Why do heat pumps need bigger cylinders?

Heat pumps heat water at lower flow temperatures (45-55C) compared to gas boilers (60-80C). This means slower reheat times and less usable hot water per litre stored. A 250-300L cylinder is recommended for heat pump systems, compared to 120-180L for gas boilers, to ensure adequate hot water availability.

What coil surface area is needed?

Heat pump cylinders need a minimum coil surface area of 2.0-3.0 m2, compared to 1.0-1.5 m2 for a boiler cylinder. The larger coil is needed because the temperature difference between the heat source and stored water is much smaller (2-10C vs 20-30C), requiring more surface area for adequate heat transfer.

How long does a heat pump take to reheat a cylinder?

An 8 kW heat pump takes approximately 2-3 hours to fully reheat a 250L cylinder from cold (10C to 48C). From partially depleted (e.g., after morning showers), reheat is typically 1-1.5 hours. This is longer than gas boilers (30-45 minutes) but manageable with a properly sized cylinder.

Can I run Legionella pasteurisation with a heat pump?

Heat pumps struggle to reach the 60C required for Legionella pasteurisation. Most HP cylinders use a small immersion heater to periodically boost temperature to 60C for Legionella compliance. Some high-temperature heat pumps can reach 65C but at reduced COP.

Should I use an unvented or vented cylinder?

Unvented cylinders are preferred for heat pump systems as they provide mains-pressure hot water and are more thermally efficient. They must be installed by a G3-qualified engineer. Vented (gravity-fed) cylinders work but may need a pump for adequate shower performance.

TradeCalculator

Heat Pump Cylinder Calculator

Size a hot water cylinder for a heat pump system. Larger cylinders with high coil surface area are essential for low-temperature heat sources.

Number of Occupants

Hot Water Usage

Heat Pump Type

Heat Pump Capacity

Rated output of heat pump

How We Calculate This

This calculator sizes a hot water cylinder specifically for heat pump systems, accounting for the lower flow temperatures and slower reheat times compared to gas boiler systems.

Sizing methodology

Daily demand: ~50 litres per person per day (at 60C equivalent)
HP cylinder: 40-60% larger than standard boiler cylinder
Coil area: 2.0-3.0 m2 for low-temp heat pump operation
Reheat time: Stored energy (kWh) / HP DHW capacity (kW)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.