Thermal Storage for Solar Heat Pumps: 10-25 L/kW

Heat pumps are increasingly paired with rooftop solar PV, and the missing piece in many designs is the buffer tank. It is the thermal storage vessel that sits between the heat pump and the building, storing surplus solar energy as hot water and protecting the compressor from short cycling. Size it wrong and the system underperforms. Too small, and the heat pump cycles on and off every few minutes. Too large, and standing heat losses waste energy year-round.

Last updated: November 2026

This guide covers buffer tank sizing for residential, commercial and institutional heat pump + solar systems. It is written for solar designers, heat pump installers and M&E engineers who need to specify tank volume, pipe configuration, insulation and PV diverter integration. Model PV yield and surplus in solar PV design software, and check the hydraulics against the heat pump manufacturer’s data before signing off on any specification.

In this guide you will learn:

• Why buffer tanks are mandatory for most heat pump + solar designs
• The four functions of a buffer tank and how they affect sizing
• The 10-25 L/kW sizing rule with adjustments for each application
• The difference between 2-pipe and 4-pipe tanks and why stratification matters
• How to integrate solar PV diverters (Eddi, myenergi) with buffer tanks
• Where to place the tank: heating circuit vs DHW circuit vs combined
• Material selection, insulation requirements, and UK regulations
• Three worked examples with full calculations

Latest Updates: Thermal Storage and Heat Pump Standards 2026

Funding and standards change often, so check each source before you quote. Three items matter for a heat pump + solar designer working in the UK in 2026.

The implication for all-in-one solar software users: never assume the existing cylinder can stay. Re-specify the buffer tank and DHW cylinder together on a retrofit. For the certification side, see MCS certification for solar installers in the UK.

What a Buffer Tank Does in a Heat Pump + Solar System

A buffer tank is not just a big hot water cylinder. In a heat pump system it performs four distinct engineering functions, and each function changes the required volume.

Function 1: Hydraulic Decoupling

Heat pumps use variable-speed compressors that modulate output across a range of rated capacity. The heating circuit, radiators or underfloor heating, has a flow rate set by pipe diameter and pump speed. When the circuit’s flow rate does not match the heat pump’s minimum flow rate, the heat pump short cycles: it starts, reaches setpoint in seconds, stops, and restarts a few minutes later.

Short cycling hurts efficiency and compressor life. Every start-up draws a high inrush current and runs the compressor at low COP until it stabilises. A buffer tank adds water volume to the primary circuit and extends the minimum run time. The heat pump runs for longer, heating the tank, then shuts off while the heating circuit draws from the stored energy.

A common rule of thumb for decoupling alone is 10-15 litres per kW of heat pump capacity. A 5 kW unit then needs 50-75 litres and a 12 kW unit needs 120-180 litres. Always check the manufacturer’s minimum system volume too. Our air source heat pump + solar PV sizing guide covers the upstream sizing decisions that feed into this calculation.

Function 2: Defrost Energy Storage

Air source heat pumps can frost up when outdoor temperature is low and humidity is high. The outdoor coil ices over, reducing airflow and heat transfer. The heat pump then reverses to melt the ice, drawing heat from the indoor side. Our deeper dive on heat pump defrost cycles in winter covers the thermodynamics in detail.

Without a buffer tank, the heat pump takes defrost heat from the building, and indoor temperature can dip while it runs. With a buffer tank, the heat pump draws defrost energy from the tank instead. The buffer acts as a small thermal battery for the defrost cycle, which lasts a few minutes.

Rules of thumb add roughly 5-10 litres per kW for defrost protection. A 5 kW unit in a cold climate then needs 75-100 litres and a 12 kW unit needs 180-240 litres. These are design heuristics, not a published standard.

Function 3: Solar PV Diverter Storage

Solar PV diverters such as the myenergi eddi, Solar iBoost and Immersun monitor grid export and redirect surplus PV electricity to a resistive immersion heater in the buffer tank. Instead of exporting solar power at the export rate, the homeowner stores it as hot water and avoids buying heat at the import rate. This is the same surplus-to-storage logic explored in our heat pump COP and solar self-consumption guide.

For diverter integration, the tank needs enough volume to absorb the typical daily solar surplus directed to heating. A 3 kW immersion heater takes several hours to heat 200 litres from 35°C to 55°C, and a larger tank stores more surplus but has higher standing losses.

A rule of thumb adds 5-10 litres per kW for diverter compatibility. A 5 kW residential system with a diverter then needs 100-125 litres, and a 12 kW system needs 240-300 litres.

