Domestic hot water cylinders

Stored hot water, heat-exchanger performance, sizing and safety.

A domestic hot water cylinder stores water for taps, showers and baths. An indirect cylinder is heated through a heat exchanger carrying water from a boiler, heat pump or solar circuit. A direct cylinder uses an immersion heater or another heat source within the vessel. Some cylinders combine several heat inputs.

A cylinder is not the same as a buffer tank. The cylinder contains domestic water that comes from the supply and reaches the outlets. A heating buffer contains primary circuit water and exists to support the space-heating hydraulics.

Vented and unvented storage

A vented cylinder is supplied from a cold-water cistern and has an open vent. Available pressure follows from the system arrangement and may be lower than mains pressure.

An unvented cylinder is supplied from the mains and stores water under pressure. It needs the expansion provision, temperature and pressure safety devices, and discharge pipework specified for the complete package. These are not optional accessories and must not be isolated, capped or replaced with arbitrary alternatives.

Part G guidance for England covers hot-water storage safety, excessive temperature, scalding, safety devices, discharge pipework, installation and commissioning. Building-control and competence arrangements vary across the UK, so the route should be confirmed for the property. Work on an unvented system belongs with someone competent for that system and its safety controls.

A heat-pump cylinder is selected by performance

A heat pump usually heats the cylinder through a coil or other heat exchanger. Because the available primary-water temperature may be lower than in a boiler system, the heat exchanger has to transfer enough power under the proposed heat-pump conditions.

Coil surface area can be useful product information, but it is not a complete rating. Geometry, primary flow, water temperatures, stored-water temperature and cylinder construction all affect heat transfer. Two coils with the same area can perform differently. The useful evidence is the manufacturer’s rated transfer and reheat data at conditions close to the design.

An existing boiler cylinder is not automatically suitable or unsuitable. The designer should establish its model, condition, volume, heat-exchanger performance, insulation and safety provision. If the manufacturer cannot support the required heat transfer at heat-pump temperatures, a long reheat or heavy immersion use is a predictable result.

Size follows from use and recovery

Cylinder volume is chosen from the expected pattern of hot-water use, not bedroom count alone. The calculation considers:

number, type and simultaneous use of showers, baths and taps
incoming cold-water conditions and desired delivery temperature
usable mixed-water volume, stratification and outlet position
heat-pump output while heating hot water
coil or heat-exchanger performance
acceptable reheat time and heating schedule
space-heating interruption during hot-water priority
backup heat and the consequence of running out

A larger cylinder stores more but has greater standing loss, weight and space demand. A smaller cylinder may recover quickly enough if the heat source and coil can genuinely transfer the required power. Stated nominal volume should not be confused with usable hot water at the outlets.

The cold main also matters. An unvented cylinder cannot deliver a sustained high flow that the incoming supply cannot provide. Pressure, flow, pipe size and simultaneous outlets should be assessed before promising hotel-style showers.

Temperature, hygiene and scald protection

Stored-water management must balance hygiene, energy and scalding risk. The appropriate control strategy depends on the building, occupancy, storage arrangement, temperatures throughout the vessel and distribution, and applicable guidance. A headline cylinder thermostat setting does not demonstrate control of the whole system.

Heat-pump systems often include a periodic higher-temperature cycle, sometimes with immersion assistance. The designer should document the target, frequency, heat source and evidence for the selected strategy. Settings copied from another product may not achieve the same temperature throughout this cylinder.

Where high stored temperatures are used, suitable temperature control at outlets may be needed to reduce scalding risk. Thermostatic mixing does not remove the need to control dead legs, stagnation or rarely used outlets in buildings where Legionella duties apply.

Heat loss, position and access

Product standing-loss data allows cylinders to be compared on a consistent declared basis. Real losses also depend on store temperature, room temperature and uninsulated valves and pipework. Locating a cylinder within the heated envelope can make some loss useful in winter, but can worsen summer overheating.

The position must support the full operating weight, allow removal and service access, and provide workable routes for primary pipework, domestic water, immersion wiring and safety discharges. A tight cupboard that fits the shell but blocks the anode, immersion or valves is not adequate.

Cylinder materials have different water-chemistry and maintenance requirements. Sacrificial anodes, powered anodes, scale control and inspection intervals should follow the exact product and local water conditions.

Controls and backup

Hot-water priority normally diverts the heat pump from space heating to the cylinder. Sensor position, hysteresis, target temperature and time schedule determine how often and how long this happens. The seasonal performance estimate should include domestic hot water rather than assuming space-heating efficiency applies unchanged.

An immersion heater can provide backup, boosting or a hygiene cycle. Its control should make unplanned use visible. A system that routinely uses resistance heat because the coil, sensor or schedule is wrong can appear to provide plenty of hot water while performing much worse than designed.

Cylinder design data

Cylinder selection and sizing use:

cylinder type, volume and declared standing loss
heat-exchanger output at the proposed primary conditions
calculated usable hot water and reheat performance
cold-main flow and pressure assumptions
hot-water priority, sensor and temperature-control strategy
immersion or other backup operation
unvented safety devices and discharge route where applicable
full weight, access, maintenance and replacement route
commissioning and building-control records

Applies to

Heat

Last reviewed

22 Jul 2026