Megaflo Eco Unvented Hot Water Cylinder Installation in Cornwall

Conventional vented hot water systems use a cold water storage tank in the loft, with hot water gravity-fed from the cylinder to taps via the cold tank. Pressure is determined by tank height above the tap — typically 0.1–0.3 bar. Showers feel weak, taps fill slowly, and multiple simultaneous draws (shower running while someone fills the kitchen sink) cause noticeable pressure drops.

Unvented systems like the Megaflo eliminate the loft tank by feeding directly from mains water pressure. Cold water enters the cylinder at mains pressure (typically 2–3 bar in Cornwall after the South West Water pressure-reducing valve at your meter), is heated, and exits at the same pressure. The result is powerful showers, fast taps, and crucially equal hot and cold flow rates throughout the home — meaning multiple bathrooms can run simultaneously without pressure crashes. For Cornish customers used to weak gravity-fed showers, an unvented Megaflo is a transformative upgrade.

Megaflo Eco Direct cylinders heat water purely via internal electric immersion elements (typically two — a primary on Economy 7 timing, a backup on always-on). Used for off-grid properties or all-electric homes without a heating boiler. Megaflo Eco Indirect cylinders include an internal coil that's heated by an external source — typically a gas boiler, oil boiler, or heat pump. The coil is the more efficient route to hot water, with the immersion element used only as backup. For homes with any kind of central heating boiler or heat pump, indirect is always the right choice.

For heat pump installations specifically, we now prefer the Megaflo Eco Plus Heat Pump model, which features an enlarged coil surface area (3.4m² vs 2.0m² in standard Indirect) optimised for the lower flow temperatures heat pumps deliver. The larger coil transfers more heat per cycle, allowing the heat pump to run efficiently at 50–55°C rather than being forced to high flow temperatures.

Megaflo capacity selection follows a simple rule of thumb: 50L per occupant plus 20L per bathroom. So a 4-person family with 2 bathrooms needs 240L (rounded to 250L Megaflo). A 6-person family with 3 bathrooms needs 360L (300L Megaflo with carefully scheduled hot water use, or pair of 200L cylinders for very high demand). For Cornish holiday lets and B&Bs we typically size 25% larger to accommodate guest peaks — a 4-bedroom holiday let might use a 300L cylinder.

The Megaflo's expansion vessel (built into the cylinder on most models, or external on larger units) accommodates the thermal expansion of water as it heats — typically 4% volume increase from cold mains to 60°C stored. This expansion vessel sizing is critical and frequently mis-sized by inexperienced installers. CCS calculates expansion vessel sizing per BS EN 12897 standards as part of every Megaflo installation.

Unvented hot water installation requires G3 qualification under Building Regulations Part G3. CCS engineers carry full G3 qualifications and notify Building Control via competent person scheme self-certification on every Megaflo install. Installation typically takes one day for a straightforward replacement of an existing cylinder, or two days where significant pipework reconfiguration is needed (for example, converting from a vented system, which requires removal of the loft cold tank and re-routing of feed pipes).

The Megaflo's tundish discharge requires routing to a safe location — typically an external soakaway or a metallic pipe to ground level. In older Cornish properties, finding an acceptable discharge route can be challenging; CCS surveys this at quotation stage to avoid surprises. For more on broader installation see our heat pump installation page.

The Megaflo Eco Plus Heat Pump model is purpose-designed for pairing with air source heat pumps — Daikin, Mitsubishi Ecodan, Vaillant and others. The enlarged coil and optimised flow rates suit heat pump operation perfectly. For solar PV households without a heat pump, we typically recommend specifying the Megaflo's optional solar input port — this allows a future Eddi solar diverter to send surplus solar to the cylinder via the immersion element.

For households wanting smart top-down heating and machine-learning usage prediction, the Mixergy is the more sophisticated alternative. Megaflo is the right choice for households prioritising proven reliability, lowest cost, and the broadest installer/parts support network. Mixergy is the right choice for households prioritising smart features, solar PV integration and absolute efficiency. We install both.

Typical fully installed pricing (May 2026):

• Megaflo Eco Indirect 145L — from £1,250
• Megaflo Eco Indirect 210L — from £1,450
• Megaflo Eco Indirect 300L — from £1,750
• Megaflo Eco Plus Heat Pump 250L — from £1,950
• Megaflo Eco Direct 210L (electric only) — from £1,550

Prices include G3 commissioning, expansion vessel and discharge plumbing, Building Regulations Part G3 self-certification, and 25-year vessel warranty registration. 0% VAT applies when installed alongside qualifying renewable technology (heat pump or solar PV); standalone installs are subject to standard 20% VAT. Conversion from vented to unvented (removing existing loft tank) typically adds £400–£600 for additional plumbing work. See finance options.