Unvented Cylinders London — Install, Service & Replacement

The cold-mains connection

An unvented cylinder is fed directly from the cold mains — no cistern in the loft, no gravity feed, no header tank. The mains-pressure water fills the cylinder body and stays under pressure throughout, meaning the hot water leaving the cylinder emerges at the same pressure as the cold mains — typically 3 bar in central London postcodes, 2.5–3.5 bar in outer boroughs.

The pre-condition is a mains supply capable of delivering enough flow. We measure this at the internal stopcock before quoting an unvented install — the standard threshold is 20 litres per minute at 3 bar. Older lead-supply Victorian terraces sometimes fall below this and need a mains upgrade before conversion.

The expansion vessel

Water expands 4% between 10°C and 65°C. In a vented system that expansion vents to the loft cistern; in an unvented system it goes into a diaphragm expansion vessel — a small steel canister with a rubber diaphragm dividing water from a nitrogen-charged air cushion. The expansion vessel is the single most maintenance-critical component in the system.

The vessel is pre-charged to a set pressure — typically 2.5 bar for a mains-pressure system, checked against the cold-mains static pressure. If the pre-charge drops (usually a slow diffusion loss over 3–5 years), water displaces the air side, the vessel loses effective volume, and the pressure relief valve on the cylinder starts discharging every time the water heats up. Annual G3 service checks and re-pressurises the vessel to specification.

The temperature and pressure relief valve (T/PRV)

The T/PRV is the safety-critical component of any unvented cylinder — legally required under Building Regulations Part G3. It opens automatically if internal cylinder temperature exceeds 90°C or internal pressure exceeds 7 bar (the actual set points vary by manufacturer). The discharge routes via a tundish (a visible air-break) and a D1/D2 pipework arrangement to an external drain.

Under G3 the discharge pipework must be visible for at least 300mm above the tundish and must terminate in a position where a discharge event would be readily noticed — typically at ground level next to an external drain, or at gutter level on an upper-storey flat. Compliant D1/D2 pipework is one of the most commonly failed elements on prior installs — every replacement we complete brings this to current compliance.

The immersion element and thermostat

Every unvented cylinder carries at least one immersion element — a resistive heating coil inserted into the cylinder body via a flanged connection. Direct cylinders rely on the immersion as the primary heat source; indirect cylinders use the immersion as a boost or backup to the boiler-driven coil.

The thermostat sits on the outside of the element head and cuts power once the water reaches the target temperature (usually 60°C — hot enough to control Legionella, cool enough to avoid scald injury). Thermostat failure is the second most common repair callout on an unvented system after expansion vessel pre-charge loss.

Megaflo Eco (Heatrae Sadia)

UK market leader — broadest range, longest warranty

Megaflo Eco Plus (indirect), Megaflo Eco Solar (twin-coil), Megaflo Eco Slimline (450mm diameter). Capacity 70 L to 300 L. 25-year cylinder warranty subject to registered G3 install and annual maintenance. Our default recommendation for London flats and terraces needing 145–250 L indirect capacity.

Range Tribune HE (Kingspan)

Very close alternative — often preferred for slim installs

Tribune HE variants across 90 L to 400 L. Excellent recovery times on the high-efficiency coil. The 210 L Slimline is the installer-preferred fit into Victorian pantry-conversion airing cupboards. 25-year cylinder warranty; comparable to Megaflo Eco.

Ariston Aures (continental premium)

Premium option — highest heat retention

Higher standing-loss performance than Megaflo or Tribune, at correspondingly higher cost and longer lead time. Chosen by customers prioritising energy efficiency in a very-well-insulated London property (Passivhaus retrofits, new-build EPC A rating). Requires specific mounting orientation — we survey before quoting.

Mixergy smart cylinder (Oxford)

Highest efficiency uplift we install

Sensored cylinder that only heats the water your daily draw pattern will actually use. 20–30% reduction in standing losses versus a standard indirect unit of the same capacity. Bluetooth + Wi-Fi commissioning, ongoing performance data via app. Best fit for Airbnb, short-let and HMO where usage patterns vary.

Vaillant uniSTOR VIH

Boiler-brand-matched premium

Selected where the customer already has a Vaillant boiler and wants a single-brand system for warranty and service simplicity. Cylinder and boiler primary sync automatically via the Vaillant eBUS communication — improves recovery efficiency modestly.

Vented to unvented conversion — Wandsworth Victorian terrace

Two-bed terrace with a legacy vented 120 L cylinder and cold-water cistern in the loft. Customer wanted mains-pressure showers throughout. Mains-flow test at internal stopcock: 22 L/min at 3.4 bar — meets threshold for unvented conversion. Removed cistern and gravity feeds, installed Megaflo Eco Plus 180 L indirect, G3 certification, LABC notified via competent-person scheme. Two engineers, two days. Cost: £2,650 all-in.

Direct unvented install in a Docklands flat

One-bed Canary Wharf flat with no combi and no cistern space. Installed a Megaflo Eco compact 120 L direct with two 3 kW immersion elements and full G3 compliance. Discharge routed to communal external drain via a compliant D1/D2 arrangement approved by the freeholder. Total cost: £1,780 all-in.

Slimline replacement — Islington Victorian conversion

Second-floor mansion-block flat with a failing 15-year-old Megaflo in a period airing cupboard just 460mm wide. Replacement with a Megaflo Eco Slimline 210 L (450mm diameter — fit clear by 10mm). Same-day install, cylinder in and running by 16:00. Cost: £2,050 all-in.