How to Install an Unvented Water Heater: A Practical Guide for Facilities Managers

Ensuring reliable hot water for your building isn’t just a convenience—it’s essential for safety, hygiene and operational continuity. That’s why more facilities managers are choosing unvented water heaters for their buildings.

These systems deliver hot water at mains pressure, without the need for a bulky cold water storage tank, offering improved performance and cleaner installs. In this guide, we’ll walk you through what you need to know to plan, install, and maintain an unvented water heater—safely and in line with UK regulations.

What Is an Unvented Water Heater?

An unvented water heater is a pressurised cylinder that connects directly to your mains water supply. Unlike vented systems, which rely on gravity-fed storage tanks, unvented systems maintain consistent high pressure throughout, making them ideal for commercial premises like offices, schools, care homes and apartment blocks.

Key Components You’ll Need to Know

Expansion vessel – absorbs the extra volume of water as it heats.
Pressure reducing valve – keeps incoming water pressure within safe limits.
Temperature and pressure relief valve (T&P valve) – a vital safety feature.
Tundish – a visible air gap that indicates if a discharge occurs.
Immersion heater(s) – electric elements used to heat the stored water.

Most unvented systems are built to comply with WRAS (Water Regulations Advisory Scheme) standards and are installed under G3 of the Building Regulations.

Planning the Installation: What You Need to Check First

Before you begin, you’ll need to assess whether your site and system are suitable.

1. Is Your Water Pressure High Enough?

Unvented systems rely on consistent water pressure—ideally 1.5 bar minimum dynamic pressure and 20 litres per minute flow rate. If your incoming supply doesn’t meet this, a booster pump or accumulator may be needed.

2. Is Your Installer G3-Certified?

Only a qualified installer with G3 certification can legally fit and sign off an unvented system in the UK. This is essential to ensure:

Safety and compliance
Valid manufacturer warranties
Proper connection of safety discharge pipes

3. Do You Have the Right Location?

Unvented cylinders are typically heavier and must be supported when full. Choose a location with:

Structural support for the unit’s weight.
Easy access for annual servicing.
Minimal distance to outlets to reduce heat loss.

Installing the Unit: Key Steps for Success

Let’s walk through what the installation process looks like for a typical direct unvented cylinder (heated by electric immersion heaters).

Mounting and Connecting the Cylinder

Mount the unit on a solid, level surface and connect the cold mains supply via an isolating valve. You’ll install a pressure reducing valve just after this, typically set to 3 bar, to ensure safe operation.

Installing Safety Controls

Install the expansion vessel, T&P relief valve, and connect all discharge pipework through the tundish. This pipe must run to a visible and safe drainage point and comply with Part G of the regulations.

Electrical Connections and Commissioning

Connect immersion heaters via an RCBO-protected circuit.
Verify earth bonding and correct fuse ratings.
Fill and pressurise the system slowly, bleed air, then power up.
Test all safety devices and record commissioning data.

We always recommend keeping a copy of the commissioning sheet for Part L compliance as part of your O&M documentation.

Aftercare: How to Keep Your System Safe and Compliant

Like any pressurised system, your unvented water heater needs annual maintenance to stay safe and efficient.

Schedule an Annual Service

This should include:

Checking and recharging the expansion vessel
Testing all pressure relief valves
Cleaning filters or strainers
Inspecting for limescale or corrosion

If your building is in a hard water area, additional descaling treatments may be advised every few years.

Watch for Discharge from the Tundish

A constant drip or flow here may mean your pressure relief valve is faulty—or your expansion vessel is waterlogged. Never ignore it. Shut down the system and call an engineer immediately.

Visit our Hot Water Systems pages to discover more about unvented water heaters.