Heatrae Sadia Megaflo Cylinders – The Installer's Guide to the UK's Best-Selling Unvented Hot Water Cylinder
If you work in UK heating, you already know the Megaflo. It is the cylinder that heating engineers reach for when they want a dependable unvented installation that goes in cleanly and stays reliable for decades. Built from duplex stainless steel and backed by a lifetime inner-tank guarantee, the Heatrae Sadia Megaflo Eco range has earned its reputation as the UK's market-leading unvented hot water cylinder through consistent engineering quality and a design philosophy that removes installation complexity wherever possible.
What makes the Megaflo Eco stand out is its patented internal floating-baffle expansion system, which eliminates the need for an external expansion vessel on the DHW side. That single feature simplifies pipework, reduces the installation footprint, and removes one of the most common maintenance points on conventional unvented systems. Combine that with mains-pressure delivery of up to 76 litres per minute, 60mm CFC-free insulation, and a complete cold-water inlet control kit supplied as standard, and you have a cylinder that ticks every box for installers and homeowners alike.
Heatrae Sadia and the Baxi Connection
Heatrae Sadia has been manufacturing electric water heating products in the UK for over 100 years. Based in Norwich, the brand specialises in electric water heaters, unvented cylinders, and commercial hot water systems, and has developed a deep understanding of what installers need from a product: reliable performance, straightforward installation, and minimal callbacks.
As part of the Baxi Group (now BDR Thermea), Heatrae Sadia benefits from significant R&D investment and a manufacturing ecosystem that also produces Baxi boilers, Potterton heating appliances, and the Santon and Redring water heater ranges. This group-level backing means Megaflo cylinders are developed and tested to standards that reflect the combined expertise of some of the most trusted names in UK heating. For installers, that translates to a product they can specify with confidence, knowing that parts availability, technical support, and warranty backing are underpinned by a major manufacturer.
Understanding the Megaflo Eco Range – Which Type Do You Need?
The Megaflo Eco range covers virtually every domestic hot water scenario through six distinct cylinder types. The key differentiator across the range is how the water is heated — by immersion heaters only (Direct), by a boiler or heat pump via an internal coil (Indirect), or by a combination of both — and how many immersion heaters are fitted for recovery speed and off-peak tariff flexibility.
Megaflo Eco Direct (DD) — Electric-Only Heating
The Direct range is designed for homes without a gas or oil boiler. Water is heated exclusively by titanium immersion heaters — one 3kW element on the 70D, and two 3kW elements (6kW total) on the DD models from 125L upwards. This makes the Direct ideal for all-electric properties, off-grid homes, or any installation where the hot water system needs to operate independently of a central heating boiler. The dual-immersion DD models give installers the flexibility to wire one element to an off-peak tariff (Economy 7 or Economy 10) and keep the second on a standard supply for boost recovery.
Megaflo Eco Indirect (i) — Boiler-Fed with Electric Backup
The Indirect range is the go-to choice for properties with a gas, oil, or heat pump system boiler. A unique coil-in-coil heat exchanger provides the primary heating circuit, delivering fast recovery times from as little as 17 minutes on the 70i. A single 3kW Incoloy 825 backup immersion heater is included for off-peak tariff use or emergency backup when the boiler is unavailable. The Indirect ships with a complete control pack including a 22mm two-port motorised valve and wiring centre — everything needed for a clean system boiler installation.
Megaflo Eco Direct Triple Immersion (DDD) — High-Demand Electric
Available in 210L, 250L, and 300L capacities, the Triple Immersion models fit three 3kW titanium heaters (9kW total). This configuration dramatically improves recovery times for larger electric-only installations where simultaneous demand from multiple bathrooms is common. The three-element setup also enables more granular tariff management — for example, two elements on off-peak and one on boost.
Megaflo Eco Direct Quad Immersion (DDDD) — Maximum Electric Capacity
The flagship electric-only option, the Quad Immersion range packs four 3kW titanium heaters (12kW total) into 210L, 250L, and 300L cylinders. This is the model to specify for large homes running entirely on electricity where absolute fastest possible recovery and maximum hot water availability are non-negotiable — particularly properties paired with solar PV and battery storage systems where surplus generation can be diverted into rapid water heating.
