If you're moving away from a gas or oil boiler — whether for a new-build flat, a rural cottage off the mains gas grid, a small commercial kitchen or a whole-house electrification project — the ThermoSphere electric boiler range covers almost every UK application from a single hand-basin up to a light commercial heating system. This guide compares the four ThermoSphere models sold at AIZO Quality Heating — the Micro, Flex Combi, Heat-Only and System boilers — so you can size the right unit for your property first time.
All four are British-made, IP-rated wall-mounted units with no flue, no gas connection and no annual gas safety inspection. What separates them is what they're designed to do: instant hot water at a single outlet, both heating and DHW from one small box, central heating on an open-vented system, or a fully pressurised system ready for a large indirect cylinder. Get that decision right and installation is straightforward. Get it wrong and the plumbing costs more than the boiler.
Which ThermoSphere Boiler Should You Fit?
Before comparing specifications, it helps to match each boiler to the job it does best. Below is the quick decision framework — use it to shortlist one or two models, then read the detailed sections that follow.
ThermoSphere Micro (TSMEB)
A4-sized instant water heater for a single outlet or single wet UFH zone. Ideal for small commercial kitchens, restaurant hand-wash stations, care home outlet temperature limiting, or a retrofit single-room underfloor heating circuit where a cylinder won't fit. No storage, no scale, no expansion vessel.
ThermoSphere Flex Combi (TSFLEX14)
A pre-wired, pre-plumbed electric combi boiler for smaller homes and flats with one shower and no bath. Adjustable output 4–14kW so it can be tuned to the property's heat loss. Direct gas-combi replacement with no flue required — installs in a kitchen cupboard at just 30 dB.
ThermoSphere Heat-Only (TSEFB)
For open-vented, gravity-fed systems with a feed-and-expansion tank in the loft. The right choice when you're replacing a regular gas or oil boiler on an older system and don't want to convert to pressurised. Pairs with an indirect cylinder for hot water.
ThermoSphere System (TSEFB-P)
A sealed-system boiler with built-in pump and expansion vessel — the widest output range at 4.5–24kW, single and three-phase. Best fit for larger homes, small commercial premises and any property with a bath fed from an indirect unvented cylinder. Includes multi-stage element modulation for efficient part-load operation.
Why electric boilers make sense in the UK — now
The Future Homes Standard has effectively ended new gas boiler installations in new-build homes, and the direction of travel is clear: electrified heating is where UK heat is going. Electric boilers are the simplest, lowest-capex electrification route — there is no outdoor unit, no refrigerant handling, no defrost cycle to manage in winter, and no flue to route. They convert ~100% of the electricity drawn into useful heat at the point of use, run silently, and pair well with solar PV, battery storage and green-electricity tariffs.
The catch has always been running cost per kWh compared with gas. That's why the sizing decisions in this guide matter: a right-sized electric boiler paired with proper insulation, weather-compensating controls and (ideally) a solar array is a very different economic proposition from an oversized unit running full-tilt on peak-rate electricity.
- No flue, no gas connection, no annual gas safety inspection
- Zero on-site combustion emissions and near-silent operation
- Compatible with solar PV, immersion diverters and Economy 7
- Compact wall-mount format — fits kitchen cupboards and utility rooms
- British-designed, British-manufactured with UK installer support
- 5-year warranty as standard; 7 years on the Flex Combi with install kit
1. ThermoSphere Micro Electric Boiler — Instant Hot Water for Compact Applications
The Micro Electric Boiler (MEB) is the outlier in the ThermoSphere range: it isn't a central heating boiler in the traditional sense. It's an instantaneous water heater built around a patented direct energy transfer system — no resistive element, no cylinder, no expansion vessel — that delivers a continuous stream of water at up to 60°C. At just 315×210×120mm, it occupies the wall footprint of a sheet of A4 paper.
