An electric system boiler heats water and pushes it around your central heating circuit — radiators, underfloor heating, or both — and pairs with a separate hot water cylinder to deliver mains-pressure hot water to every tap and shower at once. Unlike a combi, it does not heat domestic hot water on demand, which is precisely why it out-performs a combi in any home with more than one bathroom or serious hot water demand.
This guide is written for homeowners looking to switch away from gas or oil, installers speccing systems for clients, builders and developers designing plant rooms for new-builds, and housing associations or smaller landlords replacing older wet systems across their portfolio. We cover single- and three-phase options, the differences between our featured ranges (Strom, EHC, ThermoSphere and more), why you always need a cylinder alongside a system boiler, and which cylinder to pick — with a special note on horizontal models for tight spaces.
What Is An Electric System Boiler?
A system boiler is a wall-mounted, sealed-system (pressurised) heat generator that handles two jobs: it circulates hot water around your radiators or underfloor heating loops, and — when paired with an indirect cylinder — it feeds a coil inside the cylinder to heat your stored domestic hot water. All the pumps, expansion vessels and safety valves are integrated into the boiler casing, keeping the installation tidy in comparison with older open-vented systems that needed loft tanks and multiple external components.
An electric system boiler swaps a gas or oil burner for high-efficiency heating elements. The result: no flue, no combustion, no annual gas safety inspection, no on-site emissions, and 100% electric-to-heat conversion at the point of use. That last point matters — every kilowatt-hour you buy at the meter becomes useful heat inside the home.
Benefits Of An Electric System Boiler
Simultaneous Hot Water To Multiple Outlets
Because water is pre-heated and stored in a cylinder, two showers, a bath and a running tap won't fight each other for flow the way they will on a combi. Ideal for family homes and any property with more than one bathroom.
100% Electric-To-Heat Efficiency
No flue losses, no combustion inefficiency. Every unit of electricity is converted to useful heat. Pair with solar PV, a smart tariff or a battery to lower running costs dramatically over a gas alternative.
No Flue, No Gas Certification
Wall-mount anywhere with a suitable electrical supply — cupboard, utility room, garage, plant room. There's no flue routing to design around and no annual gas safety inspection (Gas Safe) required.
Quiet, Low-Maintenance Operation
Electric elements have very few moving parts compared with combustion boilers. The result: near-silent running, less to go wrong, longer service intervals and lower lifetime maintenance costs.
Off-Gas & Rural-Ready
A straightforward drop-in for rural properties, off-grid sites and any location where a gas connection isn't viable — provided the incoming electrical supply is adequate for the boiler's output.
Compatible With Renewables
Works cleanly alongside solar PV, PV diverters and smart heating controls. Some models (like the EHC Comet PV range) are designed specifically to prioritise solar-generated electricity when it's available.
System Boiler vs Combi Boiler: The Efficiency & Use-Case Difference
People often assume a combi is more efficient because it heats water “only when you need it”. In practice, an electric system boiler paired with a well-insulated modern cylinder often outperforms a combi in real-world usage, because a combi has to work continuously at its peak output every time a tap opens and cannot supply more than one hot outlet at a time. Here's how the two stack up:
| Factor | Electric System Boiler + Cylinder | Electric Combi Boiler |
|---|---|---|
| Simultaneous hot water | ✓ Two+ outlets at full flow | ✗ Flow drops with multiple outlets |
| Bathrooms supported | Two, three or more | Typically one at a time |
| Point-of-use efficiency | 100% electric-to-heat | 100% electric-to-heat |
| Real-world efficiency at peak demand | ✓ Draws off stored heat | Constrained by boiler output rating |
| Space required | Boiler + separate cylinder | ✓ Single wall-mounted unit |
| Solar PV / battery integration | ✓ Cylinder can store surplus | Limited (on-demand heating only) |
| Ideal for | Family homes, 2+ bath properties, larger flats, HMOs | Studios, 1-bed flats, single occupants |
| Reheat time (typical 210L cylinder) | Approx. 90–150 mins | N/A (on-demand) |
| Long-term flexibility | ✓ Add solar diverter, immersion, PV later | Limited — self-contained |
Verdict: A combi wins on wall-space and simplicity in a small flat with one bathroom. Everywhere else — family homes, houses in multiple occupation, developer new-builds, rural properties, anywhere with more than one hot outlet in regular use — the system boiler + cylinder combination is the right choice.
Featured System Boilers From Our Range
Four of our most-specified electric system boilers, chosen because they cover the four most common install scenarios: a mid-power single-phase workhorse for average family homes, a heavy-duty three-phase unit for larger properties and commercial jobs, a slimline space-saving option for tight cupboards, and a compact range-rateable unit that can be dialled to the exact heat loss of a smaller property.
Strom 14.4kW Single Phase Electric System Boiler
Powerful single-phase workhorse for larger family homes on a standard 230V supply. Touchscreen digital controls, integrated pump and expansion vessel, serpentine heating element for smooth modulation. 5-year warranty.
