint, or inadequate flow rate through the coil
• Insulate all DHW pipework, including the cylinder itself (factory-insulated, but any site-connected pipework should be insulated to prevent standing heat loss)

Products from APM

APM Electrics Plumbing stocks heat pump-compatible hot water cylinders, buffer tanks, motorised valves, expansion vessels, immersion heaters, and associated plumbing components for ASHP DHW systems. Browse our hot water collection and heating collection for heat pump installation components.

Visit us at APM Electricals, 24 Western Avenue, Acton, London, W3 7TZ or call 020 8896 0800 for trade and retail electrical and plumbing supplies.