The OSO Slimline HP 150SL is a 150 litre indirect unvented heat pump cylinder with an ultra-compact 435 mm diameter, making it the slimmest domestic heat pump cylinder available in the UK. Designed specifically for low-temperature renewable heating systems, it features a large 1.8 m² smooth scale-resistant coil optimised for efficient heat transfer from air source and ground source heat pumps. The cylinder is built from EVERLAST™ titanium-alloyed stainless steel with ULTRAWELD™ construction and insulated with NANOPUR foam combined with Vacuum Insulation Panels (VIP) for minimal standing heat loss. A factory-fitted 2.8 kW INCOTEC™ immersion heater provides backup boosting capability. The HP 150SL is compatible with 3-port diverter, S-plan, and Y-plan systems, and includes a 3-port motorised diverter valve, 12 litre expansion vessel, and full G3 kit as standard. Backed by OSO's 25-year warranty on the stainless steel tank.
🔴 Building Regulation G3 – Unvented Cylinder Installation
This unvented hot water cylinder must be installed by a competent person holding a current G3 unvented hot water storage qualification, in full compliance with Building Regulation G3, EN 12897:2016, and all current legislation in force at the date of installation. The discharge pipework must be routed to a safe, visible position with continuous fall to drain. This product is approved to building and water regulations by KIWA Watertec Ltd.
🟡 Immersion Heater – Backup Use Only
The immersion heater is intended for backup and supplementary use only. OSO explicitly states that using the immersion heater as the sole heat source should be avoided, as it would lead to higher energy consumption and higher operating costs. The cylinder is designed to be connected to a high-efficiency external energy source such as a heat pump or boiler.
🟡 Primary Flow Direction – Heat Pump vs Boiler
The primary coil flow direction differs depending on the heat source. When connected to a heat pump, the flow direction is top to bottom. When connected to a boiler, the flow direction is bottom to top. Ensure the primary pipework is connected to the correct flow and return positions for your system type.
🟢 Ultra-Compact 435 mm Diameter – Fits Where Others Cannot
At just 435 mm wide, the Slimline HP is designed for tight installation spaces such as airing cupboards, utility rooms, and kitchen cabinets where a standard 595 mm diameter cylinder would not fit. Despite its slim profile, the 1.8 m² coil and 137 litre actual volume deliver strong hot water performance with high flow rates suitable for mains-pressure showers and multiple outlets.
🟢 3-Port Motorised Diverter Valve Included
Unlike many cylinders that supply a 2-port zone valve, the Slimline HP includes a 3-port motorised diverter valve as standard, enabling direct use with 3-port diverter system wiring. The cylinder is also compatible with S-plan (using the supplied 3-port valve plus a separate zone valve for heating) and Y-plan (using the supplied 3-port valve plus a separate mid-position valve) configurations.
Key Features
Heat Pump Optimised 1.8 m² Coil
The large 1.8 m² smooth tube heat exchanger is purpose-built for the lower flow temperatures of modern heat pump systems. With a 70% recovery time of just 17 minutes at a primary flow rate of 15 l/min at 80°C, the coil delivers fast reheat despite the compact cylinder diameter. The smooth coil design resists limescale build-up, maintaining efficiency over the long term.
VIP + NANOPUR Insulation System
OSO's dual-layer insulation combines Vacuum Insulation Panels with NANOPUR high-performance foam to minimise standing heat loss. The cylinder achieves a standing heat loss of 76 W (1.82 kWh/day per EN 12897), keeping stored water hotter for longer and reducing the frequency of heat pump reheat cycles — improving overall system efficiency.
INCOTEC™ Backup Immersion Heater
The factory-fitted 2.8 kW INCOTEC™ immersion heater uses ultra-high-grade Incoloy 825 stainless steel elements with DZR brass fittings and zinc-free soldering for maximum durability in hard water. Positioned for supplementary boosting, it provides a reliable backup when the primary heat source is temporarily unavailable or additional capacity is needed.
Slimmest Domestic Heat Pump Cylinder
At 435 mm diameter × 1666 mm height, the HP 150SL occupies a footprint approximately 27% smaller than a standard 595 mm cylinder. This makes it suitable for retrofit projects where space is constrained, properties with narrow airing cupboards, or installations where the cylinder needs to fit alongside other equipment.
Complete G3 Kit with 3-Port Diverter Valve
The HP 150SL ships with a comprehensive G3 accessory kit including a multibloc valve (3 bar PRV with line strainer, balanced cold take-off, check valve, and 6 bar expansion relief), tundish, drain cock, 3-port motorised diverter valve, 12 litre expansion vessel with wall bracket, and stainless steel stability bracket.
EVERLAST™ 25-Year Tank Construction
The EVERLAST™ titanium-alloyed stainless steel tank with ULTRAWELD™ welded seams and advanced passivation provides class-leading corrosion resistance. OSO stainless cylinders are over 40% lighter than enamelled equivalents. The 25-year warranty on the tank reflects OSO's confidence in their Norwegian manufacturing quality, backed by ISO 9001, ISO 14001, and ISO 45001 certification.
