The Joule Invacyl Direct Unvented Hot Water Cylinder is a mains-pressure, electrically heated unvented cylinder available in seven sizes from 90 to 300 litres. It is designed for properties with no boiler or heating system coil requirement, delivering hot water heated entirely by two factory-fitted Incoloy immersion heaters — upper (boost) and lower (economy / off-peak) — at full mains pressure throughout the property. The vessel is manufactured from high-grade duplex stainless steel with a 25-year fully transferable manufacturer’s guarantee. Callefi branded safety components are factory-fitted, and the cylinder ships with a complete G3 unvented kit: cold water inlet set, tundish, T&P relief valve, expansion vessel and bracket. Compliant with Building Regulation G3 (England & Wales). All connections are female threaded with 22mm compression fittings as standard. Smart immersion compatible. Register within 28 days of commissioning to activate the full warranty.
90–300L
7 Sizes
Duplex SS
25-Yr Warranty
Mains Pressure
Unvented
2 × Immersions
Factory Fitted
Full G3
Kit Included
Callefi
Components
⚠ G3 Competent Person Installation Required: Unvented hot water systems are a controlled service under Building Regulation G3. Installation must be carried out by a person who holds a current G3 Competent Person qualification (e.g. ACS Unvented Hot Water, Level 3 City & Guilds 6161-03 or equivalent).
What’s Included in This Package
Joule Invacyl Direct Cylinder
Duplex stainless steel body with two factory-fitted immersion heaters. Colour-coded tapping collars. Callefi safety components pre-fitted.
Cold Water Inlet Set
Pre-assembled inlet control set including pressure reducing valve (set to 3 bar), line strainer, check valve and service valve. Supplied with 22mm compression fittings.
15 × 22mm Tundish
15mm inlet × 22mm outlet tundish for the discharge pipework. Provides visible indication of relief valve operation. Required by Building Regulation G3.
Temperature & Pressure Relief Valve (Fitted)
Factory-fitted T&P relief valve with plastic outer casing. Operates at 90°C / 6 bar. Pre-set — no adjustment required.
Expansion Vessel & Bracket
Dedicated expansion vessel sized for the cylinder, with wall-mounting bracket. Absorbs volume expansion from heating without discharging through the T&P valve.
2 × Immersion Heaters (Fitted)
Two Incoloy immersion heaters factory-fitted to the cylinder. Allows independent control of upper (boost) and lower (economy) heating zones. Smart immersion compatible.
ℹ G3 Compliant Package: This package includes all the components required to meet Building Regulation G3 (England & Wales), Technical Standard P3 (Scotland) and Building Regulation P5 (Northern Ireland) for unvented hot water systems. Installation must be carried out by a G3 Competent Person. Register the product online within 28 days of commissioning to validate the warranty.
Size Selection Guide
Seven sizes are available. Select based on the number of bedrooms, bathrooms and expected peak hot water demand. As a general guide, allow approximately 40 – 50 L per person per day for an all-electric direct cylinder.
The Invacyl Direct is manufactured from high-grade duplex stainless steel, which provides significantly better corrosion resistance than standard 304 stainless steel or copper. The 25-year guarantee covers faulty materials and manufacture and is fully transferable to subsequent property owners, making it a strong selling point for new-build and renovation projects. The guarantee applies provided the cylinder is installed to public mains water supply, serviced annually, and water chemistry remains within specified limits (TDS ≤600 mg/L, hardness ≤200 mg/L, chloride ≤300 mg/L). Expansion vessel and cold water controls carry a separate 2-year guarantee.
Two Factory-Fitted Immersion Heaters — Upper Boost and Lower Economy Zones
The cylinder is supplied with two Incoloy immersion heaters pre-fitted at the factory. The upper (top) immersion heats only the top portion of the cylinder for quick hot water availability during peak demand (boost mode). The lower (bottom) immersion heats the full cylinder volume and is typically wired to an economy tariff timer for overnight heating. Having both fitted and colour-coded at the factory reduces installation time significantly versus cylinders supplied with immersion heaters separately. Both are smart immersion compatible.
