Unvented Hot Water Systems: A Complete Guide

An unvented hot water system stores hot water in a sealed cylinder connected directly to the mains. Unlike a vented (gravity-fed) system, the water is delivered at mains pressure — meaning fast, powerful showers without a pump.

How an unvented cylinder works

Cold water from the mains enters the sealed cylinder and is heated by either the boiler (via a coil inside the cylinder) or an electric immersion heater. As water heats up, it expands — this is managed by an expansion vessel. Safety devices including a pressure relief valve and temperature/pressure relief valve protect against overpressure.

Benefits over vented systems

• Mains-pressure hot water to all outlets simultaneously
• No loft tank required
• Faster filling of baths
• Multiple bathrooms can use hot water simultaneously
• Ideal for large families

Requirements for installation

Unvented cylinders must be installed and commissioned by a G3 qualified engineer (an additional qualification held by our team). Annual service of the safety devices is required.

Cost

An unvented cylinder supply and install typically costs £800–£1,400 depending on cylinder size. When combined with a new system boiler, we offer package pricing.

Is it right for you?

If you have 2+ bathrooms and a family of 3+, an unvented cylinder is almost certainly the best long-term hot water solution. Book a survey and we'll advise on the ideal cylinder size for your household.