£1,085.00 Inc. VAT (£904.17 Exc. VAT)
Strom 24kW Three Phase Electric System Boiler (400V)
Heavy-duty 24kW system boiler for large houses, multi-flat blocks and commercial applications. Requires a 400V three-phase supply. Serpentine element heat exchanger, no flue required, near-silent operation.
£1,430.00 Inc. VAT (£1,191.67 Exc. VAT)
ThermoSphere Slimline Electric System Boiler With Pump (4.5–24kW)
Ultra-slim (215mm wide) wall-mounted flow boiler with integrated circulation pump. Available from 4.5kW to 24kW, single- or three-phase depending on output. Perfect where cupboard depth is at a premium.
From £1,030.00 Inc. VAT (£858.33 Exc. VAT)
EHS Flex 7kW Range-Rateable Electric System Boiler
Compact 7kW single-phase system boiler that a qualified installer can range-rate anywhere from 2.3kW to 7kW — one unit covering a wide span of property heat losses, cutting short-cycling on smaller homes. Integrated pump and 5L expansion vessel, touchscreen control, built-in frost protection, no flue, no gas. 5-year warranty when registered.
£1,100.00 Inc. VAT (£916.67 Exc. VAT)
Highlighted Ranges: EHC & EHS Electric System Boilers
Two of the UK's most-specified electric heating brands, side by side. EHC (Electric Heating Company) is the deepest range in our system boiler collection with three distinct product lines; EHS (Electric Heating Systems) is a specialist manufacturer whose Primus system boilers are engineered specifically as direct replacements for legacy gas boilers.
EHC (Electric Heating Company)
One of the UK's longest-established electric boiler makers. Two distinct product lines in our range cover almost every domestic and light-commercial requirement:
EHC SlimJim Electric Flow Boilers (4kW–12kW)
The SlimJim is EHC's compact flow boiler range — a genuinely small-footprint electric unit designed for tight cupboards and retrofit applications where wall-space is limited. Available in 4kW, 7kW, 10kW and 12kW outputs, it's ideal for small-to-medium homes, top-floor flats, garden rooms and annexes. Pair with an indirect unvented cylinder for domestic hot water.
EHC Comet System Electric Boilers (2–24kW, Single & Three Phase)
The Comet is EHC's flagship stepped-output system boiler with a 5-year warranty. Multiple output stages let the boiler modulate finely from as little as 2kW up to 8kW, 14.4kW or 24kW depending on the model chosen — smoothing out electrical demand and improving comfort. Three-phase variants (400V) extend the range up to 24kW for larger homes and commercial installs.
EHC Comet PV — Built For Solar PV Integration
The Comet PV is a purpose-designed variant of the Comet range with built-in intelligence to prioritise solar-generated electricity when it's available, reducing grid draw and maximising the value of your PV system. Available in single-phase (8kW & 14.4kW) and three-phase (24kW) configurations. If you have — or plan to add — solar panels, this is the boiler that lets your central heating benefit from the sun.
Browse The Complete EHC Electric System Boiler Lineup
Ten SlimJim, Comet and Comet PV models covering 4kW single-phase up to 24kW three-phase, with dedicated solar-PV variants for renewables integration. Every model includes a 5-year warranty on registration.
EHC Comet PV — Solar-Ready System Boiler
Multi-output boiler with dedicated solar PV integration. Prioritises free solar-generated electricity when available, cutting grid draw and improving the payback on your PV system. Single- and three-phase models.
From £1,300.00 Inc. VAT (£1,083.33 Exc. VAT)
EHS Primus & Flex Electric System Boilers (7kW & 14kW)
EHS designs its Primus S range specifically as a drop-in replacement for existing gas system boilers. The idea is simple: use the same radiators, the same pipework and the same wall position — swap the gas boiler for the EHS unit, wire it in, and you've decarbonised your heating without ripping the property apart. Two single-phase options cover most of the domestic market: the Flex 7kW is range-rateable from 2.3kW up to 7kW so a qualified installer can dial the output to the exact heat loss of a smaller-to-medium property, and the Primus 14kW covers larger multi-bathroom homes. Both use a touchscreen interface, integrated pump and expansion vessel, and pre-wired chassis to keep install time down. Combine either with an unvented indirect cylinder for mains-pressure hot water throughout the property, and add a smart or off-peak timer to heat the store on cheaper night-rate electricity where available.
EHS Flex 7kW — Range-Rateable Compact System Boiler
Single-phase 230V system boiler that a qualified installer can range-rate anywhere from 2.3kW to 7kW via the EHS portal — matching output to the actual heat loss of the property and cutting short-cycling on smaller homes. Integrated pump, 5L expansion vessel, touchscreen with boiler lock. Compact 585×380×235mm, 19kg. No flue, no gas. 5-year warranty when registered.
£1,100.00 Inc. VAT (£916.67 Exc. VAT)
EHS Primus 14kW — For Multi-Bathroom Homes
The bigger sibling — 14kW single-phase output for larger properties with higher heating demand and multiple bathrooms. Same touchscreen, same integrated pump and vessel, same direct-swap approach. 5-year warranty.