Five outputs are available — TSMEB-4 through TSMEB-9 (4.8–9.6kW) — and two units can be paralleled in tandem for combined flows up to 11 l/min. What makes the Micro genuinely useful is its flexibility: because it fires directly into whatever pipework it's plumbed to, the same unit can handle domestic hot water at a tap, act as the flow-temperature source for a single wet UFH zone, or serve as an electric top-up downstream of a heat pump or solar thermal circuit.
Where the Micro genuinely shines
Small commercial kitchens & restaurants
Café pot-wash sinks, coffee-shop hand basins, food-truck plumbing, small restaurant back-of-house. The Micro fits under a counter, delivers instant hot water at up to 5.5 l/min and eliminates the standby losses of a stored cylinder. No gas connection means no Gas Safe sign-off on the water heater.
Retrofit single-zone underfloor heating
A common problem in bathroom or extension UFH retrofits: no space for a manifold, buffer tank and boiler feed. The Micro delivers water at the exact flow temperature required for the UFH loop — no mixing valve, no buffer, no cylinder.
Care homes & assisted living
The digital outlet control locks maximum temperature at 43°C — the standard limit for care home outlets — without relying on a TMV. Precise, tamper-resistant temperature limiting for every sink and shower fed from the unit.
Solar thermal & heat pump top-up
Set your air source heat pump at its optimum flow temperature for COP; use the Micro downstream to close any temperature shortfall on cold days. The Micro only draws power when actual top-up is required.
2. ThermoSphere Flex Combi — Heating and Hot Water From One Wall-Mounted Unit
The Flex Electric Combi Boiler (TSFLEX14) is the closest thing to a direct gas-combi replacement in the ThermoSphere range. It's a self-contained unit providing both central heating and instantaneous domestic hot water from one 585×380×235mm wall box — no flue, no gas, no separate cylinder, no separate expansion vessel.
The “Flex” name refers to its defining feature: digitally adjustable output on both circuits. The heating circuit runs 4–14kW; the DHW circuit 10–14kW. Both can be set independently to the calculated heat loss of the property, which matters because oversized boilers cycle constantly, wasting energy and shortening component life. The unit arrives pre-wired with 10mm² supply cable and pre-plumbed with an integrated pump and 5-litre expansion vessel, which cuts on-site labour significantly compared with a system boiler build-up.
Best-suited properties
The Flex Combi is designed for smaller homes and flats with one bathroom, one shower and no bath. At full 14kW output the DHW circuit delivers approximately 6 l/min at a 30°C temperature rise — comfortable for a shower, but slow for filling a bath. For bath-equipped properties, step up to the System boiler paired with an indirect cylinder.
Off-grid rural cottages
Direct replacement for LPG combi boilers in rural properties without mains gas. Zero on-site emissions and no ongoing LPG deliveries. Pairs well with a solar PV package for load offset.
Flats and apartments
No external flue penetration, no gas riser, no shared-service risk assessment. Installs in a kitchen cupboard or utility void. At 30 dB it can be sited on a bedroom-adjoining wall without noise issues.
New-build homes under Future Homes Standard
A compliant, low-capex electrified heating option for small new-build units where a heat pump is over-specified for the load. Compatible with radiators, wet UFH and skirting heating; supports volt-free external controls.
Gas boiler replacement on existing pipework
Reuses the existing radiator circuit. No flue, no gas — just a dedicated 63A single-phase electrical supply with 10mm² cable. Registered installation with the ThermoSphere Installation Kit extends the warranty from 5 to 7 years.
3. ThermoSphere Heat-Only Boiler — For Open-Vented Central Heating Systems
The Electric Heat-Only Boiler (TSEFB) is a slimline wall-mounted electric boiler built around a cluster of resistive heating elements inside an insulated steel cylinder. Efficiency is quoted at 99.8% with zero flue gas emissions. Cold water enters the base, heated water leaves via the top flow connection at up to 65°C, and the entire assembly measures just 220×575×115mm and weighs 13kg regardless of the output selected.
Five outputs are available — TSEFB-4.5, TSEFB-6, TSEFB-9, TSEFB-12 (all switchable single-phase 230V or three-phase 400V) and TSEFB-15 (three-phase 400V only). This is the right ThermoSphere model when you're replacing a regular gas or oil boiler on an open-vented, gravity-fed heating system — the type with a small feed-and-expansion tank in the loft.
