Heat Pump Cylinder Coil Sizing UK 2026

The internal coil is a copper tube spiralled inside the cylinder that carries heat pump flow water + transfers heat to the stored hot water. Heat transfer rate depends on:

• Coil surface area (square metres of copper exposed to stored water).
• Temperature differential between flow water + stored water (small with heat pumps - flow 45-50°C, target tank 50-55°C, delta only 5-10°C).
• Flow rate through the coil.

For gas-boiler systems at 75°C flow, the temperature differential to 55°C tank is ~20°C - enough to drive fast heat transfer even with modest 1.5 m² coil. For heat pumps at 45-50°C flow, the 5-10°C differential needs ~2x larger coil surface area to deliver equivalent reheat time.

Standard gas-boiler cylinders fitted to heat pump installs result in unacceptably slow reheat (4-6 hours) + the heat pump runs at constant low output for hours. Heat-pump-specific cylinders match the heat pump's design + deliver reheat in 60-90 min.

Standard UK gas-boiler cylinder (e.g. Megaflo Eco):

• Coil surface area: ~1.5-2.5 m²
• Design flow temperature: 75°C
• Reheat 200L cylinder cold → 55°C at 7kW heat pump 45°C flow: 4-6 hours
• Heat pump SCOP during cylinder reheat: ~2.5 (poor due to extended low-COP operation)

Heat-pump-compatible cylinder (e.g. Telford Tempest HP, Mixergy iHP):

• Coil surface area: 3.5-5.0 m²
• Design flow temperature: 45-50°C
• Reheat 200L cylinder cold → 55°C at 7kW heat pump 45°C flow: 60-90 minutes
• Heat pump SCOP during cylinder reheat: ~3.5 (much better)

The reheat time difference (4-6 hours vs 60-90 min) is what makes a heat-pump-specific cylinder essential. Anything else is a false economy.

Heat pump cylinder volume sizing typical UK 2026:

• 1-2 person household: 150-200L cylinder. Smaller cylinders allow more frequent reheats + slightly improved SCOP (better stratification, less stand-by loss).
• 3-4 person household: 200-250L cylinder. The standard UK retrofit size.
• 5+ person household OR significant simultaneous-use patterns (multiple showers): 250-300L cylinder.
• Very large households (7+) or commercial use: 300-400L or twin-cylinder setup.

Note: too-large cylinders waste energy on stand-by losses; too-small cylinders mean cold showers during peak usage. The above ranges are well-tested UK norms.

• Telford Tempest HP / Cyclone Aerocyl: the UK heat-pump retrofit default. Wide installer awareness + strong availability. 150-300L typical range. Manufactured in Telford.
• Mixergy iHP: smart cylinder with real-time monitoring of temperature stratification, dynamic heating zones, Mixergy app integration. Higher cost (~GBP 1,500-2,000 vs GBP 800-1,200 for standard) but smart-tariff coordination potential.
• Joule Cyclone: Irish-manufactured, popular with Octopus Cosy installations. Standard heat-pump coil sizes.
• Newark Cylinders Hercules: UK manufacturer with reputation for high-quality stainless steel construction. Slightly premium pricing.
• Range Tribune HE: mid-market UK manufacturer with heat-pump-specific range. Solid choice for installers not specifying any of the above.

For UK retrofits where you have an existing standard cylinder:

• Heat-pump-compatible cylinder replacement cost: GBP 800-1,500 (cylinder + install) for standard sizes; GBP 1,500-2,000 for Mixergy smart cylinders.
• Bundled with heat pump install: typically included in the installer quote at slightly lower cost than retrofit.
• Standalone retrofit (heat pump already installed): 1-day job, ~GBP 1,200-2,000 total.

Confirm with your installer:

• Specific cylinder model + coil surface area (m²).
• Reheat time spec for your heat pump flow temperature.
• Manufacturer warranty (typically 2-5 years on cylinder).
• Anode rod type + service interval (standard 4-6 years for hard-water areas).