PRACTICAL GUIDE · UPDATED 2026-10-31
Hot-water cylinders: vented, unvented and heat-pump options
See how stored hot water works, which questions to ask about pressure and capacity, and why heat-pump compatibility matters.
| Feature | Vented cylinder | Unvented cylinder |
|---|---|---|
| Cold-water feed | Normally a separate storage cistern | Incoming water mains |
| Pressure / flow | Affected by cistern height and pipework | Depends on mains supply and pipework |
| Things to assess | Cistern, space, suitability and heat source | Safety controls, discharge route and supply |
| Heat-pump suitability | Needs a system-specific assessment | Needs a system-specific assessment |
A cylinder stores heated water ready for taps and showers. A combi boiler normally heats tap water as it is needed instead. Cylinder capacity and reheating time matter when several people want hot water close together; a larger tank alone does not guarantee strong showers.
A vented cylinder is normally fed from a cold-water storage cistern, often in the loft. Available pressure depends partly on the height of that cistern. An unvented cylinder is supplied from the water mains and does not need that separate feed cistern. Its performance still depends on the incoming mains flow and pressure and the pipework; it cannot guarantee unrestricted simultaneous showers.
Direct cylinders use electric immersion heating. Indirect cylinders receive heat through a coil supplied by another heat source, such as a boiler or compatible heat pump; an immersion may provide backup. A heat-pump installation may need a different cylinder or heat exchanger, appropriate controls and a suitable hot-water schedule. Ask the designer whether the existing cylinder is suitable and how capacity, reheating, hygiene protection and backup have been assessed.
Compare the complete specification: usable hot-water capacity, reheat performance, standing heat loss, dimensions, access for future maintenance and warranty conditions. Ask which valves, controls, pipework, electrical work and disposal are included. A competent installer should check the existing supply and explain the proposed system rather than choosing only by tank size or brand.
Stored hot water can scald, and unvented systems need correctly designed safety controls and discharge arrangements. Use a competent specialist for installation and servicing, following the manufacturer’s requirements and applicable building regulations. Never cap a discharge pipe, adjust relief valves or disable safety devices. Have unexplained discharge, overheating, excessive noise or leaks assessed promptly; keep clear of hot discharge. Gas registration alone is not evidence of competence for every cylinder task.
An existing cylinder is not automatically wasteful or unsuitable. Good insulation, suitable controls and a correctly designed hot-water routine all matter. Get advice before changing stored-water temperatures or safety schedules, particularly with a heat pump; an energy-saving adjustment must not defeat the system’s hygiene and safety provisions.
Your checklist
- Identify whether the current cylinder is vented or unvented and how it is heated.
- Discuss household demand, mains flow and reheating rather than size alone.
- Confirm compatibility before keeping a cylinder with a new heat pump.
- Use a competent specialist and retain handover and servicing records.
Sources & further reading
- Hot Water Association: Cylinder terminology ↗
- GOV.UK: Approved Document G — hot-water safety ↗
- HSE: Domestic hot-water scalding safety alert ↗
- Energy Saving Trust: Air-source heat pumps ↗
General information, not a property assessment, booking or assurance about a listed business. Dates indicate content updates; they are not invented publication dates.