The Vaillant uniSTOR is a high-grade unvented hot water cylinder built for domestic properties on a sealed or open-vented system with a boiler or heat pump. Unlike a vented cylinder that relies on a cold water storage tank and gravity, the uniSTOR connects directly to the rising mains, storing and supplying hot water at full mains pressure — delivering a powerful, balanced flow at taps, power showers, and bath fillers simultaneously. The cylinder is constructed from Duplex stainless steel — significantly more corrosion-resistant and stronger than standard austenitic stainless — with thermal-injected polyurethane insulation keeping standby heat losses among the lowest in class. Available in five capacities: 150, 180, 200, 250 and 300 litres, covering 1-bedroom flats through to 5-bedroom family homes. The uniSTOR is compatible with gas, oil, LPG, and electric system boilers, and with air-source and ground-source heat pumps. 2-year guarantee on parts and labour; 25-year guarantee on the cylinder shell.
⚠ G3 / Unvented Qualified Installation Required — Unvented hot water cylinders must be installed by a competent person holding a current G3 (Unvented Hot Water Storage) qualification, in accordance with Building Regulations Part G3 and the manufacturer's installation instructions. The system must be notified to Building Control. Incorrectly installed unvented cylinders present a serious risk of scalding and pressure vessel failure. The Vaillant uniSTOR is supplied with an integrated unvented cylinder kit (tundish, T&P expansion relief valve, expansion vessel, and motorised valve), but correct installation, commissioning, and annual servicing remain the responsibility of a G3 registered installer.
Choosing the right capacity — guide by property size
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150L — 1–2 bedrooms, 1 bathroom, low demand (1–2 occupants)
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180L — 2–3 bedrooms, 1 bathroom (2–3 occupants)
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200L — 3 bedrooms, 1–2 bathrooms (3–4 occupants)
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250L — 3–4 bedrooms, 2 bathrooms (4–5 occupants)
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300L — 4–5 bedrooms, 2–3 bathrooms, high simultaneous demand
When using a heat pump as the primary heat source, err toward the larger capacity for the bedroom count — heat pumps heat more slowly than gas boilers, so more stored volume compensates for longer recovery times.
Mains pressure
Full-flow at all outlets
Duplex SS
25-yr shell guarantee
Features & Benefits
✔ Duplex Stainless Steel — 25-Year Shell Guarantee
The uniSTOR tank is manufactured from Duplex stainless steel — a two-phase microstructure combining austenitic and ferritic stainless that delivers roughly twice the strength and significantly better chloride corrosion resistance compared to the standard 304-grade stainless used in many competitor cylinders. This means the tank does not require a sacrificial anode (unlike copper or mild steel cylinders) and is not susceptible to the pitting corrosion that can occur in standard stainless in hard-water areas. The 25-year shell guarantee reflects Vaillant's confidence in the tank's longevity.
✔ Full Mains Pressure — All Outlets Simultaneously
Being an unvented cylinder, the uniSTOR stores hot water under full mains supply pressure rather than relying on a header tank. The result is powerful, balanced pressure at every hot water outlet — bath fillers, thermostatic bar showers, and kitchen mixer taps all run at the same pressure as the cold water supply. There is no risk of the flow dropping to a trickle when someone turns on a tap downstairs, and no need for a separate pump to boost shower pressure — particularly valuable in multi-bathroom properties where simultaneous demand is common.
✔ Large Smooth-Bore Coil — Heat Pump Optimised
The large diameter, smooth-bore coil heat exchanger provides the surface area needed for efficient heat transfer from low-temperature primary circuits — particularly important when the heat source is a heat pump running at 45–55°C flow temperatures rather than a boiler at 70–80°C. The smooth bore also reduces the rate of limescale accumulation and aids passive descaling during temperature cycling. For heat pump installations, the 250L and 300L cylinders have an increased coil size that extends compatibility to a wider range of ground source heat pump systems.