Mains-Pressure Hot Water — Consistent Flow Across the Whole Property
As an unvented cylinder, the Invacyl Direct operates at mains pressure (typically 2–3 bar after the inlet set) rather than relying on gravity head from a cold water tank. This means equal pressure at every outlet: showers, kitchen taps, bath taps and basins all receive the same head pressure regardless of where they sit in relation to the cylinder. Minimum recommended supply is 1.5 bar static and 25 L/min. The cylinder is capable of delivering over 50 L/min at the rated operating pressure of 3 bar.
Mains pressure>50 L/min deliveryNo cold tank needed
Complete G3 Unvented Kit Supplied — One Delivery, Ready to Install
This package includes all the statutory safety and control components required for a Building Regulation G3 compliant unvented installation: cold water inlet control set (PRV set to 3 bar, line strainer, check valve, service valve), 15 × 22mm tundish, T&P relief valve (pre-fitted), and expansion vessel with wall bracket. All supplied with 22mm compression fittings. Having all components in one delivery saves procurement time, reduces the risk of component mismatches, and simplifies site logistics.
Full G3 kit included22mm compression fittingsPRV pre-set to 3 bar
Callefi Safety Components — High Flow Rate Performance
The Invacyl Direct uses Callefi branded safety and control components. Callefi is one of the leading European manufacturers of hydraulic components for heating and plumbing systems. The high-quality inlet control set is specifically designed to maximise flow rate while minimising pressure drop, giving excellent performance particularly in areas with lower mains pressure. All safety devices are pre-set at the factory — no field adjustment is required.
All tapping ports on the Invacyl Direct are fitted with colour-coded collars that visually identify each connection at a glance: cold in, hot out, immersion positions, T&P valve and secondary return. This significantly reduces the risk of mis-connection during installation and speeds up pipe routing, particularly useful in confined airing cupboard spaces. All connections are female threaded throughout.
The Invacyl Direct is ERP (Energy-related Products Directive) compliant. Both immersion heaters are compatible with smart immersion controllers, such as those used in solar PV diversion systems and time-of-use tariff controllers (e.g. Eddi, Zappi, iBoost), allowing excess solar generation or cheap-rate electricity to be used for water heating automatically. This makes the Invacyl Direct well suited to properties with solar PV installations.
CFC/HCFC-Free Polyurethane Foam Insulation — Minimal Standing Heat Loss
The cylinder shell is insulated with approved CFC/HCFC-free polyurethane foam with an ozone depletion factor of zero. The foam is injected to fully encapsulate the vessel, eliminating cold bridges and minimising standing heat loss. This is important for a direct electric cylinder where all energy consumed is paid for at the electricity rate — low standing loss directly reduces running costs.
CFC/HCFC-free foamZero ODPLow standing loss
Technical Specifications
Cylinder
Type
Direct unvented (electrically heated, no coil)
Material
High-grade duplex stainless steel
Insulation
CFC/HCFC-free polyurethane foam, zero ODP
Operating Pressure
3 bar (after inlet set PRV)
Max Cold Mains Inlet Pressure
10 bar
Min Recommended Supply
1.5 bar static, 25 L/min
Max Flow Rate
>50 L/min at rated pressure
Orientation
Vertical only
Immersion Heaters
Number Fitted
2 (factory-fitted)
Element Material
Incoloy
Positions
Upper (boost) + Lower (economy/off-peak)
Smart Immersion Compatible
Yes
Safety Components (Callefi)
T&P Relief Valve
Factory-fitted — 90°C / 6 bar
Cold Water Inlet Set
PRV (set 3 bar), line strainer, check valve, service valve
Tundish
15mm inlet × 22mm outlet (15 × 22mm)
Expansion Vessel
Included with wall bracket
Connections
Female threaded, 22mm compression fittings
Dimensions by Size
90 L (TRBMVD-0090LFC)
913 mm H × 545 mm Ø
120 L (TRBMVD-0120LFB)
1085 mm H × 545 mm Ø
150 L (TRBMVD-0150LFB)
1310 mm H × 545 mm Ø
180 L (TRBMVD-0180LFB)
1490 mm H × 545 mm Ø
210 L (TRBMVD-0210LFC)
1600 mm H × 545 mm Ø
250 L (TRBMVD-0250LFC)
1760 mm H × 545 mm Ø
300 L (TRBMVD-0300LFC)
1980 mm H × 545 mm Ø
Water Chemistry Limits (Warranty)
Total Dissolved Solids
≤600 mg/L
Total Hardness
≤200 mg/L
Chloride
≤300 mg/L
pH Range
6.5 – 9.5
Iron
≤1 mg/L
Compliance and Warranty
Building Regulation
G3 (England & Wales) / P3 (Scotland) / P5 (NI)
Standards
BS 5449, BS 5546, BS 6700, BS EN 12897
Certifications
KIWA independently tested and certified
Cylinder Warranty
25 years (fully transferable — register within 28 days)
Expansion Vessel & Cold Water Controls
2-year guarantee
All Other Components
2-year guarantee
Installation Requirement
G3 Competent Person
Manufacturer
Joule (UK unit: Pontefract, West Yorkshire; IE: Dublin)
Frequently Asked Questions
What is the difference between a direct and an indirect unvented cylinder?
