The OSO Slimline RI150 is the slimmest domestic unvented cylinder available — just
430 mm in diameter, yet it delivers full mains-pressure hot water for an entire household.
Built on over 90 years of Norwegian engineering expertise, this indirect cylinder pairs a
0.7 m² smooth scale-resistant coil with OSO's proprietary NANOPUR foam and Vacuum Insulation Panel (VIP) technology
to achieve an exceptionally low standing heat loss of only 1.8 kWh per 24 hours —
saving up to 300 kWh per year compared to conventionally insulated cylinders. Whether you are retrofitting a tight airing cupboard,
fitting out a flat or loft conversion, or specifying for a renovation where a standard 595 mm cylinder simply will not go,
the Slimline RI150 solves the space problem without compromising on performance or reliability.
430 mm
Diameter — slimmest domestic unvented
150 L
Nominal capacity (141 L actual)
1.8 kWh
Standing heat loss per 24 hrs (EN12897)
ErP C
Energy efficiency rating
5 yr
Stainless steel tank warranty
⚠ Installer Notice — G3 Unvented Qualification Required
Unvented cylinders must be installed by a competent person holding a valid G3 unvented qualification (e.g. Part P registered, G3 certified).
Installation must comply with Building Regulation G3, the current OSO Installation and Maintenance Manual, and all relevant codes of practice.
Failure to comply may invalidate the warranty and breach building regulations.
⚠ Compatibility Notice — Pressurised Primary Required
The Slimline RI is not suitable for gravity-fed (open-vented) primary systems. It requires a pressurised primary circuit
from a system or regular boiler with a pump. In hard water areas, install a scale inhibitor or water treatment device on the cold mains supply
in line with Building Regulation Part L Domestic Heating Compliance Guide. A minimum cold mains standing pressure of 2.5 bar and
flow rate of 20 l/min at 1 bar dynamic pressure is recommended for optimum performance.
ℹ Space-Saving Advantage
At just 430 mm diameter, the Slimline RI150 is 165 mm narrower than a standard 595 mm unvented cylinder.
It fits in cupboards, alcoves and service voids where no conventional cylinder would go — making it the go-to choice for flats, conversions and
compact new-build properties across the UK.
🌞 NANOPUR + VIP Insulation
OSO's NANOPUR foam combined with Vacuum Insulation Panels (VIP) achieves insulation performance far surpassing conventional polyurethane foam. The result is a standing heat loss of just 76 W / 1.8 kWh per 24 hours — delivering genuine energy savings and faster reheat cycles throughout the year.
🔄 Large 0.7 m² Smooth Scale-Resistant Coil
The 0.7 m² smooth-bore heat exchanger coil maximises heat transfer from the boiler primary while resisting limescale accumulation. At a primary flow rate of 15 l/min at 80°C, the coil heats the full cylinder in just 24.3 minutes and achieves a 70% recovery in only 17 minutes.
⚡ INCOTEC Incoloy Immersion Backup
A 3 kW Incoloy 825 immersion heater — OSO's own INCOTEC element — provides reliable backup and supplementary heating. The working thermostat is adjustable between 40–70°C, with an independent safety cut-out at 85°C ±3°C. An upper thermostat controls the primary circuit via the motorised valve.
🔒 Complete G3 Factory Safety Pack
Supplied fully kitted for G3 compliance: multibloc pressure reducing valve (3 bar) with integral expansion relief valve (6 bar), temperature & pressure relief valve (90–95°C / 10 bar), 12-litre expansion vessel with wall bracket, motorised 2-port zone valve, tundish, drain cock and cable clamp — everything needed for a compliant, neat installation.
🏗 EVERLAST Stainless Steel Construction
The inner vessel is manufactured from 1.3 mm high-grade stainless steel using OSO's proprietary EVERLAST titanium-alloyed composition and ULTRAWELD robotised welding system. This delivers class-leading corrosion resistance without cathodic protection, anodes or annual chemical treatments. KIWA Watertec approved to UK building and water regulations.
📋 S-Plan & Y-Plan Compatible
Works with both S-plan (2-port zone valve) and Y-plan (mid-position valve) central heating configurations, with factory-supplied wiring diagrams for standard and thermistor-controlled boilers. Internal wiring is factory fitted; the installer simply connects the mains supply, motorised valve, and boiler primaries. Compatible with all modern system and regular boilers.
📈 221 Litres Mixed to 40°C
Despite its compact footprint, the RI150 delivers an impressive 221 litres of usable water mixed to 40°C — enough for multiple simultaneous showers or a full bath with capacity to spare. This blended output figure demonstrates the real-world performance advantage of storing water at higher temperatures with proper pressure control.
🏅 ISO-Certified Norwegian Manufacturing
OSO Hotwater has manufactured stainless steel cylinders in Norway since 1932. The company is certified to ISO 9001 (Quality), ISO 14001 (Environment), ISO 45001 (Occupational Safety) and welding standard ISO 3834-2, ensuring every cylinder meets exacting quality and sustainability benchmarks before it leaves the factory.
