Heat pump water storage: how to choose
and size the right cylinder for your installation

When a heat pump installation goes wrong, the water storage selection is one of the most common culprits. Specifying a boiler cylinder on a heat pump system, mixing up DHW and buffer roles, or selecting a vessel with a coil too small for the heat pump's low flow temperature — any of these errors produces either poor hot water performance or a system that short-cycles and wears out its compressor inside five years. This heat pump installation guide for water storage covers every decision an installer needs to make: vessel type and role, sizing rules for both DHW and buffer, unvented vs vented implications under Building Regulations, ErP rating requirements, and material selection for hard water areas.

DHW cylinder vs buffer tank: two different jobs in the same system

The most fundamental distinction is one of function. A domestic hot water (DHW) cylinder stores potable water that residents use at the tap. It must comply with water hygiene standards — the water inside must be heated to at least 60 °C periodically to control legionella risk, and the vessel material must be food-safe. In heat pump systems, a DHW cylinder is typically an indirect type with an internal heat exchanger coil connected to the heat pump's heating circuit.

A buffer tank (also called a buffer vessel or accumulator tank in continental European markets) stores non-potable heating circuit water between the heat pump and the distribution system. It contains no internal coil serving DHW. Its role is to provide hydraulic separation, thermal mass against short-cycling, and defrost cycle energy for air-source heat pumps. No potable water compliance applies — but it must be correctly sized for the heat pump output.

Many domestic heat pump installations require both vessels. The buffer tank sits on the primary (heat pump) circuit; the DHW cylinder sits on a secondary circuit fed by the buffer or connected directly to the heat pump via a diverter valve or plate heat exchanger. Some modern heat pumps use a combisystem in which a single vessel with multiple connections serves both roles — these are known as combination or dual-function cylinders.

Sizing a DHW cylinder for heat pump systems

Heat pump cylinders cannot be sized using the same rules as boiler cylinders. The reason is flow temperature. A gas boiler delivers water at 65–80 °C; a heat pump runs at 45–55 °C in standard operation. A lower flow temperature means a smaller temperature difference (delta-T) across the coil, which means the coil transfers heat more slowly. If the coil is sized for a boiler and you connect a heat pump, reheat times lengthen significantly — sometimes to four or five hours for a 200 L cylinder at 45 °C flow.

Coil surface area: the critical specification

A heat pump DHW cylinder must have a coil surface area of at least 4.5–6.0 m² to deliver adequate heat transfer at heat pump flow temperatures. Standard boiler cylinders typically have coil areas of 1.0–2.5 m². Using a boiler cylinder on a heat pump system is the single most common specification error in the market.

When reviewing cylinder datasheets, look for the coil surface area in square metres and the manufacturer's stated reheat time at a specific flow temperature. Reject any cylinder that quotes reheat performance at boiler-circuit flow temperatures only. For UK installers sourcing cylinders at wholesale, this spec point — coil area at heat pump flow temps — is a direct quality filter for the product.

Volume by household demand

Once the coil specification is confirmed, size the vessel volume by hot water demand:

• 1–2 people: 150–180 L
• 3–4 people: 200–250 L
• 4–5 people: 250–300 L
• 5+ people or high-demand properties: 300–400 L

These figures assume the cylinder is heated once daily to 55 °C at heat pump operating temperature. Properties with simultaneous high-draw requirements — for example, two bathrooms used at the same time — should move to the top of the applicable range. For installers working under the Boiler Upgrade Scheme (BUS), the heat pump manufacturer's published minimum cylinder specification must be met for grant eligibility.

Heat-up time and recovery rate

Recovery rate is the volume of water the cylinder can reheat from cold in one hour. For a heat pump system, recovery rate is directly determined by the heat pump's kW output and the coil area. At a flow temperature of 50 °C and a draw-off temperature of 40 °C, a 10 kW heat pump with a 5.0 m² coil can typically recover approximately 130–150 L per hour. A 200 L cylinder reheated from 10 °C cold will therefore take roughly 80–100 minutes — acceptable for overnight setback scheduling but slow for same-day recovery if hot water is exhausted mid-day.