A direct cylinder is heated solely by its own immersion heaters — there is no coil inside for connection to a boiler or heat pump. It is the right choice for all-electric properties, properties without a central heating boiler, or where solar PV diversion is the primary heating method. An indirect cylinder has one or more internal coils fed by a boiler or heat pump, with immersion heaters as backup. The Invacyl Direct is for direct electric heating only.
Why does this cylinder need a G3 Competent Person to install it?
Unvented hot water cylinders operate at mains pressure and store large volumes of hot water. If the safety devices fail, stored energy can cause serious scalding or vessel failure. Building Regulation G3 therefore requires installation by a trained and qualified person who can correctly size, install and commission the safety components (inlet set, T&P valve, expansion vessel, discharge pipe and tundish). The installer must complete the Benchmark log and notify the local authority. Using an unqualified installer will invalidate the warranty and may constitute a Building Regulations offence.
What is the expansion vessel for?
When water is heated, it expands in volume by approximately 4% between cold fill and 60°C. In a sealed (unvented) system, this expansion has nowhere to go unless it is absorbed. The expansion vessel contains a rubber bladder pre-charged with nitrogen: as water expands, it compresses the nitrogen in the vessel. This prevents pressure from rising to the T&P valve trip point during normal heating. Without a correctly sized expansion vessel, the T&P valve would open on every heat cycle, wasting water and eventually blocking the valve with scale.
What does the tundish do?
The tundish is a visible break in the discharge pipework connected to the T&P relief valve. It serves two purposes: it provides an air gap that prevents backflow from the drain into the potable water system, and it provides a visual indicator that a relief valve has operated — a drip or flow of water through the tundish tells the homeowner or installer that the valve has lifted and the system needs attention. Building Regulation G3 requires a tundish to be installed.
Can I connect this cylinder to a solar PV diversion system?
Yes. Both immersion heaters are smart immersion compatible, meaning they work with solar PV diversion devices such as the myenergi Eddi, Zappi (with Harvi), iBoost or similar. These devices detect excess generation on the circuit and divert power to the immersion rather than exporting it, heating water using free electricity. The lower immersion is typically connected to the diverter device for economy-rate/PV heating, and the upper immersion to a boost timer or manual switch for peak demand top-up.
What water quality does the warranty require?
The 25-year warranty on the stainless steel vessel covers standard UK mains water supplies within these limits: Total Dissolved Solids ≤600 mg/L, Total Hardness ≤200 mg/L, Chloride ≤300 mg/L, pH 6.5 – 9.5. If your mains water has high mineral content (particularly in hard-water areas of the UK), check that Total Hardness is below 200 mg/L. In areas approaching this limit, a descaler or scale filter on the cold mains feed is recommended. Joule Wellmaster cylinders are available for well water, borehole, or supplies that exceed these parameters.
Related Products
Joule Invacyl Indirect Unvented Cylinder
Boiler/heat pump heated version with internal coil
Aquabion Scale Conditioner
Electrochemical scale protection for hard water areas
myenergi Eddi Solar Diverter
Diverts excess solar PV generation to the immersion heater
Strom Electric Heat-Only Boiler + Cylinder Package
Alternative pre-plumbed electric hot water package
BTU & WATT Calculator
Room Dimensions
Room Type
Living Room
Bedroom
Kitchen
Bathroom
Conservatory
Hallway
Glazing Type
Single Glazed
Double Glazed
Triple Glazed
External Walls
0 Wall
1 Wall
2 Walls
3 Walls
4 Walls
Insulation Level
Poor
Average
Good (New Build)
Required Heating
0
Watts
0
BTUs
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