Function 4: DHW Pre-Heat

In combined space heating and DHW systems, the buffer tank can pre-heat cold mains water before it enters the DHW cylinder. Cold water at 10°C entering a DHW cylinder makes the heat pump work harder and longer. Pre-heating to 30°C in the buffer covers 20 K of a 45 K rise to 55°C, which is about 44% of the heating energy.

DHW pre-heat is most valuable in homes with high hot water demand or in commercial settings with continuous DHW loads. A rule of thumb adds 5-10 litres per kW if the tank has a DHW pre-heat coil. For dedicated DHW configurations, see our hot water heat pump + solar PV sizing guide.

The 10-25 L/kW Sizing Rule Explained

The 10-25 L/kW range is a widely used installer rule of thumb, not a published standard. The exact figure within it depends on which functions the tank must perform, so confirm it against the heat pump manufacturer’s minimum system volume.

These figures assume a monobloc or split heat pump with a design flow temperature in the 35-45°C range. Higher flow temperatures, such as legacy radiators at 50-55°C, can push the lower bound up even for basic decoupling.

Why the Range Is Wide

The 10-25 L/kW range exists because heat pump systems vary enormously in hydraulic design. Factors that push you toward the upper end of the range:

• Small heating circuit volume (microbore pipes, few radiators)
• High heat pump modulation ratio (units that cannot throttle below 50%)
• Cold climate with frequent defrost cycles
• Solar PV diverter installed
• DHW pre-heat coil fitted
• Underfloor heating with low flow temperature and high thermal mass

Factors that let you use the lower end:

• Large heating circuit volume (22 mm pipes, many radiators, high thermal mass)
• Heat pump with wide modulation range (30-100%)
• Mild climate with few frosts
• No solar diverter
• Separate DHW cylinder with its own heat pump or immersion

Volume vs Surface Area: The Hidden Tradeoff

A larger tank stores more energy but also has more surface area, which increases standing heat losses. The relationship is not linear, because surface area grows more slowly than volume. The surface areas below are approximate for a cylindrical tank.

Standing heat loss = Surface area (m²) x U-value (W/m²K) x Temperature difference (K)

The table below is an illustrative calculation, not product data. It assumes a U-value of 0.30 W/m²K, a tank at 55°C and ambient air at 20°C (a 35 K difference). Real tanks publish a declared standing loss in watts on the product data sheet or energy label, so use that figure in your design.

At an assumed 30p/kWh, the annual cost of standing losses in this illustration is about £41 to £105 depending on tank size. Heat loss falls in proportion to the U-value, so a better-insulated tank loses proportionally less.

2-Pipe vs 4-Pipe Buffer Tanks: Stratification Matters

The pipe configuration of a buffer tank determines whether the water mixes fully or forms temperature layers. This choice affects heat pump efficiency, comfort, and solar diverter performance.

2-Pipe Buffer Tanks

A 2-pipe tank has one connection at the top and one at the bottom. Water enters at one point, mixes with the existing volume, and exits at the other. The entire tank tends toward a single uniform temperature.

Advantages:

• Simpler installation, only two pipe connections
• Lower cost than a 4-pipe equivalent
• Smaller footprint, no extra tappings

Disadvantages:

• Destroys stratification, the whole tank sits at the mean temperature
• Forces the heat pump to reheat the entire volume, not just the top layer
• Reduces effective storage capacity because the bottom half is too cool for heating
• Raises the return temperature the heat pump sees, which lowers COP

2-pipe tanks work for basic decoupling in small residential systems where cost is the primary constraint. They are not recommended for solar diverter integration or systems with underfloor heating.

4-Pipe Buffer Tanks

A 4-pipe tank has separate supply and return connections for both the heat pump circuit and the heating circuit. Hot water from the heat pump enters at the top. Cool return water from the heating circuit enters at the bottom. The heating circuit draws from the top and returns to the bottom. Stratified layers form naturally.

Advantages:

• Maintains temperature stratification, hot at top, cool at bottom
• Lets the heat pump operate at lower return temperatures, raising COP
• Lets the heating circuit draw water at the temperature it needs
• Maximises usable storage volume, because less of the tank sits at an unusable temperature
• Improves solar diverter efficiency because the immersion heats the top layer first

Disadvantages:

• More complex installation, four pipe connections plus tundish and valves
• Higher cost than a 2-pipe equivalent
• Requires careful pipe sizing to avoid turbulence that disrupts stratification

The Science of Stratification

A heat pump’s COP rises as the temperature it must lift falls. In a stratified tank, the heat pump can draw return water from the cool bottom layer, so it works against a lower return temperature. In a mixed tank the whole volume sits near the setpoint, so the return temperature is higher. The size of the gain depends on the tank, flow rates and controls, so this guide does not quote one efficiency figure. Ask the tank manufacturer for stratification performance data.