Technical Specifications
| Product Identification |
| Manufacturer |
OSO Hotwater AS (Norway) |
| Product Name |
Slimline HP 150 |
| MPN |
11014614 |
| Product Code (ErP) |
Slimline HP 150 – 3 kW / 1×240V + HX 1.8 m² |
| EAN / Barcode |
Not publicly listed — contact OSO UK |
| Capacity & Dimensions |
| Nominal Capacity |
150 litres |
| Actual / Storage Volume |
137 litres |
| Diameter (ø) |
435 mm |
| Height (H) |
1666 mm |
| Weight (Empty) |
43 kg |
| Weight (Full) |
180 kg |
| Thermal Performance |
| ErP Energy Rating |
C |
| Insulation Type |
NANOPUR foam + Vacuum Insulation Panels (VIP) |
| Standing Heat Loss (EN 12897) |
76 W |
| Heat Loss per 24 Hours |
1.82 kWh/day |
| Recovery Rate at 70% |
17 minutes (at 15 l/min primary, 80°C) |
| Primary Heating (Coil) |
| Primary Coil Surface Area |
1.8 m² |
| Coil Type |
Smooth, scale-resistant tube heat exchanger |
| Primary Flow Rate (Test) |
15 l/min at 80°C |
| Primary Flow Direction (Heat Pump) |
Top to bottom |
| Primary Flow Direction (Boiler) |
Bottom to top |
| Primary Connections |
3/4" BSP female (flow & return) |
| Max Primary Flow Temperature |
82°C |
| Coil Pressure Drop at 0.15 L/s |
50 mbar |
| Coil Operating Pressure |
2.5 bar |
| Immersion Heater |
| Immersion Power |
2.8 kW (1-phase, 240 V) — backup use only |
| Element Type |
INCOTEC™ (Incoloy 825, scale-resistant) |
| Immersion Thermostat Range |
Adjustable 40 – 70°C |
| Boiler / Cylinder Thermostat Range |
Adjustable 50 – 75°C |
| Safety Cut-Out |
85°C ± 3°C (manual reset) |
| Part Number (Immersion) |
71214 |
| Pressure & Water Supply |
| Max Design Pressure |
6 bar |
| Cold Water Operating Pressure |
3 bar (via multibloc PRV) |
| Max Supply Pressure |
12 bar (PRV required above 12 bar) |
| Minimum Supply Pressure |
2.5 bar (recommended) |
| Recommended Flow Rate |
20 litres/min at 1 bar dynamic |
| T&P Relief Valve |
7 bar / 90 – 95°C |
| Expansion Relief Valve |
6 bar |
| Expansion Vessel |
12 litre (3 bar pre-charge, with wall bracket) |
| Controls & System Compatibility |
| Motorised Valve (Supplied) |
3-port diverter valve |
| System Compatibility |
3-port diverter, S-plan, and Y-plan |
| Junction Box Compatibility |
Honeywell 10-way (42002116-001) |
| Sensor Pockets |
2 × sensor clamp brackets (factory fitted) |
| Stability Bracket |
Stainless steel (included) |
| Construction & Compliance |
| Tank Material |
EVERLAST™ titanium-alloyed stainless steel |
| Welding System |
ULTRAWELD™ |
| Pressure Vessel Standard |
EN 12897:2016 |
| Safety Standard |
EN 60335-2-21 |
| Welding Standard |
ISO 3834-2 |
| Certification |
UKCA marked |
| Approval Body |
KIWA Watertec Ltd |
| Directives |
LVD 2014/35/EU, EMC 2014/30/EU, PED 2014/68/EU |
| Tank Warranty |
25 years |
| Country of Manufacture |
Norway |
Frequently Asked Questions
Will this cylinder fit in a standard airing cupboard?
The Slimline HP 150SL has an ultra-compact 435 mm diameter (approximately 17 inches), making it significantly narrower than standard cylinders which are typically 580–595 mm wide. At 1666 mm tall, it will fit in most standard airing cupboards. Always verify the available space including clearance requirements: 400 mm in front of the electrical junction box covers and 150 mm above the highest point.
What heat pumps is the HP 150SL compatible with?
The cylinder is compatible with all thermostatically controlled air source heat pumps (ASHP) and ground source heat pumps (GSHP). The 1.8 m² coil is optimised for low-temperature heat transfer. Two sensor clamp brackets are factory fitted to accept 6 mm or 8 mm temperature sensors from the heat pump controller. The cylinder can also be connected to gas or oil boilers for faster recovery.
Why is the ErP rating C rather than A?
The ErP rating for indirect cylinders is based on standing heat loss relative to volume. The HP 150SL's slim 435 mm diameter gives it a higher surface-area-to-volume ratio than wider cylinders of the same capacity, which results in proportionally higher standing losses (76 W). The VIP + NANOPUR insulation still delivers class-leading thermal performance for a slimline cylinder — the ErP C rating reflects the physical geometry rather than insulation quality.
What is included in the box?
The HP 150SL is supplied with: a multibloc valve (3 bar PRV with line strainer, balanced cold take-off, check valve, and 6 bar expansion relief), tundish with screws, 1/4" drain cock, 3-port motorised diverter valve, 12 litre expansion vessel with wall bracket, stainless steel stability bracket, plastic cable clamp, and installation manual with service logbook. The immersion heater, thermostats, T&P relief valve, and sensor clamp brackets are factory fitted.
Can I use this with an S-plan or Y-plan system?
Yes. While the cylinder is supplied with a 3-port motorised diverter valve for direct 3-port wiring, it is also fully compatible with S-plan and Y-plan configurations. For S-plan, you will need an additional 2-port zone valve (e.g. Honeywell V4043H) for the heating circuit, which is not included. For Y-plan, you will need a mid-position valve (e.g. Honeywell V4073A1039), also not included.
Is this cylinder suitable for use with solid fuel or wood-burning stoves?
No. OSO Slimline HP cylinders are not suitable for use with non-thermostatically controlled heat sources such as solid fuel stoves, wood-burning stoves, or any other heat source that cannot be thermostatically regulated. The cylinder must be connected to a thermostatically controlled heat pump, gas boiler, or oil boiler to comply with unvented cylinder safety regulations.