Megaflo Eco SystemReady (SB) — Boiler + Enhanced Electric
The SystemReady bridges the gap between Indirect and Direct by combining a boiler heating coil with enhanced electric immersion capacity. Available in 170L to 300L, this model is designed for installations that need both boiler-fed heating and meaningful electric capacity — perfect for properties transitioning from gas to renewable energy, or homes with solar PV that want to divert surplus energy into hot water via an immersion controller.
Megaflo Eco Plus Indirect — Large-Capacity Commercial
For properties that need serious hot water storage, the Eco Plus Indirect range extends the Megaflo platform to 250L, 300L, 400L, 500L, and 570L. These larger cylinders use the same duplex stainless steel construction and internal expansion technology, making them suitable for large family homes, multi-bathroom properties, and light commercial applications like B&Bs and small hotels.
Megaflo Eco Immersion Heater Comparison
One of the most important decisions when specifying a Megaflo cylinder is the immersion heater configuration. The table below summarises the key differences across the range to help you match the right model to the installation requirements.
| Megaflo Model | Heat Source | Immersions | Total kW | Capacities | Best For | From |
|---|---|---|---|---|---|---|
| Eco Direct (D/DD) | Electric only | 1 or 2 x 3kW Titanium | 3–6kW | 70–300L | All-electric homes, off-grid, Economy 7 | £995 |
| Eco Indirect (i) | Boiler coil + backup | 1 x 3kW Incoloy 825 | 3kW | 70–300L | Gas/oil/heat pump system boiler setups | £1,200 |
| Eco Direct Triple (DDD) | Electric only | 3 x 3kW Titanium | 9kW | 210–300L | Large electric homes, multiple bathrooms | £1,595 |
| Eco Direct Quad (DDDD) | Electric only | 4 x 3kW Titanium | 12kW | 210–300L | Maximum recovery, solar PV diversion | £1,650 |
| Eco SystemReady (SB) | Boiler coil + electric | Enhanced immersion | Variable | 170–300L | Hybrid setups, gas-to-renewable transition | £1,900 |
| Eco Plus Indirect | Boiler coil + backup | 1 x 3kW backup | 3kW | 250–570L | Large homes, B&Bs, light commercial | £2,335 |
Why Installers Choose the Megaflo Eco
Internal Expansion — No External Vessel
The patented floating-baffle system handles all DHW expansion inside the cylinder. No external expansion vessel to fit, no additional pipework, no extra servicing point. This is one of the biggest time-savers on any Megaflo installation.
Duplex Stainless Steel + Lifetime Guarantee
The inner vessel is fabricated from high-grade duplex stainless steel, pressure-tested to 15 bar. Heatrae Sadia backs this with a lifetime guarantee on the inner tank for domestic first-owner installations — transferring to 30 years for subsequent owners.
76 l/min Mains-Pressure Delivery
Connected directly to the incoming mains, the Megaflo delivers up to 76 litres per minute at 3 bar pressure. No shower pumps, no header tanks, no compromises on performance — even with multiple outlets running simultaneously.
Complete Safety Kit as Standard
Every cylinder ships with the full cold-water inlet control kit (3 bar PRV, 8 bar relief valve, isolator, strainer, non-return valve, drain cock), factory-fitted T&P valve, tundish, and insulation kit. Nothing extra to source or order separately.
Quick Sizing Guide — Which Capacity Do You Need?
Choosing the right Megaflo cylinder capacity comes down to the number of occupants, bathrooms, and whether you want simultaneous hot water delivery to multiple outlets. Here is a general guide based on the Direct range — the same sizing applies to Indirect models.
70L: Studio flat or single-person home, 1 bedroom, 1 bathroom
125–145L: 1–2 bedroom property, 2–3 occupants, 1–2 bathrooms
170L: 3 bedroom property, 3–4 occupants, 1–2 bathrooms
210L: 4 bedroom property, 4–5 occupants, 2 bathrooms including en-suite
250–300L: 4–6 bedroom property, 5+ occupants, 2–3+ bathrooms, high simultaneous demand
400–570L (Eco Plus): Large family homes, B&Bs, guest houses, light commercial applications
Shop Heatrae Sadia Megaflo Eco Cylinders
Heatrae Sadia
Megaflo Eco Direct (DD) — 70L to 300L
All-electric unvented cylinder with 1–2 titanium immersion heaters. Duplex stainless steel construction, internal expansion, lifetime tank guarantee. Ideal for off-grid and all-electric homes.