The table shows the logic with illustrative COP values. They are assumptions for the arithmetic, not measured data.

In this illustration the mixed tank needs 493 kWh more electricity a year than the stratified one, about £148 at an assumed 30p/kWh. Use that gap to weigh the extra cost of a 4-pipe tank, then repeat the calculation with your own heat pump’s COP curve.

Internal Baffles and Diffusers

Some buffer tanks use internal baffles or diffusers to maintain stratification. A diffuser plate at the inlet spreads incoming water horizontally, reducing turbulent mixing. A baffle plate separates the tank into upper and lower zones with limited mixing between them.

For smaller residential systems, a standard 4-pipe tank with correct pipe sizing may be enough. For larger commercial systems or systems with solar diverter integration, ask the manufacturer whether diffuser plates or baffles are available.

Solar PV Diverter Integration: Eddi, myenergi, and Alternatives

A solar PV diverter is the bridge between rooftop solar and thermal storage. It turns surplus PV electricity into hot water, raising self-consumption without the cost of a battery.

How PV Diverters Work

A current transformer (CT) clamp sits on the main incoming supply cable, measuring grid import and export in real time. The diverter unit reads the CT signal and controls a resistive immersion heater in the buffer tank. When export is detected above the unit’s minimum load, the diverter ramps up the heater to absorb the surplus. When import is detected, the diverter reduces or stops the heater.

Check the product data sheet for response time and for how the unit handles cloud transients.

myenergi eddi Specifications

The myenergi eddi is one of the best-known PV diverters in the UK. Per the myenergi eddi product page, the published specifications include:

myenergi says the eddi can work alongside heat pumps and can switch between a heat pump and its immersion heater depending on available generation. The page does not list SG Ready certification for the eddi, so check with the manufacturer before you specify it for heat pump signalling.

Other PV Diverters

Other diverters on the UK market include Solar iBoost, Immersun and GivEnergy’s options. Compare maximum load, number of outputs, CT type and control integration on each manufacturer’s own data sheet. This guide does not rank them, because specs and prices change and are best taken from the maker’s own page.

Buffer Tank Requirements for Diverter Integration

When a PV diverter is installed, the buffer tank needs:

1. An immersion heater port, typically 1.5“ BSP thread at the top of the tank, sized for a 3 kW element. Some tanks come with dual immersion ports for 3 kW + 1.5 kW elements.

2. Sufficient volume, the diverter sends surplus PV to the tank over 4-8 hours on a sunny day. A 200-litre tank heated from 35°C to 55°C stores roughly 4.7 kWh of usable heat. At 3 kW diverter output, that takes 1.5-2 hours. A larger tank stores more surplus but also has higher standing losses.

3. Stratification-friendly design, the immersion heater should heat the top layer only, not the whole tank. In a 4-pipe stratified tank, the immersion sits in the top zone. In a 2-pipe tank, the immersion heats the entire volume, which is less efficient.

4. Temperature limit control, the diverter must not overheat the tank. Most units include a tank thermostat input that stops heating at 60-65°C. The heat pump’s own control should also limit tank temperature to prevent excessive pressure and scaling.

Self-Consumption Impact

A diverter raises self-consumption by absorbing midday PV surplus that would otherwise be exported. The improvement tends to be largest in spring and autumn, when PV surplus is substantial and heating demand is moderate. In summer the tank may reach its temperature limit before all surplus is absorbed, and in winter there is little surplus to divert. This guide does not quote a single percentage, because the result depends on array size, load profile, tank volume and controls. Model the full-year curve in a generation and financial tool before deciding whether to pair the tank with a battery.

Tank Placement: Heating Circuit vs DHW Circuit vs Combined

The physical location of the buffer tank in the hydraulic system determines what it can do and how it interacts with other components.

Option 1: Buffer Tank on the Heating Circuit Only

This is the most common configuration. The buffer tank sits in the primary circuit between the heat pump and the heating emitters (radiators or underfloor heating). DHW is handled by a separate cylinder with its own heat pump or immersion heater.

Hydraulic layout:

Heat pump flow → Buffer tank top inlet → Buffer tank → Buffer tank bottom outlet → Heating circuit pump → Emitters → Return → Buffer tank bottom inlet → Heat pump return

Advantages:

• Simplest design, the buffer only sees heating water, not potable water
• No legionella concerns because the buffer is a closed heating circuit
• Easy to add solar diverter, immersion heater in the buffer heats heating water
• Standard off-the-shelf buffer tanks work without modification

Disadvantages:

• DHW demand is not buffered, the heat pump must respond to hot water calls directly
• Two separate heat sources (heat pump for heating, immersion or second heat pump for DHW)
• Higher capital cost if a separate DHW heat pump is specified

In this configuration the buffer is commonly a modest volume sized as shown above. The DHW cylinder has its own immersion heater for legionella protection.

