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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.
Rated output of heat pump
For DHW, typically 50-55C
HP typically stores at 48-50°C; the legionella-pasteurisation check needs ≥60°C per L8 ACoP
UK average 10C
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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: ~35-70 litres per person per day (at 60C equivalent), scaled by usage level — 35 L for low (quick showers only), 50 L for average (showers plus the occasional bath) and 70 L for high (daily baths and long showers). Daily bathing pushes demand well above the standard ~40-50 L/person figure
- HP cylinder: typically one or two sizes larger than a boiler cylinder (heat pump 200-300 L vs boiler 120-180 L) to cope with slower low-temperature reheat
- Coil area: roughly 3 m² recommended for low-temp heat pump operation (vs ~1-1.5 m² for a boiler). This is a rule of thumb — size the coil from the manufacturer's kW rating at the actual flow/return temperatures, as coil type (plain, finned or corrugated) affects real heat transfer, not area alone
- Reheat time: Stored energy (kWh) ÷ assumed HP DHW output (≈80% of nominal kW). Real DHW output derates further at low flow temperature and cold ambient
Frequently Asked Questions
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.
As a rule of thumb, heat pump cylinders need a coil of roughly 3 m² (commonly 2.0-3.0 m²), compared to about 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 (heat pump ~50-55C flow vs boiler 70-80C), so more surface area is required for the same heat transfer. Surface area is only a proxy, though — coil type (plain, finned/Integron or corrugated stainless) changes the real kW output, so size the coil from the manufacturer’s rated output at your flow and return temperatures.
An 8 kW heat pump takes roughly 1.5-2 hours to fully reheat a 250L cylinder from cold (about 10C to 48C) — the energy needed is 250 × 4.186 × 38 / 3600 ≈ 11 kWh, divided by an assumed DHW output of about 6.4 kW (~80% of nominal). From partially depleted (e.g. after morning showers), reheat is typically under an hour. This is slower than a gas boiler (20-30 minutes) but manageable with a properly sized cylinder. Reheat lengthens in cold weather because the heat pump derates. Independent figures agree: a 250L heat pump cylinder reheats in about 1-2 hours (73 minutes at 10 kW per energy-stats.uk).
HSE L8 ACoP and BS 8580-1:2019 advise storing hot water at 60C or above; where it is stored below 60C (as heat pumps often do, around 48-50C for a better COP) a weekly pasteurisation cycle raising the whole store to at least 60C for at least one hour is required. Most HP cylinders use a small immersion heater, or the heat pump’s own high-temperature boost, to run this cycle automatically. Some high-temperature heat pumps can reach 65C directly, but at a reduced COP. Hot water should still be distributed to outlets at 50C or above, with thermostatic mixing valves limiting tap temperature to prevent scalding.
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.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.