Heat-only vs system: how to check which you have
Look in the loft. If there are one or two small cold-water tanks feeding the heating circuit, you have an open-vented system and the Heat-Only is the right boiler. If there are no tanks and the heating has a pressure gauge and filling loop on the boiler itself, the system is already sealed and pressurised — go to the System boiler in section 4.
Conversion from open-vented to sealed is possible at the same time as boiler replacement, but it adds significant cost and complexity: additional pipework, system flushing, pressure test and typically the removal of the loft tanks. For a straight like-for-like boiler swap, keeping the open-vented configuration and fitting the Heat-Only is much simpler.
Traditional radiator systems
Works directly with your existing radiators, programmers, thermostats and thermostatic radiator valves. If your F&E tank in the loft has been serving the property for years, the pipework is already set up for this style of boiler.
Wet underfloor heating on open-vented systems
The 65°C max flow suits both radiator and UFH circuit temperatures. Compatible with wet UFH manifolds and TRVs from the standard heating controls range.
Off-grid properties keeping legacy pipework
A cost-effective, low-disruption route to remove oil or LPG from an older rural property without re-piping. Pairs with an existing indirect cylinder for hot water via the coil.
Flats where flue routing is impossible
For older converted flats where an external flue can't be routed but the heating system already uses a header tank arrangement, the Heat-Only is often the only compliant replacement option.
4. ThermoSphere System Boiler — The Widest Output Range With Built-In Pump
The Electric System Boiler with Pump (TSEFB-P) is the workhorse of the ThermoSphere range and the model that scales furthest. Seven outputs are offered — from 4.5kW right up to 24kW — and it includes an integrated circulation pump and expansion vessel, ready for direct connection to a sealed pressurised heating circuit and, via a coil, an indirect unvented hot water cylinder for high-flow domestic hot water.
Its 3-stage heating element configuration provides multi-step modulation, so part-load operation matches actual demand rather than cycling a single element hard. Single-phase 230V versions cover 4.5–12kW; the 15kW, 18kW and 24kW models are three-phase 400V only and are the appropriate spec for larger properties or light commercial premises. Compliance with ErP Lot 20 and UK electrical installation standards is stated on the datasheet.
Why it's the flexible middle-to-large-home choice
Because it drives a sealed pressurised system, the System boiler pairs with any modern indirect unvented cylinder — which is what delivers proper mains-pressure hot water and rapid bath filling. That's the key limitation of the Flex Combi resolved. For any property with more than one shower, more than one bathroom, or a bath the family actually uses, this is the right ThermoSphere model.
Family homes with baths
Paired with an indirect unvented cylinder, this configuration fills a bath in the time you'd expect from a modern gas system boiler. The cylinder handles the DHW flow rate; the boiler handles the space heating.
Larger heat loads, larger properties
The 15, 18 and 24kW three-phase models cover heat loads a single-phase electric boiler can't. Standard use case: rural five-bedroom off-gas properties, small hotels, converted farmhouses.
Multi-zone wet underfloor heating
The built-in pump drives multi-zone manifold-based UFH systems across large floor plates without a separate secondary circulator, and the 65°C max flow suits both UFH and radiator circuits.
Light commercial & small offices
The 24kW three-phase variant handles small workshops, converted units, dental practices and similar SME premises where a heat pump install is impractical but a fossil-fuel boiler is off the table.