Option 2: Buffer Tank as DHW Pre-Heat

In this configuration, the buffer tank pre-heats cold mains water before it enters the DHW cylinder. The buffer is heated by the heat pump or solar diverter. Cold water at 10°C passes through a coil in the buffer and exits at 30-35°C. The DHW cylinder then heats it to 55-60°C.

Advantages:

• Cuts the temperature rise the DHW cylinder must supply (a pre-heat from 10°C to 30°C covers 20 K of a 45 K rise)
• Uses the buffer tank for both heating and DHW functions
• Solar diverter energy can pre-heat DHW indirectly

Disadvantages:

• Requires a buffer tank with a DHW coil, more expensive and complex
• The coil adds pressure drop and requires a secondary pump in some designs
• Legionella risk if the buffer temperature drops below 25°C for extended periods
• The DHW cylinder still needs an immersion heater for legionella boost

This configuration is common in continental European systems but less common in the UK. It works best in homes with high DHW demand and a heat pump that struggles to reach legionella-control temperatures.

Option 3: Combined Buffer and DHW Tank

Some manufacturers offer combined tanks with internal baffles or separate chambers. The upper chamber acts as a DHW cylinder. The lower chamber acts as a heating buffer. A heat pump coil or solar coil heats the lower chamber. An immersion heater in the upper chamber provides legionella protection.

Advantages:

• Single tank reduces footprint and installation cost
• Integrated design from the manufacturer ensures compatibility
• DHW and heating loads are thermally separated by the baffle

Disadvantages:

• Limited choice of manufacturers, so check availability early
• If the tank fails, both heating and DHW are lost
• DHW volume is often smaller than a standalone cylinder
• Higher replacement cost if the tank needs changing

Combined tanks are popular in new-build apartments and compact homes where space is limited. For larger homes and commercial buildings, separate tanks are preferred for redundancy and capacity.

Legionella Protection Requirements

Legionella control is a design requirement for stored hot water. The reference is HSE guidance on legionella control (see the HSE legionella guidance), which sets expectations for storage and distribution temperatures. Most heat pumps cannot reach legionella-control temperatures efficiently, so the standard solution is an immersion heater boost. The heat pump heats the DHW cylinder to its normal setpoint, and the immersion heater periodically raises the temperature for the required hold time. This is often called the legionella cycle or anti-legionella boost.

In a buffer tank + DHW system, the legionella cycle applies to the DHW cylinder, not the buffer tank. The buffer is a closed heating circuit and does not store potable water. If the buffer is used for DHW pre-heat, make sure the pre-heat coil is designed for potable water, and follow the manufacturer’s water treatment guidance for the buffer.

Materials, Insulation, and UK Regulations

Tank Materials

Match the material to local water quality and the manufacturer’s guidance. Soft or aggressive water often points to stainless steel.

Insulation Standards

Insulation requirements for hot water storage vessels sit in the Building Regulations (Approved Document L) and in product standards, and they change between editions. Do not rely on a generic thickness. Compare tanks on the declared standing loss shown on the product data sheet or energy label.

Buffer tanks usually run cooler than DHW cylinders, so losses are lower than at a 60°C test condition. Heat loss scales with temperature difference. A tank at 45°C in 20°C air (25 K) loses about 71% as much as one at 55°C (35 K), because 25 / 35 is 0.71.

UK Regulations and Standards

• MIS 3005 (MCS heat pump standard): read the current published version for buffer vessel, low-loss header and minimum run time requirements. Do not rely on a summary.
• Building Regulations Approved Document L: sets expectations for insulation and heat loss of storage vessels in new-build and major renovation work.
• Water Supply (Water Fittings) Regulations 1999: cover backflow prevention and fittings on anything connected to the mains. They apply to DHW cylinders, not to closed heating circuit buffers.
• Heat Networks (Metering and Billing) Regulations 2014: may apply to communal buffer tanks serving multiple dwellings.

The full UK compliance picture, including MCS installer audits and Boiler Upgrade Scheme rules, is covered in our MCS heat pump + solar UK guide.

Pressure and Safety

Unvented buffer tanks and DHW cylinders must include the safety devices required by the manufacturer and the Building Regulations:

• Temperature relief valve to prevent boiling
• Pressure relief valve to prevent over-pressurisation
• Tundish as a visible discharge point for relief valve water
• Expansion vessel to absorb thermal expansion of heated water
• Pressure reducing valve to keep mains pressure within the cylinder’s rating

Closed heating circuit buffers operate at heating system pressure and follow the heat pump manufacturer’s pressure and safety requirements.