From £995.00 inc. VAT
Heatrae Sadia
Megaflo Eco Indirect (i) — 70L to 300L
Boiler-fed unvented cylinder with coil-in-coil heat exchanger and single 3kW Incoloy 825 backup immersion. Ships with motorised valve, wiring centre, and full control pack. Compatible with gas, oil, and heat pump system boilers.
From £1,200.00 inc. VAT
Heatrae Sadia
Megaflo Eco Direct Triple Immersion (DDD) — 210L to 300L
Three titanium immersion heaters delivering 9kW total for faster recovery in large electric-only installations. Available in 210L, 250L, and 300L capacities. Excellent for multi-bathroom properties on off-peak tariffs.
From £1,595.00 inc. VAT
Heatrae Sadia
Megaflo Eco Direct Quad Immersion (DDDD) — 210L to 300L
Four titanium immersion heaters totalling 12kW for maximum recovery capacity. The ideal partner for solar PV systems where surplus energy is diverted into rapid water heating. Available in 210L, 250L, and 300L.
From £1,650.00 inc. VAT
Megaflo Eco Direct — Full Specifications at a Glance
| Specification | 70D | 125DD | 170DD | 210DD | 250DD | 300DD |
|---|---|---|---|---|---|---|
| Nominal Capacity | 70L | 125L | 170L | 210L | 250L | 300L |
| Immersions | 1 x 3kW | 2 x 3kW | 2 x 3kW | 2 x 3kW | 2 x 3kW | 2 x 3kW |
| Mixed Water V40 at 40°C | 78L | 167L | 218L | 241L | 307L | 359L |
| Max Flow at 3 Bar | 76 l/min | 76 l/min | 76 l/min | 76 l/min | 76 l/min | 76 l/min |
| Heat-Up Time (6kW) | — | 71 min | 93 min | 102 min | 121 min | 146 min |
| Standing Loss (kWh/24h) | 0.91 | 1.05 | 1.20 | 1.42 | 1.61 | 1.69 |
| Height (mm) | 802 | 1102 | 1384 | 1486 | 1738 | 2053 |
| Diameter (mm) | 579 | 579 | 579 | 579 | 579 | 579 |
| ErP Class | B | B | C | C | C | C |
| Tank Guarantee | Lifetime (inner tank) · 2 years (components) | |||||
Installer Requirement: All Megaflo Eco cylinders must be installed and commissioned by a G3 competent person (England & Wales), or under Building Warrant (Scotland) / Building Regulation P5 (Northern Ireland). Electrical connections must be made by a qualified electrician to current IET Wiring Regulations. The Benchmark commissioning checklist must be completed and left with the householder.
Frequently Asked Questions
Do I need an external expansion vessel with a Megaflo Eco?
What is the difference between Direct and Indirect Megaflo cylinders?
Can the Megaflo Eco be used with solar PV?
What mains pressure do I need for a Megaflo to work properly?
Is the Megaflo Eco compatible with heat pumps?
What guarantee does the Megaflo Eco carry?
Compatible System Boilers — Pair with a Megaflo Eco Indirect
The Megaflo Eco Indirect range works with any heat-only or system boiler that has thermostatic output control. Here are three popular options available at AIZO Quality Heating that pair perfectly with a Megaflo cylinder.
Baxi
Baxi Amptec S Electric System Boiler — 4kW to 18kW
Compact UK-manufactured electric system boiler with 9:1 modulating ratio, OpenTherm compatibility, and built-in pump and expansion vessel. No flue, no gas — ideal for off-gas homes paired with a Megaflo Eco Indirect.
From £1,190.00 inc. VAT
Baxi
Baxi 800 System 2 Gas Boiler — 18kW / 24kW / 30kW
Premium condensing gas system boiler with 93% seasonal efficiency, stainless steel heat exchanger, and 10-year parts and labour warranty. Ships with Adey MagnaClean Micro2 filter. An excellent Megaflo Eco Indirect partner.
From £1,495.00 inc. VAT
Worcester Bosch
Worcester Greenstar 4000 System Boiler — 12kW to 24kW
Compact ErP A-rated condensing gas system boiler with built-in pump and expansion vessel. Available in five outputs from 12kW to 24kW with a 7-year warranty. A trusted Megaflo Indirect companion from one of the UK's most respected boiler brands.
From £1,395.00 inc. VAT
Need help choosing the right Megaflo cylinder for your project?
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