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TOPIC HUB 07 · HOT WATER & CYLINDERS
Heat Pump Hot Water & Cylinders
Heat pumps heat domestic hot water differently to a combi boiler, and almost always need a cylinder to do it well. Getting cylinder size, reheat time and priority settings right matters just as much as the space-heating design.
- Topic hub
- Updated August 2026
- By Phil · Heat Loss Hub
WHY A CYLINDER IS USUALLY NEEDED
Heat pumps produce hot water much more slowly than a combi boiler’s instant burner. A cylinder stores heated water in advance so it is ready when needed, rather than trying to heat water instantly on demand.
How heat pumps heat domestic hot water
The heat pump raises the water flowing through a coil inside the cylinder, gradually heating the stored water to its target temperature. This is a slower, steadier process than a combi’s instant heat exchanger, which is why storage volume matters so much.
Cylinder sizing
Cylinder size should reflect household size, bathroom count and usage pattern — not just be copied from the old boiler’s setup. A property with two bathrooms and back-to-back showers in the morning needs a different cylinder to a single-occupant flat.
Reheat time and coil size
Reheat time — how long it takes to bring a full cylinder back up to temperature after use — depends on the coil surface area and the heat pump’s hot-water output, not just the cylinder’s litre capacity. A large cylinder with an undersized coil can still leave the household waiting.
Legionella cycles and hot-water temperature
Cylinders periodically run a legionella cycle, raising stored water to a higher temperature to control bacterial growth. Many heat pumps handle this with an immersion heater boost rather than running the heat pump itself at that temperature, since heat pumps are generally less efficient at very high output temperatures.
Immersion heaters and hot-water priority
An immersion heater is normally kept as a backup or legionella-cycle assist, not the main way of heating water — relying on it regularly usually means the heat-pump hot-water design needs a second look. Hot-water priority mode pauses space heating briefly so the cylinder reheats faster when needed.
High hot-water demand and space constraints
Larger households or properties with multiple bathrooms should have hot-water demand assessed explicitly rather than assumed. Where airing-cupboard space is limited, cylinder choice can become a real constraint on the whole design — worth checking early, not after equipment has already been chosen.
Hot water deserves the same design attention as space heating. It’s easy to focus entirely on radiators and flow temperature and treat hot water as an afterthought — but a poorly sized cylinder can undermine an otherwise well-designed system.
Continue exploring hot water & cylinders
CONTENTS
RELYING ON THE IMMERSION?
Get the hot-water side of the system checked independently.
NOT SURE THE CYLINDER IS RIGHT FOR YOUR HOUSEHOLD?
Hot water needs designing too, not just guessing.
Heat Loss Hub can assess hot-water demand alongside the space-heating design to make sure the whole system, not just the radiators, is sized correctly.