Worked Example 1: 5 kW Heat Pump + 6 kWp PV: Residential Semi-Detached

Consider a hypothetical household in a 110 m² semi-detached house in Birmingham. All inputs in this example are assumptions for illustration, including an import price of 30p/kWh and an export rate of 5p/kWh. They have a 5 kW monobloc air source heat pump with underfloor heating downstairs and oversized radiators upstairs. Assume a heat loss calculation shows annual space heating demand of 8,500 kWh and DHW demand of 2,500 kWh. Total: 11,000 kWh. Projected SCOP: 3.5. They also have a 6 kWp solar PV array with a myenergi Eddi diverter.

Step 1: Determine Buffer Tank Functions

The tank must perform three functions:

1. Hydraulic decoupling, the underfloor heating manifold has low volume
2. Defrost protection, assume frequent defrost days in winter
3. Solar diverter storage, the Eddi sends surplus PV to the tank

No DHW pre-heat is needed because they have a separate 200-litre DHW cylinder with its own heat pump coil.

Step 2: Apply the Sizing Rule

Base sizing for decoupling + defrost + diverter: 20-25 L/kW

5 kW x 20 L/kW = 100 litres (minimum) 5 kW x 25 L/kW = 125 litres (recommended)

The underfloor heating has very low circuit volume, roughly 10 litres for the manifold and pipework. This pushes toward the upper end of the range. The Eddi diverter needs enough volume to absorb summer surplus.

Selected tank: 120-litre 4-pipe stratified buffer tank with 50 mm PU foam insulation, 3 kW immersion heater port, and inlet diffuser plate.

Step 3: Calculate Standing Heat Loss

Tank surface area (120 L, cylindrical): approximately 1.8 m² Insulation: 50 mm PU foam, U-value 0.30 W/m²K Average tank temperature: 42°C (heating circuit setpoint) Ambient temperature: 20°C (utility room) Temperature difference: 22 K

Standing loss = 1.8 m² x 0.30 W/m²K x 22 K = 11.9 W Annual standing loss = 11.9 W x 8,760 hours = 104 kWh/year Annual cost at 30p/kWh = £31/year

Step 4: Calculate Solar Diverter Contribution

Assume the 6 kWp array in Birmingham generates 5,400 kWh/year. Without storage, assume self-consumption is 40% = 2,160 kWh. Assume the Eddi diverter raises self-consumption to 55-60%.

Additional self-consumed energy: 15-20% of 5,400 kWh = 810-1,080 kWh/year Value at 30p/kWh instead of 5p/kWh export = £203-270/year Assumed Eddi cost: £400 Assumed buffer tank cost: £650 Total upgrade cost: £1,050 Payback: £1,050 / £237 (midpoint) = 4.4 years

Step 5: Verify Defrost Protection

The 120-litre tank stores usable heat energy calculated as:

Usable heat = Volume (L) x Temperature difference (K) x Specific heat capacity (kWh/L·K)

At 45°C tank temperature and 35°C minimum useful temperature: Usable heat = 120 L x 10 K x 0.00116 kWh/L·K = 1.4 kWh

A 5 kW heat pump defrost cycle draws roughly 1.5-2.5 kW of heat for 3-5 minutes. Energy per defrost: 0.075-0.21 kWh. The tank stores enough energy for 7-19 defrost cycles without drawing from the building. This is ample protection.

Verdict

The 120-litre 4-pipe stratified buffer tank is correctly sized for this application. It prevents short cycling, protects against defrost heat theft, stores solar diverter energy, and costs only £31/year in standing losses. In this illustration, the solar diverter upgrade pays back in 4.4 years.

Worked Example 2: 12 kW Heat Pump: Small Commercial Office

Consider a hypothetical 350 m² open-plan office near Bristol. All inputs are assumptions for illustration. They have a 12 kW monobloc air source heat pump serving underfloor heating throughout. The heat loss calculation shows annual space heating demand of 28,000 kWh. DHW is minimal, a small kitchenette with an instantaneous electric heater. Projected SCOP: 3.2. They plan to add a 10 kWp solar array with Eddi diverter in 2026.

Step 1: Determine Buffer Tank Functions

The tank must perform:

1. Hydraulic decoupling, large underfloor heating zone with multiple manifolds
2. Defrost protection, Bristol has mild winters but frequent frost at dawn
3. Solar diverter storage, the 10 kWp array will produce substantial surplus

No DHW pre-heat is needed.