Technical Specifications
| Brand |
OSO Hotwater AS |
| Model / Product Code |
Slimline RI150 / 1326200 |
| Type |
Indirect unvented mains-pressure hot water cylinder |
| Nominal Capacity |
150 litres (141 L actual storage volume) |
| Energy Efficiency Rating (ErP) |
C (EU 812/2013 / 814/2013) |
| Standing Heat Loss |
76 W / 1.8 kWh per 24 hours (EN12897:2016) |
| Dimensions (Ø × H) |
430 mm diameter × 1,610 mm height |
| Weight (Empty / Full) |
43 kg / 184 kg |
| Primary Heating Coil |
0.7 m² smooth bore, scale-resistant (HX coil) |
| Primary Flow Rate |
15 l/min at 80°C |
| Coil Heat-Up Time (ΔT 55°C) |
24.3 minutes (full) / 17 minutes (70% recovery) |
| Coil Heating Power |
16.5 kW |
| Volume Mixed to 40°C |
221 litres |
| Immersion Heater |
3 kW Incoloy (INCOTEC), 1 × 240 V |
| Thermostat Range |
40–70°C working thermostat; 85°C ±3°C safety cut-out |
| Max Design Pressure |
6 bar |
| Operating Pressure — CW Inlet |
3 bar (via PRV) |
| Operating Pressure — Primary Coil |
2.5 bar |
| T&P Relief Valve |
90–95°C / 10 bar (factory fitted) |
| Expansion Relief Valve |
6 bar (integral to multibloc PRV) |
| Expansion Vessel (Supplied) |
12 litres with wall mounting bracket (AX 12) |
| Primary Connections |
¾" BSP flow and return |
| Cold Water Inlet |
¾" BSP (22 mm recommended) |
| Hot Water Outlet |
¾" BSP |
| Inner Tank Material |
1.3 mm EVERLAST titanium-alloyed stainless steel (ULTRAWELD welding) |
| Insulation Technology |
NANOPUR polyurethane foam + Vacuum Insulation Panels (VIP) |
| Regulatory Approval |
KIWA Watertec — UK Building & Water Regulations |
| Standards Compliance |
EN 12897:2016, EN 60335-2-21, ISO 3834-2, UKCA marked |
| Manufacturer Certifications |
ISO 9001 · ISO 14001 · ISO 45001 |
| Wiring Plans Included |
S-plan and Y-plan (standard and thermistor-controlled boilers) |
| Min. Clearances Required |
400 mm front and sides; 150 mm rear; 40 mm above |
| Warranty — Tank |
5 years (stainless steel inner vessel) |
| Warranty — Components |
2 years |
Installation & Compliance Summary
The Slimline RI150 is supplied with all G3 components in a separate accessory kit for site fitting plus P&T valve factory fitted to the vessel.
Installation must be carried out by a G3-qualified competent person on an S-plan or Y-plan pressurised primary system.
A dedicated fused spur with a double-pole isolating switch (minimum 3 mm contact separation) is required for the immersion heater circuit.
All cables must comply with BS6141 Table 8 (HOFR 85°C rating). The discharge pipework must be installed with a continuous fall to a safe visible discharge point per Building Regulation G3.
Annual maintenance by a competent person — including checking the PRV strainer, expansion relief valve, T&P relief valve, and expansion vessel pre-charge — is a condition of the warranty.
Frequently Asked Questions
What makes the Slimline RI150 different from a standard unvented cylinder?
The defining feature is its 430 mm diameter — the narrowest footprint of any domestic unvented cylinder on the UK market.
A standard cylinder is typically 595 mm in diameter, meaning the Slimline is 165 mm slimmer. It fits in locations that would otherwise require an expensive structural alteration or force a compromise to a smaller vented tank.
Despite its slim profile, it uses the same NANOPUR + VIP insulation technology, EVERLAST stainless steel, and INCOTEC immersion elements as OSO's full-size range — there is no performance penalty for the reduced diameter.
Is the Slimline RI150 compatible with a combi boiler?
No. The Slimline RI is an indirect unvented cylinder and requires a system boiler or regular (heat-only) boiler to heat water through the primary coil.
Combi boilers do not have a separate hot water circuit and cannot drive a cylinder.
If you have a combi boiler and need to add stored hot water, you would need to change your boiler to a system or heat-only model as part of the installation project.
Can the immersion heater be used as the sole heat source?
The OSO manual specifically cautions against this. The 3 kW immersion heater is designed for backup and supplementary use only — for example, during boiler servicing, summer periods, or as a boost when solar or heat-pump energy is available.
Using the immersion as the sole heat source is possible but results in significantly higher running costs and energy consumption compared to primary coil heating, and OSO advise it should be avoided.
Does the Slimline RI150 require an anode or any corrosion protection?
No. OSO's EVERLAST titanium-alloyed stainless steel is naturally corrosion-resistant to mains water supply without the need for sacrificial anodes, magnesium rods, or chemical corrosion inhibitors.
The manual confirms no specific corrosion maintenance is required. The warranty does not cover use with a private (non-mains) water supply or water chemistries outside OSO's permissible parameters — your local water authority can provide water quality data if required.
What is the real-world hot water output — will it cope with a busy household?
At storage temperature and mixed to a comfortable 40°C, the RI150 delivers 221 litres of usable water from a single heated charge —
more than enough for two simultaneous showers plus a kitchen load with capacity remaining.
With a fast 17-minute 70% recovery time via the primary coil, hot water is rapidly replenished after high-demand periods.
For households of 2–4 people with a system or regular boiler, the RI150 provides reliable mains-pressure performance.
What annual maintenance does the Slimline RI150 require?
Annual maintenance by a competent person is a warranty condition. The required checks are: inspect and clean the line strainer on the PRV;
test the expansion relief valve and T&P relief valve to confirm they operate freely; repressurize the expansion vessel air charge to its set level;
and visually check the thermostat and immersion heater.
The cylinder must also be recommissioned following any maintenance that requires draining. OSO recommend using original OSO spare parts for any replacements.
