Megaflo & unvented cylinders

Done properly, unvented cylinders deliver mains-pressure hot water to multiple bathrooms simultaneously. When they go wrong, they go wrong dramatically (steam-explosion risk). So they’re regulated tighter than vented systems, and only G3-qualified engineers can install or service them. Every install is Building Regs notifiable and requires a G3 commissioning certificate.

Megaflo & unvented cylinder installation

For a typical 4-bed house with two bathrooms, a 210L unvented cylinder install with primary pipework runs £1,650–£2,400 fitted, depending on whether you’re replacing an existing vented cylinder or doing a fresh install.

Brands we fit:

• Megaflo (Heatrae Sadia): the industry standard. 25-year tank warranty, decent parts availability, robust track record.
• Telford Tempest: quieter operation, better insulation, slightly more compact. 25-year warranty.
• Range: premium choice, particularly the Tribune HE with intelligent immersion control.
• Worcester Greenstore: pairs natively with Worcester boilers; convenient if you’re upgrading both.
• Joule: for heat-pump pairings, the Joule Cyclone is hard to beat.

We’ll talk you through the trade-offs at survey. The right choice depends on your boiler/heat-pump pairing, the cupboard space available, and the household’s hot-water demand profile.

Sizing: the bit most installers get wrong

Cylinder sizing is based on simultaneous demand, not total household occupancy.

• Single bathroom, modest household: 150L is typically enough.
• Two bathrooms, family of 4: 180–210L for combi-boiler-fed, 200L–250L for heat-pump-fed.
• Three bathrooms or large family: 250–300L.
• Four bathrooms or HMO: 300L+ or paired smaller cylinders.

Heat-pump-fed cylinders need to be one size larger than gas-fed equivalents because the reheat is slower (4–6 hours versus 30–60 minutes). We size for the worst-case usage pattern, not the average.

What G3 safety devices do

Every unvented cylinder must have:

• Temperature relief valve: opens at 90–95°C to prevent boil-over. Tested individually at commission and annually.
• Pressure relief valve: opens at 7 bar to prevent over-pressure. Tested at commission.
• Pressure reducing valve (PRV): caps incoming mains pressure (typically 3 bar). Required if mains pressure exceeds the cylinder rating.
• Expansion vessel: accommodates thermal expansion of heated water. Pre-charge tested annually.
• Tundish: visible discharge point so you can see if any of the relief valves are dripping. Discharge pipe routed to safe outdoor termination.
• Immersion thermostat with thermal cut-out: backup safety on the immersion heater itself.

Annual G3 inspection

Required by Building Regulations on every unvented cylinder. We inspect the safety valves, check the expansion vessel pre-charge (typically 1.5 bar), test the temperature cut-out, and certify in writing.

£85 standalone or £20 added to a same-visit boiler service or CP12. Most homeowners bundle it with annual boiler service for the small charge, which saves a separate visit.

Replacement & repair

Most unvented cylinder failures are:

• Temperature/pressure relief valve drip: usually a worn seat or grit in the valve. Replaced during a single visit. £140 fitted.
• Expansion vessel pre-charge loss: re-pressurise (or replace if punctured). £90–£180 fitted.
• Failed immersion: heating element burnt out. £140 fitted including new element + thermostat + cut-out.
• Cylinder leaking from the tundish: signal that one of the relief valves has opened. Investigate and rectify before re-using.

Cylinder body failures (leaking from the steel) are rare in the warranty window (25 years) but possible in older units. Replacement is the only option.

Common mistakes other firms make

• Skipping the discharge pipe routing. The tundish discharge pipe must terminate visibly outdoors at low level. Some installers route it into a hidden drain, which is wrong. Building Regs explicit on this.
• Ignoring the cold mains flow rate. An unvented cylinder feeds at mains pressure but only at the rate the mains supplies. If mains flow is < 16 L/min, an unvented system won’t deliver multi-bathroom performance. We measure first.
• Skipping G3 commissioning. Some installers fit and walk away. The G3 commissioning certificate is a legal requirement and a manufacturer warranty condition.
• Pairing wrong-sized cylinder to heat pump. Most common failure mode in heat-pump retrofits: boiler-spec cylinder doesn’t reheat fast enough on lower flow temperatures. We always upsize for heat-pump pairings.

Workmanship and warranty

12-month workmanship warranty. Cylinder warranties typically 25 years (Megaflo, Telford, Range) or 30 years (Joule premium models) on the tank itself. Components (immersion, valves) typically 2–5 years.