Step 2: Apply the Sizing Rule

Base sizing for decoupling + defrost + diverter: 20-25 L/kW

12 kW x 20 L/kW = 240 litres (minimum) 12 kW x 25 L/kW = 300 litres (recommended)

The underfloor heating has moderate volume, roughly 40 litres across three manifolds. This is above the 10-15 L/kW danger zone, so short cycling is less of a risk. However, the large solar array (10 kWp) will produce 15-25 kWh of surplus on summer weekdays when the office is unoccupied. A larger tank stores more of this surplus.

Selected tank: 300-litre 4-pipe stratified buffer tank with 60 mm high-density PU foam, dual immersion ports (3 kW + 1.5 kW), and internal baffle plate.

Step 3: Calculate Standing Heat Loss

Tank surface area (300 L, cylindrical): approximately 2.8 m² Insulation: 60 mm high-density PU, U-value 0.25 W/m²K Average tank temperature: 40°C (low-temperature underfloor heating) Ambient temperature: 18°C (plant room, partially heated) Temperature difference: 22 K

Standing loss = 2.8 m² x 0.25 W/m²K x 22 K = 15.4 W Annual standing loss = 15.4 W x 8,760 hours = 135 kWh/year Annual cost at 30p/kWh = £41/year

Step 4: Calculate Solar Diverter Contribution

Assume the 10 kWp array in Bristol generates 9,500 kWh/year and the office baseload (servers, lighting, equipment) is 12,000 kWh/year. On summer weekends and bank holidays, the office is empty and all PV generation is surplus.

Weekend summer surplus: approximately 40-60 kWh per sunny weekend Weekday surplus (shoulder months): approximately 10-20 kWh per day

Total annual surplus available to diverter: roughly 2,500-3,500 kWh Diverter capture efficiency (tank limited): 60-70% = 1,500-2,450 kWh Value at 30p/kWh instead of 5p/kWh export = £375-613/year

The 300-litre tank heated from 35°C to 50°C stores: Usable heat = 300 L x 15 K x 0.00116 = 5.2 kWh

At 3 kW immersion output, the tank heats in 1.7 hours. On a summer day with 20 kWh surplus, the tank fills in the morning and the remaining surplus is exported. A larger tank (400-500 L) would capture more surplus but the payback diminishes.

Step 5: Verify Defrost Protection

At 40°C tank temperature and 35°C minimum useful temperature: Usable heat = 300 L x 5 K x 0.00116 = 1.7 kWh

A 12 kW heat pump defrost draws 3-5 kW of heat for 4-6 minutes. Energy per defrost: 0.2-0.5 kWh. The tank stores enough for 3-8 defrost cycles. This is adequate for Bristol’s mild climate but would be marginal in Scotland or the North East.

For colder climates, increase to 350-400 litres or add a second buffer tank in series.

Verdict

The 300-litre 4-pipe stratified buffer tank with 60 mm insulation is correctly sized for this commercial application. It stores a meaningful share of summer PV surplus, protects against defrost, and costs only £41/year in standing losses. In this illustration the diverter has a short payback because the PV surplus is large, but model your own surplus.

Worked Example 3: 20 kW Heat Pump: Primary School

Consider a hypothetical 600 m² single-storey primary school near Leeds. All inputs are assumptions for illustration. They have a 20 kW monobloc air source heat pump serving a mix of underfloor heating in classrooms and radiators in corridors and offices. Annual space heating demand: 52,000 kWh. DHW is served by a separate gas boiler that the school plans to replace with a second heat pump in 2027. Projected SCOP: 3.0. They have a 25 kWp solar array on the south-facing roof, installed in 2024.

Step 1: Determine Buffer Tank Functions

The tank must perform:

1. Hydraulic decoupling, mixed emitter types (UFH + radiators) with different flow temperatures
2. Defrost protection, assume a colder site with many frost days
3. Solar diverter storage, 25 kWp array produces substantial surplus during school holidays
4. DHW pre-heat, the existing gas boiler DHW cylinder can be pre-heated by the buffer

This is the most demanding application of the three examples.

Step 2: Apply the Sizing Rule

Base sizing for decoupling + defrost + diverter + DHW pre-heat: 25-30 L/kW

20 kW x 25 L/kW = 500 litres (minimum) 20 kW x 30 L/kW = 600 litres (recommended)

The mixed emitter types create a hydraulic challenge. The underfloor heating needs 35°C flow. The radiators need 45-50°C flow. A stratified tank can supply both: underfloor draws from the middle layer at 38°C, radiators draw from the top layer at 48°C.

Assume the 25 kWp array generates 22,000 kWh/year and school consumption (lighting, IT, kitchen) is 18,000 kWh/year. During the 13 weeks of school holidays, nearly all generation is surplus. Holiday surplus: approximately 6,000-8,000 kWh/year.