£1,170.00 Inc. VAT (£975.00 Exc. VAT, sale price)
Three-Phase Electric System Boilers Explained
The single-most-important thing to check before ordering an electric boiler over about 15kW is your incoming electrical supply. Standard UK domestic properties are on a single-phase 230V supply, which supports electric boilers up to roughly 14.4kW (drawing about 62A on a 63A breaker with 10mm² cable). Anything above that typically requires a three-phase 400V supply.
Where three-phase is available:
- Newer developments & new-builds — many modern builds are wired three-phase as standard
- Rural properties with dedicated overhead lines from the DNO
- Commercial buildings, offices & blocks of flats where higher aggregate demand is normal
- Farms, workshops & light industrial units already equipped for machinery loads
- Housing association large-format properties and older estates that were originally wired for storage heating
Three-phase models in our range include:
- Strom SBTP18S, SBTP21S, SBTP24S — 18kW, 21kW, 24kW at 400V with serpentine element heat exchanger
- EHC Comet 3-Phase (EHC.CS24KW) — Multi-output 12/16/20/24kW three-phase system boiler with 5-year warranty
- EHC Comet PV 3-Phase (EHCEPV24KW) — 12/16/20/24kW three-phase with solar PV integration and app control
- ThermoSphere TSEFB-P18 & TSEFB-P24 — Slimline three-phase-only variants
- Biasi Pro 16kW Three Phase (BIPROELEC16S) — Compact commercial-grade three-phase system boiler
- Elterm Battalion Commercial (30–48kW) — Heavy-duty commercial range for larger buildings, offices, blocks of flats
⚠️ Always confirm your supply type with your DNO or a qualified electrician before specifying a three-phase model. Retrofitting a three-phase supply where none exists is a significant civils job.
You'll Need A Cylinder — Here's Why
A system boiler cannot store or deliver domestic hot water on its own. This is not optional: every system boiler installation is paired with a hot water cylinder that stores pre-heated water at mains pressure, ready to run showers, baths and taps on demand. Which cylinder you choose depends on how you want the boiler to interact with it.
Indirect Unvented Cylinder
Boiler heats the cylinder via a coil. The most common configuration — the system boiler pumps hot water through a heat exchanger coil inside the cylinder, warming the stored water without mixing the two circuits. Best for anyone wanting the boiler to manage both space heating and hot water.
Direct Unvented Cylinder
Cylinder heats itself via its own immersion element. The boiler focuses purely on radiators and underfloor heating; the cylinder is essentially a large mains-pressure water heater with its own thermostat and 3kW element. Good for splitting loads or running hot water on a cheaper off-peak tariff.
Featured Cylinder Range: Indirect & Direct Unvented
Our unvented cylinder range spans compact 90L units for small properties up to 500L high-flow tanks for larger family homes and commercial applications. Below are the models we recommend most often — indirect models (where the system boiler heats the store via a coil) and direct models (where the cylinder heats itself with built-in immersion elements, decoupled from the boiler). We also feature a pre-plumbed OSO 210L direct cylinder — a compelling standalone hot water solution for properties where the system boiler is dedicated purely to space heating.
Featured Direct Cylinder: OSO Super Xpress S2X 210L (Pre-Plumbed)
OSO Super Xpress S2X 210L Direct Unvented Cylinder — Pre-Plumbed
Direct electric unvented cylinder engineered to deliver up to 30% more usable hot water than comparable competitors of the same nominal volume, thanks to OSO's patented S-DESIGN and 75°C stratification. Twin 3+3 kW titanium immersion heaters support Economy 7/10 off-peak tariffs. The entire G3 kit — expansion vessels, valves, tundish, drain cock — is factory-fitted, saving up to 70% of on-site plumbing time. Ultraweld Duplex stainless steel construction. 25-year cylinder warranty — one of the best on the UK market. Washing-machine footprint (1375mm H × 595mm Ø) fits utility rooms and airing cupboards.
£1,170.00 Inc. VAT (£975.00 Exc. VAT)
Indirect & Direct Cylinder Range
The full recommended list — indirect cylinders for pairing directly with a system boiler, direct cylinders for standalone electric hot water, plus the horizontal specials for tight installs.
Browse All Indirect Cylinders Browse All Direct Cylinders
Specifying System Boilers For A Client, Site Or Portfolio?
If you're an installer, builder, property developer or housing association manager working on multiple properties, we support trade buyers with volume pricing, staged deliveries, technical support and package deals that combine the boiler, cylinder, thermostats, valves and expansion vessels in one order.
What we can do for the trade:
- Package pricing — matched boiler + cylinder + controls + kit bundles
- Volume discounts for multi-unit orders across a portfolio or development
- Staged deliveries aligned to a build programme, so units arrive when the site is ready
- Technical support on sizing, phase requirements, cylinder capacity and Part L compliance
- Same-day & next-day delivery options for UK mainland on most orders
- Custom quotes on any project — no obligation, quick turnaround
Need Help Picking The Right Boiler & Cylinder Combination?
Our expert team can size your system, confirm your electrical supply requirements and quote a full package — boiler, cylinder, thermostats and all — in one go.