The Joule Invacyl Indirect Standard Unvented Hot Water Cylinder range provides mains-pressure domestic hot water for properties heated by a gas, oil or electric boiler, with seven capacities from 90 to 300 litres to suit anything from a one-bathroom flat to a large family home. Each cylinder is built around a high-grade Duplex stainless steel vessel and a smooth-tube, high-efficiency indirect coil, and is factory-supplied as a complete G3 unvented kit — including expansion vessel, inlet control group, temperature & pressure relief valve, dual cylinder thermostat, two-port motorised zone valve and a 3kW Incoloy backup immersion heater — ready for a competent installer to fit and commission.
The range is rated ErP Energy Class C and the stainless steel vessel carries a 25-year manufacturer's warranty, with a 1-year warranty on the supplied expansion vessel and unvented kit components.
Duplex Steel
Vessel Material
25 Years
Cylinder Warranty
Class C
ErP Energy Rating
26–48 min
Indirect Reheat Time
3kW Incoloy
Backup Immersion
Notifiable installation – Gas Safe / G3-qualified installer required: Unvented hot water cylinders are a controlled service under Building Regulations (Part G3 in England & Wales, Technical Standard P3 in Scotland, Building Regulation P5 in Northern Ireland). This cylinder must be installed, commissioned and serviced only by a competent G3-trained installer, who must notify the local building control authority and complete the Benchmark Commissioning Checklist on handover. The temperature & pressure relief valve discharge pipe must always be left open to the atmosphere and routed to a safe, frost-free termination – occasional dripping during normal operation is expected and the pipe must never be capped or restricted.
Primary circuit must be thermostatically controlled: The boiler feeding the primary coil (gas, oil or electric) must be under effective thermostatic control and positively pumped. Uncontrolled heat sources – such as AGA-type cookers, back boilers or solid-fuel stoves – must not be connected directly to the primary coil. The dual cylinder thermostat and two-port motorised zone valve supplied with this Indirect kit must be wired into the primary circuit exactly as shown in the installation manual to prevent overheating.
Single-coil model – not solar-ready: This Standard Indirect cylinder has one primary coil for boiler heating, plus the 3kW immersion as electric backup. If you need to integrate a solar thermal panel array as well, this is not the right variant – you'll need an Invacyl Solar Indirect Twin Coil cylinder, which adds a dedicated lower solar coil.
Balanced cold connection needed for showers & mixer taps: If the system feeds any showers, bidets or mono-bloc mixer taps, the balanced cold connection on the supplied inlet control group must be used. Feeding these outlets from an unbalanced cold supply can back-pressurise the cylinder and trigger discharge from the safety valves.
Mains water chemistry affects the warranty: The 25-year vessel warranty applies only where the water supply meets Joule's published chemistry limits (for example, total dissolved solids ≤600mg/litre, total hardness ≤200mg/litre, pH 6.5–9.5). Properties on a private well, borehole or other non-mains supply – or any supply outside these limits – require a Joule Wellmaster cylinder instead of this Standard model.
Everything in the box: Each cylinder is supplied as a complete, factory-matched unvented kit – expansion vessel, 3 bar inlet control group, 7 bar T&P relief valve, 22mm tundish, dual cylinder thermostat and two-port zone valve are all included, sized to the cylinder. There's nothing extra to source for a compliant first fix beyond pipework, isolation valves and (where needed) a thermostatic mixing valve.
Consistent footprint across the range: The 120–300 litre models share the same 545mm diameter, so moving up in capacity only increases the cylinder's height – useful when planning airing cupboard or utility room space ahead of delivery.
Duplex Stainless Steel Vessel
The inner vessel is manufactured from high-grade Duplex stainless steel, an alloy that pairs the corrosion resistance of austenitic grades with the mechanical strength of ferritic steel. This combination gives strong resistance to pitting and stress-corrosion over decades of use – the basis for Joule's 25-year warranty on the vessel.
Smooth-Tube, High-Efficiency Indirect Coil
Heat from the boiler is transferred via a smooth-tube primary coil holding 3.25–4.26 litres of primary fluid depending on cylinder size. The smooth bore reduces turbulent flow losses compared with corrugated coil designs, helping the range achieve reheat times of 26–48 minutes from a cold fill across the 90–300L range.
CFC/HCFC-Free Insulation
A polyurethane foam jacket with zero ozone-depletion potential wraps the vessel, holding standing heat loss to between 52W (90L) and 93W (300L). This is what underpins the cylinder's ErP Class C rating, keeping background energy use down between draw-offs.
Complete Factory-Matched G3 Kit
Rather than sourcing components separately, every cylinder arrives with its expansion vessel, 3 bar inlet control group, 7 bar T&P relief valve, 22mm tundish, dual cylinder thermostat and two-port motorised zone valve already matched to the cylinder size – everything a G3 installer needs for a Building Regulations-compliant first fix.
Dual Thermostat & Two-Port Zone Valve
Because this is an indirect model, the kit pairs a dual cylinder thermostat with a two-port motorised zone valve in the primary circuit. Together they stop the cylinder overheating if the boiler keeps firing once the stored water reaches its set temperature – a safeguard that's specific to indirect cylinders and supplied as standard here.
3kW Incoloy Backup Immersion
A factory-fitted 3kW Incoloy immersion heater, wired through its own thermostat, lets the cylinder keep producing hot water electrically if the boiler is off for servicing or breaks down – useful resilience where this cylinder is the property's only source of domestic hot water.