Selected tank: 500-litre 4-pipe stratified buffer tank with 80 mm high-density PU foam, dual immersion ports (3 kW + 3 kW), internal baffle plate, and DHW pre-heat coil.

Step 3: Calculate Standing Heat Loss

Tank surface area (500 L, cylindrical): approximately 3.8 m² Insulation: 80 mm high-density PU, U-value 0.20 W/m²K Average tank temperature: 45°C (mixed emitter setpoint) Ambient temperature: 15°C (plant room, unheated but within building envelope) Temperature difference: 30 K

Standing loss = 3.8 m² x 0.20 W/m²K x 30 K = 22.8 W Annual standing loss = 22.8 W x 8,760 hours = 200 kWh/year Annual cost at 30p/kWh = £60/year

With 80 mm insulation, the standing loss is well below the Part L maximum of 80 W for a 500-litre tank at 60°C. At the actual operating temperature of 45°C, the loss is even lower than calculated.

Step 4: Calculate Solar Diverter Contribution

The 25 kWp array produces holiday surplus of 6,000-8,000 kWh/year. The 500-litre tank heated from 35°C to 55°C stores:

Usable heat = 500 L x 20 K x 0.00116 = 11.6 kWh

At 6 kW dual immersion output, the tank heats in 1.9 hours. On a summer holiday day with 120 kWh generation and 10 kWh school baseload, 110 kWh is surplus. The tank absorbs 11.6 kWh. The remaining 98.4 kWh is exported.

Diverter capture over the year: 11.6 kWh/day x 50-60 sunny holiday days = 580-696 kWh Value at 30p/kWh instead of 5p/kWh = £145-174/year

This seems modest, but the tank’s primary value is hydraulic decoupling and defrost protection. The solar diverter is a bonus function. In term time, the tank also absorbs midday PV surplus on weekends.

Step 5: Verify Defrost Protection

At 45°C tank temperature and 35°C minimum useful temperature: Usable heat = 500 L x 10 K x 0.00116 = 5.8 kWh

A 20 kW heat pump defrost draws 5-8 kW of heat for 5-8 minutes. Energy per defrost: 0.4-1.1 kWh. The tank stores enough for 5-14 defrost cycles. This is ample for Leeds climate.

Step 6: DHW Pre-Heat Calculation

The school DHW demand is roughly 300 litres/day at 55°C. Cold mains at 10°C. Pre-heat in the buffer coil to 30°C.

Heat required without pre-heat: 300 L x 45 K x 0.00116 = 15.7 kWh Heat required with pre-heat: 300 L x 25 K x 0.00116 = 8.7 kWh Savings: 7.0 kWh/day = 1,400 kWh/year (200 school days) Value at 30p/kWh = £420/year

If the pre-heat coil adds an assumed £200-300 to the tank cost and saves £420/year, payback in this illustration is under 1 year.

Verdict

The 500-litre 4-pipe stratified buffer tank with 80 mm insulation and DHW pre-heat coil is correctly sized for this school application. It handles hydraulic decoupling for mixed emitters, provides ample defrost protection, stores holiday PV surplus, and pre-heats DHW to save £420/year. Total standing losses are £60/year, a small price for the system stability the tank provides.

Sizing Checklist for Installers

Use this checklist on every heat pump + solar buffer tank specification:

• Confirm heat pump capacity (kW) and minimum modulation percentage
• Calculate heating circuit volume (L) and compare to 10-15 L/kW minimum
• Determine required functions: decoupling, defrost, diverter, DHW pre-heat
• Apply 10-25 L/kW rule based on functions required
• Select 4-pipe stratified tank unless budget forces 2-pipe
• Specify minimum 50 mm insulation (80 mm for outdoor or unheated locations)
• Verify tank material compatibility with local water quality
• Include immersion heater port if solar diverter is planned
• Add DHW pre-heat coil if DHW cylinder is separate and heat pump serves heating only
• Calculate standing heat loss and confirm it is acceptable
• Verify defrost protection: usable heat stored vs typical defrost energy demand
• Check Part L compliance for insulation and heat loss
• Confirm MIS 3005-D buffer vessel requirement is met
• Specify temperature relief valve, tundish, and expansion vessel if unvented
• Locate tank within building thermal envelope where possible
• Plan pipe sizing to avoid turbulence that disrupts stratification

Common Buffer Tank Sizing Mistakes

Many buffer tank guides repeat the same advice: bigger is always better, follow the L/kW rule, fit a 4-pipe tank. That advice is incomplete. Three mistakes come up repeatedly in heat pump + solar designs.

Mistake 1: Oversizing without checking standing losses. A generous tank looks safe on paper, but annual loss depends on the declared standing-loss value, insulation, pipework, ambient temperature and controls. Compare declared product data and model the actual installation location.

