Best Boiler Deals Best Boiler Deals

Vented vs Unvented Hot-Water Cylinders: Capacity, Pressure and Replacement

Written by

[email protected]

in

  1. Home
  2. Guides
  3. Vented vs Unvented Hot-Water Cylinders: Capacity…

Vented vs Unvented Hot-Water Cylinders: Capacity, Pressure and Replacement

A vented cylinder is supplied from a cold-water storage cistern; an unvented cylinder is supplied directly from the mains and uses safety controls for expansion and pressure.

Published 7 September 2026By the bestboilerdeals.com editorial teamHow this site is funded

↓ Jump to the answer
In this guide
  1. Capacity is demand plus recovery
  2. Pressure and flow
  3. Space and discharge
  4. Replacement questions
  5. Frequently asked questions
  6. Suggested on-page component

A vented cylinder is supplied from a cold-water storage cistern; an unvented cylinder is supplied directly from the mains and uses safety controls for expansion and pressure. Neither is universally better. The choice depends on measured supply, simultaneous demand, space, the existing system and competent installation.

Feature Vented cylinder Unvented cylinder
Cold supply Storage cistern, commonly at higher level Incoming mains
Delivery Influenced by cistern height and pipework Influenced by dynamic mains pressure, flow and controls
Loft cistern Usually required Not required
Safety arrangement Open vent and feed/expansion design Expansion and pressure-relief controls
Main constraint Space and potentially lower pressure Adequate mains supply, discharge route and servicing

Capacity is demand plus recovery

Cylinder litres alone do not tell you how many showers it provides. Stored temperature, mixing, outlet flows, simultaneous use and reheating rate all matter. Ask the designer to state the demand assumptions and recovery performance.

Oversizing increases stored volume and standing heat loss; undersizing can leave the household waiting. Capacity should match anticipated use, not bedroom count alone.

Pressure and flow

A larger cylinder does not cure weak pressure. A vented system depends on head and pipework. An unvented cylinder can deliver mains-fed performance only if incoming dynamic pressure and flow support it. Measure the supply under realistic demand before specifying the system.

Space and discharge

Both types need service access. A vented system also needs a suitable cistern and pipe routes. An unvented system requires safety devices and a compliant discharge arrangement; locating one involves more than finding a cupboard with enough floor area.

In England and Wales, unvented hot-water storage is controlled building work and should follow the appropriate competent-person or notification route. Other nations have their own standards.

Replacement questions

  • Is the existing cylinder sound and adequately sized?
  • Are its coil and controls compatible with the heat source?
  • What are measured dynamic mains pressure and flow?
  • Where will expansion and safety discharge be routed?
  • Can the floor support the filled weight?
  • Is there service and replacement access?
  • Which records and notifications will be supplied?

Frequently asked questions

Will a larger cylinder solve low water pressure?

No. Storage volume and delivery pressure are separate. Measure the relevant supply and assess the pipework and system design.

What space and maintenance does an unvented cylinder need?

It needs the manufacturer’s clearances, access to controls and a compliant discharge route. Safety devices require periodic inspection under the manufacturer’s instructions by a competent person.

Suggested on-page component

Use a Cylinder Demand and Evidence Planner that records outlet use, recovery expectations, mains measurements, plant space and discharge questions. It must not specify capacity automatically.

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write.

More posts