For high-demand applications where same-day recovery is required, either increase cylinder volume so the reserve is less likely to be exhausted, or ensure the heat pump output is matched to the combined DHW and space heating load rather than the space heating load alone.

Unvented vs vented cylinders: what MCS and Building Regs require

The majority of new heat pump DHW installations in the UK use unvented cylinders — sealed, pressurised vessels connected directly to the mains water supply. They deliver mains-pressure hot water to all outlets without a gravity tank, which is both a performance advantage and a space saving. However, they carry strict regulatory requirements that every MCS-registered installer must understand.

Under Approved Document G (Section G3) of the Building Regulations for England and Wales — and equivalent regulations in Scotland (Technical Standard P3) and Northern Ireland (Building Regulation P5) — unvented hot water systems above 15 litres must be installed, commissioned, and serviced only by a person who holds a current G3 qualification. The G3 qualification covers the mandatory safety device set: expansion vessel, pressure reducing valve (PRV), temperature and pressure (T&P) relief valve, and tundish discharge pipework.

Annual inspection of the safety set is a regulatory obligation for unvented systems. Building owners must evidence this inspection; failure to do so can void building insurance and creates liability exposure in the event of a system failure. For a full treatment of the technical differences between unvented and vented systems — including safety mechanism specifications and selection guidance — read our article on unvented vs vented water systems.

ErP rating: what it means for product selection and compliance

The EU Energy-related Products (ErP) Directive — and its retained equivalent in UK law post-Brexit — requires that all hot water storage products placed on the market carry an energy efficiency rating. For DHW cylinders used in heat pump systems, the relevant regulation is EU 812/2013, the Energy Labelling Regulation for water heaters and hot water storage tanks.

Under EU 812/2013, cylinders are rated from A (most efficient) to G (least efficient) based on standing heat loss — the rate at which a fully heated cylinder loses energy to the surrounding air while idle. The rating depends on cylinder volume and insulation quality. For heat pump applications in the EU and UK, products must achieve a minimum ErP class C. Class B and A are achievable with thicker PU foam insulation (typically 50 mm or more) and minimised fitting penetrations through the foam layer.

Why this matters for product selection: an ErP C-rated cylinder that loses 1.5 kWh per day to standby heat loss will require the heat pump to run an additional cycle every day just to maintain the set temperature. At a typical heat pump seasonal COP of 3.0, that 1.5 kWh of heat costs 0.5 kWh of electricity — small per day, but over a year it represents a meaningful operating cost. Specifying an ErP B or A cylinder over a C-rated equivalent reduces standby losses by 30–60% and is particularly important for lightly insulated plant rooms or utility spaces. For a detailed explanation of how PU foam insulation thickness affects ErP class, see our article on PU foam insulation and ErP energy ratings.

For wholesale buyers, ErP class must appear on the product declaration and energy label shipped with each cylinder. Distributors placing cylinders on the EU or UK market are responsible for ensuring ErP documentation is present and compliant. Heatlyt HC series cylinders are supplied with full ErP product datasheets and energy labels as standard.

Hard water and corrosion: material selection for DHW cylinders

In hard water areas — broadly, anywhere east of a line from the Humber Estuary to Dorset in England, and across much of central and eastern Europe — limescale accumulation inside DHW cylinders is a significant maintenance and performance issue. Scale builds up on the coil heat exchanger surface, increasing thermal resistance and reducing effective reheat performance. A 1 mm scale layer on a coil surface reduces heat transfer efficiency by approximately 10%; a 3 mm layer reduces it by 25–30%.