✔ Thermal-Injected Insulation — ErP Band B
Thermal-injected polyurethane insulation — applied under pressure to completely fill the void between the inner tank and outer shell — delivers some of the lowest standby heat losses in its class, as low as 1kWh per 24 hours on the 150L model. This is significantly better than a traditional copper cylinder with a foam jacket, which can lose 2–3kWh per day. Achieving ErP Band B across the range means the uniSTOR meets modern energy efficiency standards and reduces the cost of maintaining stored water at temperature between heating cycles.
✔ Unvented Kit Included — Ready to Commission
The standard uniSTOR is supplied with all the mandatory unvented system components pre-packaged: tundish, temperature and pressure expansion relief valve (T&P ERV), expansion vessel, three-way motorised valve, and ¾" × 28mm MI-copper compression fittings. The 250L and 300L also include a secondary return T-piece connection for recirculating systems. This means the G3 installer has every required safety component in one delivery — there is no need to source a separate unvented kit, reducing the risk of specifying incompatible components and simplifying the installation quote.
✔ Factory-Fitted 3kW Immersion Heater Backup
A factory-fitted 3kW immersion heater provides emergency hot water backup in the event of a primary heat source failure — if the boiler breaks down or the heat pump requires servicing, the immersion heater can maintain domestic hot water supply independently. The immersion heater also enables Legionella thermal disinfection cycling (periodic heating to 60°C+) as required by HSE L8 guidance, without relying on the primary heat source. Wired to standard 13A connection via the 1¼" BSP boss.
✔ Boiler & Heat Pump Compatible
The uniSTOR is compatible with all sealed system and open-vented boilers (gas, oil, LPG, and electric) and with Vaillant aroTHERM, flexoTHERM, and geoTHERM heat pumps, as well as other ASHP and GSHP installations via the standard ¾" coil connections. This makes it a future-proof choice for properties considering decarbonisation — the same cylinder can remain in use if the primary heat source is later changed from a gas boiler to a heat pump, provided the appropriate coil capacity is selected at the time of initial installation.
✔ Adjustable Feet — Thermodynamic Sensor Pocket
Adjustable levelling feet allow the cylinder to be installed on uneven floor surfaces without shimming. A thermodynamically positioned dry sensor pocket (8mm diameter sleeve) allows precise temperature sensing at the point where accurate DHW temperature measurement has the greatest system efficiency impact — enabling the connected controller or heat pump to correctly identify when the cylinder has reached setpoint and stop primary heating before overshooting. Both features are small details that meaningfully affect both installation ease and long-term performance.
Dimensions & Key Data by Capacity
| Capacity |
Height × Width |
Standby loss (kWh/24h) |
Expansion vessel |
ErP band |
| 150L |
1,060 × 580mm |
~1.0 |
12L |
B |
| 180L |
1,190 × 580mm |
~1.1 |
18L |
B |
| 200L |
1,310 × 580mm |
~1.2 |
18L |
B |
| 250L |
1,540 × 650mm |
~1.4 |
25L |
B |
| 300L |
1,740 × 650mm |
~1.6 |
25L |
B |
Technical Specifications — All Sizes
| Manufacturer |
Vaillant |
| Range |
uniSTOR — unvented indirect hot water cylinder |
| Available capacities |
150L · 180L · 200L · 250L · 300L |
| Tank material |
Duplex stainless steel |
| Insulation |
Thermal-injected polyurethane — GWP <5, ozone depletion 0 |
| Max. DHW temperature |
85°C |
| Max. primary circuit temperature |
95°C |
| Primary coil connections |
¾" BSP female (flow and return) |
| Cold water / hot water connections |
¾" BSP female |
| Sensor sleeve |
8mm diameter dry pocket |
| Immersion heater |
3kW factory-fitted — 13A, 1.5mm² cable — 1¼" BSP boss |
| Supplied with (standard) |
Tundish · T&P expansion relief valve · expansion vessel · three-way motorised valve · ¾" × 28mm MI-copper compression fittings · documentation. 250L & 300L also include secondary return T-piece |
| Compatible heat sources |
System and regular boilers (gas, oil, LPG, electric) · air source heat pumps · ground source heat pumps |
| Energy-related Products band |
B (all sizes) |
| Cylinder shell guarantee |
25 years |
| Parts & labour guarantee |
2 years |