Caleffi Safety & Control Components
The safety valves and inlet controls are supplied by Caleffi, a specialist manufacturer of hydronic safety devices used widely across the European unvented cylinder market. All units are factory pre-set, so the installer doesn't need to adjust pressure or temperature settings on site.
Screwed Connections, Compression Fittings
All pipework connections are female threaded with compression fittings supplied as standard, speeding up first-fix plumbing without compromising the watertight seal needed for a 3 bar mains-pressure system.
| Capacity |
Product Code |
Weight Empty |
Weight Full |
Reheat Time¹ |
Standing Heat Loss² |
Primary Coil Fluid Volume |
ErP Class |
| 90L |
TRBMVI-0090LFC |
23 kg |
109 kg |
26 min |
52 W |
— |
C |
| 120L |
TRBMVI-0120LFB |
30 kg |
146 kg |
33 min |
60 W |
3.25 L |
C |
| 150L |
TRBMVI-0150LFB |
38 kg |
184 kg |
36 min |
68 W |
3.25 L |
C |
| 180L |
TRBMVI-0180LFB |
42 kg |
218 kg |
41 min |
76 W |
3.76 L |
C |
| 210L |
TRBMVI-0210LFC |
45 kg |
250 kg |
45 min |
80 W |
3.76 L |
C |
| 250L |
TRBMVI-0250LFC |
51 kg |
296 kg |
47 min |
91 W |
4.26 L |
C |
| 300L |
TRBMVI-0300LFC |
60 kg |
355 kg |
48 min |
93 W |
4.26 L |
C |
¹Reheat time is the time taken to heat the cylinder from cold to 60°C via the primary coil at full output. ²Standing heat loss is the cylinder's idle heat loss in Watts. Source: Joule Invacyl technical datasheet (SAL-0005-1).
| Dimensions & Connection Heights (mm, measured from floor) |
| Capacity |
Height (A) |
Diameter (B) |
Cold Water Inlet (C) |
Primary Coil Flow (D) |
Primary Coil Return (E) |
Aquastat Pocket (F) |
T&P Relief Valve (G) |
| 120L |
900 |
545 |
195 |
440 |
200 |
360 |
710 |
| 150L |
1100 |
545 |
195 |
520 |
200 |
395 |
890 |
| 180L |
1280 |
545 |
195 |
560 |
200 |
395 |
1085 |
| 210L |
1470 |
545 |
195 |
600 |
200 |
475 |
1265 |
| 250L |
1720 |
545 |
195 |
600 |
200 |
475 |
1520 |
| 300L |
1980 |
545 |
195 |
600 |
200 |
475 |
1980 |
Dimensions and connection heights for the 90L Indirect model (TRBMVI-0090LFC) are not published in Joule's selection guide alongside the 120–300L range. Confirmed weight, reheat time and standing loss data for the 90L model are shown in the table above; please refer to the official Joule technical datasheet or contact us for a full dimensional drawing before finalising pipework on the 90L model. Source: Joule technical datasheet (SAL-0005-1).
What's the difference between this Indirect cylinder and the Direct version?
The Direct Invacyl heats water purely by electric immersion elements and has no primary coil, which suits properties without a boiler. This Indirect model adds a smooth-tube primary coil so it can be heated from a gas, oil or electric boiler's central heating circuit, with the 3kW immersion fitted as backup only.
Can this cylinder be heated by an air source heat pump?
This Standard Indirect model is designed around boiler-style heat sources, and the dual thermostat and two-port zone valve supplied in the kit are matched to that arrangement. For heat pump systems, Joule offers dedicated heat pump cylinder ranges with coil sizing and controls suited to lower flow temperatures – please check our heat pump cylinder range if that's your heat source.
Is everything needed for a G3-compliant installation included?
Yes – each cylinder is supplied with the full unvented kit: expansion vessel, 3 bar inlet control group, 7 bar T&P relief valve, 22mm tundish, dual cylinder thermostat and two-port zone valve, all sized to the cylinder. A competent G3 installer will still need to provide pipework, isolation valves and, where needed, a thermostatic mixing valve for scald protection.
Does this cylinder need annual servicing?
Yes. Joule's 25-year warranty requires the cylinder to be serviced annually by a competent person, with the Benchmark service record completed at each visit. Regular servicing – including operating the T&P relief valve to clear lime deposits – is also important for safe day-to-day operation, not just the warranty.
What size cylinder do I need?
As a rough guide, 90–120L typically suits a one-bathroom property, 150–180L a two-bathroom home, and 210–300L larger households with multiple bathrooms or higher simultaneous demand. Final sizing should account for the number of outlets, boiler output and peak usage patterns – a heating engineer can confirm the right capacity for your property.
Can I connect a solar thermal panel to this cylinder?
No – this Standard Indirect model has a single primary coil intended for boiler heating plus electric backup. For solar thermal integration, Joule's Invacyl Solar Indirect Twin Coil cylinders add a second, lower coil dedicated to the solar circuit.
What if my property isn't on the mains water supply?
This cylinder's warranty depends on the water supply meeting Joule's published chemistry limits (total dissolved solids, hardness, pH and so on), which most UK mains supplies satisfy. Properties on a private well, borehole or other non-mains supply should use a Joule Wellmaster cylinder instead, regardless of how the water tests.
