The Strom 21kW Electric System Boiler (three-phase, model SBTP21S) sits at the top end of Strom's mid-range output, built for large homes and commercial buildings with high-demand heating needs. It runs central heating only, paired with a vented or unvented indirect hot water cylinder, and uses Strom's patented serpentine heat exchanger to spread heat evenly across the whole element surface rather than concentrating it at a single hotspot-prone heating rod.
Featuring an integrated pump and expansion vessel, a full-colour touchscreen control panel, and zero on-site carbon monoxide emissions, the SBTP21S needs no flue and is a future-proof choice for electric central heating. At 99%+ efficiency with self-modulating output down to as little as 1kW, it is designed to run near-silently and cost-effectively on Economy 10 and similar time-of-use electricity tariffs.
21 kW
Heating Output
400V 3PH
Power Supply
32 A
Rated Current
99%+
Efficiency
15 dB
Sound Pressure
5 Yr
Warranty*
Qualified Electrician Required
This boiler must be installed by a qualified, competent electrician registered with a competent persons scheme, in accordance with BS 7671 (IEE Wiring Regulations). An electrical installation certificate and a Part P certificate are legally required for any new boiler circuit, and this documentation is also a condition of the manufacturer's warranty.
Correct Sizing Is Essential
Strom advise that a proper heat loss calculation is carried out before purchase. Electric boilers are 100% efficient at the point of use, so a straight like-for-like swap with an older gas or oil boiler will typically oversize the unit. Part L 2022 of the Building Regulations requires that boilers are not significantly oversized for the property.
System Boiler, Not Combi
This is a heating-only system boiler: hot water is provided separately via a vented or unvented indirect hot water cylinder connected to the boiler's heating circuit, not by the boiler itself. If instant combination hot water and heating from one unit is needed, the equivalent Strom Combi Boiler range should be considered instead.
Patented Serpentine Heat Exchanger
Heat is transferred evenly across the whole surface area of the element rather than at isolated hot points, speeding up heat-up time and reducing limescale build-up that would otherwise reduce performance over time.
Multiple Independent Heating Strands
Where many competing boilers rely on a single heating strand for all their output, Strom's element uses multiple strands, each independently controlled, spreading electrical stress and enabling self-modulation down to as little as 1kW.
Water-Isolated Heating Element
The heating strands are kept physically separate from the system water, sharply reducing the corrosion that shortens the life of conventional immersion-style heating elements.
Single-Unit Replaceable Element
Unlike competitor designs that bury an element inside a small internal tank, Strom's heat exchanger is a single replaceable unit, simplifying any future servicing.
Integrated Pump & Expansion Vessel
A circulation pump and expansion vessel are built into the boiler casing, reducing the number of separate components an installer needs to source, fit and commission.
Touchscreen Control With LED Display
A full touchscreen control panel displays live flow temperature, clock, and system status, with clear icons for ignition, pump running, frost protection and thermostat demand.
Wall-mounted, integrated bracket, upright position only
ErP Rating (Heating)
D
Compatible Hot Water
Vented or unvented indirect cylinder
Compatible With
Solar panel diverters, underfloor heating, Economy 10 tariffs
Does this boiler provide hot water as well as heating?
No. The SBTP21S is a system boiler for central heating only. Hot water must be provided by connecting a vented or unvented indirect hot water cylinder to the boiler's heating circuit.
Do I need a flue for this boiler?
No. As an all-electric boiler, the SBTP21S produces zero carbon monoxide at the point of use and requires no flue, chimney or external venting of any kind.
What size breaker and cable does this boiler need?
Strom's specification calls for a 32A rated current, a minimum 40A breaker and a minimum 3 x 6mm² cable. An electrician should always confirm the correct breaker and cable size on site, since supply voltage can vary property to property.
How does this compare to the 24kW model?
The SBTP21S sits just below the range-topping 24kW SBTP24S. Choosing between them comes down to the property's calculated heat loss; Strom recommend against oversizing, so the smaller of two suitable outputs is normally the better choice.
Is professional installation required?
Yes. Installation must be carried out by a qualified, competent electrician in line with BS 7671 wiring regulations, with an electrical installation certificate and Part P certificate provided as a condition of both building regulations compliance and the manufacturer's warranty.
What warranty does this boiler come with?
Strom boilers are supplied with an out-of-box warranty that can be extended by completing product registration within 45 days of purchase; warranty duration can vary by model, supplier or promotional package, so the registration documentation supplied with the unit should be checked for exact terms.
