The Strom 21kW Electric Combi Boiler (three-phase, model SBTP21C) is a compact, high-efficiency unit that provides both central heating and instant hot water for properties without a gas supply. Running on a 400V three-phase supply, its higher output over the 18kW model suits larger homes and higher simultaneous hot water demand, such as multiple bathrooms or back-to-back showers.
The boiler uses Strom's patented serpentine heat exchanger for rapid, even heat transfer, and includes a full-colour touchscreen interface, integrated pump and expansion vessel, and over 99% efficiency with zero carbon monoxide emissions at the point of use. Ideal for off-grid homes, flats, or properties on Economy 10-style tariffs, it removes the need for a flue, gas connection or separate hot water cylinder entirely.
21 kW
Heat Output
400V 3PH
Power Supply
32 A
Rated Current
99%+
Efficiency
IPX4
Protection Class
2 Yr
Warranty*
Qualified Electrician & Plumber Required
Installation must be carried out by a qualified electrician and plumber in accordance with BS 7671 wiring regulations and current building regulations. An electrical installation certificate and Part P certificate are legally required for the new circuit, and are also a condition of the manufacturer's warranty.
Three-Phase Supply Required
This model requires a three-phase 400V electrical supply. Where only single-phase power is available, Strom's single-phase system and combi boilers (up to 14.4kW) should be considered instead. Actual delivered hot water flow rate also depends on incoming mains water pressure, so this should be checked alongside the electrical supply before ordering.
Correct Sizing Matters
A proper heat loss calculation should be carried out before purchase. Electric boilers are 100% efficient at the point of use, so replacing an older gas or oil boiler like-for-like will typically oversize the new unit, which Part L 2022 of the Building Regulations advises against.
Combi Design, No Cylinder Needed
Heating and instant hot water from one compact unit, removing the need for a separate hot water cylinder and freeing up airing cupboard or plant room space.
Patented Serpentine Heat Exchanger
Heat spreads evenly across the whole element surface rather than at a single hotspot, improving heat-up speed and reducing limescale-related performance loss over time.
Full-Colour Touchscreen Interface
An intuitive touchscreen gives precise control over flow temperature and system status, simplifying day-to-day operation for the end user.
Integrated Pump & Expansion Vessel
The circulation pump and expansion vessel are built in, simplifying installation and reducing the number of separate parts to source and fit.
Over 99% Efficient, Zero Emissions
With no flue and no on-site carbon monoxide, the boiler runs at over 99% efficiency, converting almost all its electrical input directly into usable heat.
Solar & Economy Tariff Compatible
Works alongside solar PV systems and is well suited to Economy 10 and similar time-of-use electricity tariffs to help manage running costs.
General
Model / MPN
SBTP21C
Boiler Type
Electric Combi Boiler (heating & instant hot water)
Heating Output
21 kW
Heating Element
Patented multi-strand serpentine heat exchanger
Efficiency
99%+, zero carbon monoxide at point of use
Protection Class
IPX4
Electrical Supply
Phase
Three-phase
Voltage
400V AC
Rated Current
32 A
Minimum Recommended Breaker
40 A
Minimum Recommended Cable Size
3 x 6mm²
Heating & Hot Water Circuit
Max. Heating Pressure
1.5 bar
DHW Flow Rate Note
Limited by incoming mains supply; three-phase supply recommended for bath filling
Max. Flow Temperature
Suitable for domestic hot water and central heating
Physical & Compliance
Dimensions (H x W x D)
700 x 425 x 260mm
Weight
29.1 kg
Mounting
Wall-mounted, no flue required
Warranty
2-year manufacturer warranty (extendable to 24 months upon registration)
Compatible With
Solar panels, Economy 10 tariffs, most UK domestic heating systems
Does this boiler need a flue or gas connection?
No. The SBTP21C is fully electric with no flue, no gas supply and no carbon monoxide risk at the point of use, making it suitable for off-grid homes, flats, or properties where a flue can't be fitted.
What electrical supply does this boiler need?
A three-phase 400V supply drawing 32A, with a minimum 40A breaker and 3 x 6mm² cable recommended; an electrician should always confirm this against the property's actual supply and load.
Can it also run underfloor heating?
Yes, the boiler is compatible with underfloor heating systems as well as standard radiator central heating circuits.
Is this suitable for a property with multiple bathrooms?
The 21kW output is intended for larger homes with higher simultaneous hot water demand than the 18kW model, such as multiple baths or showers used close together, though delivered flow rate also depends on the property's mains water supply.
Can it work with solar panels?
Yes, it is compatible with solar PV systems and Economy 10-style tariffs to help reduce running costs.
Who can install this boiler?
Installation must be carried out by a qualified electrician and plumber in accordance with BS 7671 and current UK building regulations, with full electrical and plumbing checks completed before use.