Mistake 2: Fitting a diverter on every job. A diverter pays back only if there is enough surplus PV and a heat load to absorb it. Small arrays, low hot water demand or a heat pump that already uses smart control may leave little to divert. Run the numbers project by project. The eddi’s published minimum load is 100 W, so very small surpluses may not be usable.

Mistake 3: Expecting stratification in an undersized tank. If the tank volume turns over quickly in cold weather, the layers mix and the efficiency benefit shrinks. If the budget forces a smaller tank, compare a well-insulated 2-pipe unit with a poorly stratified 4-pipe one using manufacturer data, rather than assuming the 4-pipe wins.

The pattern across all three: rule-of-thumb sizing breaks down when site-specific factors dominate. Real performance depends on tank location, PV array size, heating demand profile and emitter type, not on a single L/kW number.

Conclusion

Buffer tank sizing for heat pump + solar systems starts with a rule of thumb. The 10-25 L/kW range, adjusted for the four functions the tank must perform, gives a reasonable first estimate. Then check the manufacturer’s minimum volume, the standing loss on the data sheet and an hourly model of PV surplus.

The three hypothetical worked examples show the range: a 5 kW residential system needs about 100-125 litres for decoupling, defrost and diverter storage. A 12 kW commercial system needs about 240-300 litres for the same functions plus holiday surplus capture. A 20 kW school needs about 500-600 litres for mixed emitters, defrost, diverter and DHW pre-heat.

For heating-dominated loads, a buffer tank is often the cheapest way to store surplus solar energy, with no cycle-count wear on the vessel. Specify it before you consider battery capacity, and compare both with local quotes.

• Use the 10-25 L/kW rule as your starting point, then adjust for climate, emitter type and solar diverter plans. For ground source heat pump + solar systems, the same rule applies but defrost protection drops out.
• Specify a 4-pipe stratified tank for larger systems and any system with solar diverter integration, and confirm shading impact on summer surplus with solar shadow analysis software before sizing the tank.
• Locate the tank inside the thermal envelope and choose on declared standing loss.
• Model the PV side in solar PV design software, then carry the assumptions into your solar proposal software so the homeowner sees the same numbers you signed off on.

To model PV surplus and payback for a heat pump project, book a SurgePV demo and bring a real site.

Frequently Asked Questions

How do you size a buffer tank for a heat pump and solar PV system?

A common rule of thumb is 10 to 25 litres per kW of heat pump heating capacity. Use the low end for decoupling alone and the high end when the tank also covers defrost, a PV diverter and DHW pre-heat. For a 5 kW unit that is roughly 50 to 125 litres. Treat it as a starting point and check the heat pump manufacturer’s minimum system volume.

What is the purpose of a buffer tank in a heat pump + solar system?

A buffer tank does four jobs: hydraulic decoupling between the heat pump and heating circuit, defrost energy storage, storage for surplus PV via a diverter and immersion heater, and DHW pre-heat. Decoupling prevents short cycling, which protects the compressor. Each added job increases the volume you need.

What is the difference between a 2-pipe and 4-pipe buffer tank?

A 2-pipe tank has one inlet and one outlet and tends to mix to one temperature, while a 4-pipe tank has separate heat pump and heating circuit connections that let temperature layers form. Stratification lets the heat pump see a cooler return temperature, which helps COP. The 4-pipe tank costs more and needs careful flow rates to keep its layers.

How does a solar PV diverter work with a buffer tank?

A diverter reads grid import and export through a current transformer and sends surplus PV to a resistive immersion heater in the tank. The tank stores that energy as hot water, which the heating or DHW system uses later. The myenergi eddi, for example, publishes a maximum heater load of 3.68 kW and a minimum load of 100 W.

Can a buffer tank replace a battery for solar self-consumption?

No, because a tank stores heat and not electricity, so it cannot run lights or appliances. For a heating-dominated load, it can be a cheaper way to soak up surplus PV than a battery. Compare both using local quotes and an hourly model, and use both where loads are mixed.

Should the buffer tank be on the heating circuit or the DHW circuit?

For space heating, the buffer tank normally sits on the primary heating circuit between the heat pump and the emitters, while a separate cylinder handles DHW. Combined tank designs exist, but the buffer should not replace a DHW cylinder in a home because stored hot water needs legionella control. Follow the manufacturer’s hydraulic schematic.

What are the UK regulations for buffer tanks in heat pump systems?

Check the current MCS standard (MIS 3005), Building Regulations Approved Document L for insulation, the Water Supply (Water Fittings) Regulations 1999 for connections to mains water, and the heat pump manufacturer’s installation manual. Requirements change, so read the current published versions before you specify a tank.