Material selection affects how scale interacts with the vessel over time:

• Stainless steel (SUS304): Scale deposits on the coil surface but does not bond with the steel substrate. The surface can be descaled mechanically or chemically without damaging the vessel. No sacrificial anode is required. Expected vessel lifespan is 30–50 years in hard water areas with normal maintenance.
• Vitreous enamel (glass-lined): Enamel is chemically inert to limescale but is prone to thermal expansion cracking in areas of very high hardness (above 400 ppm CaCO₃). A sacrificial magnesium anode must be inspected and replaced every 3–7 years to protect the steel shell where the enamel cracks. Failure to replace the anode leads to internal corrosion and premature cylinder failure, typically after 5–8 years.

For heat pump DHW installations in hard water regions, stainless steel cylinders are the lower whole-life-cost option. They eliminate anode replacement cost and are not vulnerable to enamel cracking under thermal cycling. For a full analysis of the lifespan and total cost difference between stainless and enamel vessels, see our comparison article on stainless steel vs enamel water tanks.

In coastal and aggressive-water environments (elevated chloride content), SUS316 stainless steel — which contains 2–3% molybdenum for enhanced pitting corrosion resistance — is preferable to the standard SUS304 grade. For the vast majority of inland UK and EU sites, SUS304 is adequate. For grade selection guidance, see our article on SUS304 vs SUS316 water tanks.

System type guide: which vessels do you need?

The correct vessel combination depends on the system configuration. The three most common installation types are:

DHW-only installation

The heat pump provides domestic hot water only — no space heating. Typically a retrofit to supplement an existing boiler heating system. The primary vessel is a DHW cylinder with a large heat exchanger coil (minimum 4.5 m²). A buffer tank may or may not be required depending on whether the heat pump manufacturer mandates minimum system volume — always check the installation manual. Many monobloc heat pumps designed specifically for DHW do not require a separate buffer vessel.

DHW plus space heating (separate vessels)

The most common full-system configuration in the UK domestic market. A buffer tank sits on the primary heating circuit for short-cycle protection and defrost energy storage; a separate DHW cylinder serves domestic hot water via a diverter valve or plate heat exchanger. Size the buffer vessel using the 25 L/kW rule for ASHP — see our detailed buffer tank sizing guide for the full calculation — and the DHW cylinder by household demand as described above.

Combination buffer and DHW cylinder

Some installations use a single vessel that combines the buffer and DHW functions — typically a dual-coil cylinder with one coil serving the heating circuit and a second coil or internal section for potable water. These are space-efficient but must be sized carefully: the DHW volume must meet household demand, and the non-potable buffer volume must meet the heat pump's minimum volume requirement. Heatlyt's HDC-300 dual-coil cylinder is engineered for this configuration.

Heatlyt product recommendations by system type

The table below maps common heat pump installation scenarios to the appropriate Heatlyt vessel combination. All HC DHW cylinders feature oversized heat exchanger coils suitable for heat pump flow temperatures; all HB buffer tanks are manufactured from SUS304 stainless steel with PU foam insulation.

For installations outside the standard ranges above — including light-commercial projects, multi-unit developments, or systems requiring larger buffer volumes — Heatlyt supplies OEM vessels from 100 L to 500 L with custom connection layouts, SUS316 steel, and OEM labelling available from a 20 ft container minimum order.

Key takeaways

• A DHW cylinder and a buffer tank serve different functions — never substitute one for the other.
• Heat pump DHW cylinders need a coil surface area of at least 4.5–6.0 m²; standard boiler cylinders are not suitable.
• Unvented cylinders are standard for new heat pump DHW installations; G3 qualification and annual safety inspection are mandatory under Approved Document G.
• ErP class C is the minimum for EU/UK market compliance; class B or A reduces standby heat loss by 30–60% and lowers operating cost over the cylinder's life.
• Stainless steel outperforms vitreous enamel in hard water areas — no anode replacement, 30–50 year lifespan, lower whole-life cost.
• ASHP buffer tank minimum: 25 L/kW per BS EN 14511 for defrost cycle energy. GSHP: 10 L/kW for cycling prevention.
• Document all sizing calculations in the system design file for MCS compliance.