An electric system boiler heats water and circulates it around your central heating circuit — radiators, underfloor heating, or both. Unlike a combi boiler, it does not produce domestic hot water directly from the tap. It works in conjunction with a separate hot water cylinder (either a direct unvented cylinder with its own immersion heater, or an indirect unvented cylinder heated by the boiler itself via a coil). This is an important distinction that catches many buyers out — if you want instant hot water from a single unit with no cylinder, you want an electric combi boiler instead.
System Boiler vs Combi Boiler — Which Do You Need?
The right choice depends on your hot water demand and available space. A combi boiler heats water on demand as it flows through the unit, making it compact and simple — but the flow rate is limited by the boiler's kW output, and it struggles to supply multiple outlets simultaneously. A system boiler with a cylinder stores a larger volume of pre-heated water, meaning you can run multiple showers, baths, and taps at the same time without a drop in performance. For family homes with two or more bathrooms, or any property with higher hot water demand, a system boiler and cylinder combination is almost always the better choice.
What Does a System Boiler Actually Do?
The system boiler's job is to heat the central heating circuit and, if paired with an indirect cylinder, to also heat the stored hot water via a coil inside the cylinder. The boiler itself contains a pump, an expansion vessel, and the heating element — meaning the installation is relatively clean, with fewer components needed in the airing cupboard or plant room compared to an older open-vented system. All of the boilers in this range are sealed system (pressurised) units, which means they work with modern unvented cylinders at mains pressure.
⚠️ You Will Need a Cylinder — This Is Not Optional
A system boiler cannot store or deliver domestic hot water on its own. Every installation requires a compatible hot water cylinder. If you want the boiler to heat both the radiators and the hot water cylinder, you need an indirect unvented cylinder with a coil. If you prefer the cylinder to heat itself independently using its own immersion heater (allowing the boiler to focus purely on heating), a direct unvented cylinder is the alternative. Both approaches are valid — the right choice depends on your system design and whether you want the boiler to manage hot water or not.
Single Phase vs Three Phase — Electrical Supply
As with electric combi boilers, the kW output of a system boiler determines what electrical supply is required. Single-phase models in our range run from 4kW up to 14.4kW, all suitable for a standard 230V domestic supply provided your consumer unit has sufficient capacity and your installer fits a correctly rated dedicated circuit. At 14.4kW single phase, the draw is approximately 63 amps — requiring a 10mm² cable and a 63A or 80A MCB as a minimum.
For higher outputs — 16kW, 18kW, 21kW, 24kW and above — a three-phase 400V supply is required. Three-phase supplies are more common in commercial buildings, newer developments, and rural properties with three-phase grid connections, but are not universally available in domestic settings. Always confirm your supply type with your DNO or electrician before ordering a three-phase model.
For commercial, industrial, or large residential applications, the Elterm Commercial range extends up to 48kW on three phase, making it suitable for office buildings, blocks of flats, and larger heating loads.
Brands in This Collection
Our electric system boiler range includes models from EHC (SlimJim flow boiler and Comet PV with solar PV integration), Strom (single and three phase with touchscreen controls), Heatrae Sadia (Amptec — a long-established name in electric flow boilers), Biasi Pro, E.C.A Arceus, and Elterm (covering both domestic and commercial three-phase applications). All are wall-mounted, require no flue, and are 100% efficient at point of use.
Browse the full range below, or contact our team for help matching the right boiler output and cylinder to your property's heating and hot water requirements.
