Unvented Cylinder Installation Cost London 2025 — Megaflo and G3 Prices

Unvented Cylinder Costs in London 2025

An unvented hot water cylinder stores and delivers hot water at mains pressure — eliminating the need for a header tank in the loft and providing powerful flow rates to multiple outlets simultaneously. London installation costs:

• 150 litre unvented cylinder (1-2 bed property), supply and install: £1,200 - £1,900
• 180 litre cylinder (2-3 bed property): £1,400 - £2,100
• 210 litre cylinder (3-4 bed property): £1,600 - £2,500
• 300 litre cylinder (large house or HMO): £2,200 - £3,500
• Annual servicing (safety valve and expansion vessel check): £80 - £150

Leading brands include Megaflo (Heatrae Sadia), Vaillant Unistor, Joule Cyclone, and Range Tribune Xe. Megaflo remains the most commonly specified brand in London due to its reliability record and availability of service parts.

G3 Building Regulations — The Legal Requirement

Installation and servicing of unvented hot water systems is restricted work under Building Regulations Part G3. It must be carried out by a competent person who holds a current G3 qualification (typically the BPEC or City & Guilds 6189 unvented hot water qualification). G3-qualified engineers self-certify the work — no separate Building Control notification is required when using a competent person scheme member.

All Prestige Engineers technicians who install unvented systems hold current G3 qualifications. On completion you receive a Building Regulations compliance certificate and the manufacturer's warranty documentation.

Key Safety Components

An unvented cylinder stores water under mains pressure at temperatures up to 95°C. The following safety components are mandatory under G3 and BS 7206:

• Expansion vessel: Accommodates the volume increase as water heats from cold. Typically 2-18 litres depending on cylinder size. Must be inspected annually
• Pressure reducing valve (PRV): Reduces incoming mains pressure to the design working pressure of the cylinder (typically 3.5 bar). Set at the time of installation
• Temperature and pressure relief valve (T&P valve): Opens if the cylinder overheats beyond 90°C or if pressure exceeds 6 bar. Discharges via a tundish and drain — this pipe must be correctly routed to a safe visible location
• Thermostatic control: Thermostat and high-limit thermostat (overheat cut-out) are factory-set. The immersion thermostat should be set to a minimum of 60°C
• Line strainer: Protects the PRV and cylinder from particulate contamination in the mains supply

Flow Rate Advantages vs Combi Boiler

A modern combi boiler delivers hot water at typically 10-14 litres per minute — adequate for one shower but insufficient for simultaneous use. A 210L unvented cylinder can deliver 25-40 litres per minute (pressure and flow dependent), making it genuinely capable of supplying two or three showers simultaneously. For family homes and HMOs in London, this is a significant practical advantage.

When an Unvented Cylinder Is the Right Choice

• Properties with 2+ bathrooms where simultaneous hot water demand is high
• Properties where a combi boiler cannot deliver adequate flow rate to showers
• Properties being converted to HMO use
• Loft conversions where a cold water tank cannot easily be installed above the level of fittings
• Properties with solar thermal panels (most unvented cylinders have a solar coil option)