Side-by-Side Comparison: All Four ThermoSphere Boilers
Use this table to compare the four models at a glance. Prices shown are the starting variant on qualityheating.co.uk at time of publication and include VAT.
| Model | Purpose | Output Range | Best For | Warranty | From |
|---|---|---|---|---|---|
| Micro (TSMEB) | Instant hot water / UFH source | 4.8–9.6kW single (up to 11 l/min tandem) | Commercial kitchens, single UFH zone, care home outlets | 5 years | £570 |
| Flex Combi (TSFLEX14) | Heating + DHW (combi) | 4–14kW heating / 10–14kW DHW | Flats, small homes, single-shower properties | 5 yr (7 yr with kit) | £997 |
| Heat-Only (TSEFB) | Central heating — open-vented | 4.5–15kW single / three phase | Legacy F&E tank systems, gas regular replacements | 5 years | £730 |
| System (TSEFB-P) | Central heating — sealed pressurised | 4.5–24kW single / three phase | Larger family homes, baths, multi-zone UFH, light commercial | Up to 5 years | £1,130 |
Buy the ThermoSphere Range
ThermoSphere
Micro Electric Boiler — Instant Hot Water, 4.8–9.6kW
A4-sized wall-mounted instantaneous heater. No cylinder, no scale, no expansion vessel. Ideal for commercial kitchens, single-zone UFH, care home outlet limiting and solar/heat pump top-up.
£570.00 Inc. VAT (£475.00 Exc.)
ThermoSphere
Flex Electric Combi Boiler 4–14kW (TSFLEX14)
Pre-wired, pre-plumbed electric combi with digitally adjustable output. Heating + instant DHW from a single 585mm-tall unit. No flue, no gas — 30 dB, 7-yr warranty with install kit.
£997.00 Inc. VAT (£830.83 Exc.)
ThermoSphere
Electric Heat Only Boiler 4.5–15kW (TSEFB)
99.8% efficient resistive-element boiler for open-vented, gravity-fed systems. 220×575×115mm wall footprint, 65°C max flow. The right answer for regular gas boiler replacement without pipework conversion.
From £730.00 Inc. VAT (£608.33 Exc.)
ThermoSphere
Electric System Boiler with Pump 4.5–24kW (TSEFB-P)
Sealed-system boiler with built-in pump and expansion vessel. 3-stage element modulation, single or three-phase, up to 24kW. The right choice for family homes, baths, multi-zone UFH and light commercial.
From £1,130.00 Inc. VAT (£941.67 Exc.)
Full Technical Specification
Full electrical and hydraulic specifications for each ThermoSphere variant. All models are wall-mounted, indoor-installed, and require a dedicated protected circuit sized to the maximum current draw. Confirm your consumer unit's spare capacity with a qualified electrician before ordering.
| Model / SKU | Output | Voltage | Current | Dimensions (H×W×D) | Max Temp | Max Pressure |
|---|---|---|---|---|---|---|
| TSMEB-4 (Micro) | 4.8kW | 240V 1Φ | 20A | 315×210×120mm | 60°C | 10 bar |
| TSMEB-6 (Micro) | 6.0kW | 240V 1Φ | 25A | 315×210×120mm | 60°C | 10 bar |
| TSMEB-9 (Micro) | 9.6kW | 240V 1Φ | 40A | 315×210×120mm | 60°C | 10 bar |
| TSFLEX14 (Combi) | 4–14kW | 230V 1Φ | 63A | 585×380×235mm | 80°C | 1.5 bar |
| TSEFB-4.5 (Heat-Only) | 4.5kW | 230/400V | 21A | 575×220×115mm | 65°C | 2.0 bar |
| TSEFB-9 (Heat-Only) | 9.0kW | 230/400V | 40A | 575×220×115mm | 65°C | 2.0 bar |
| TSEFB-15 (Heat-Only) | 15.0kW | 400V 3Φ | 22A / phase | 575×220×115mm | 65°C | 2.0 bar |
| TSEFB-P4.5 (System) | 4.5kW | 230/400V | ~21A | 755×215×150mm | 65°C | 2.0 bar |
| TSEFB-P12 (System) | 12.0kW | 230/400V | ~53A | 755×215×150mm | 65°C | 2.0 bar |
| TSEFB-P24 (System) | 24.0kW | 400V 3Φ | ~35A / phase | 755×215×150mm | 65°C | 2.0 bar |
Frequently Asked Questions
Do I need a three-phase electricity supply for an electric boiler?
What's the difference between the Heat-Only and the System boiler?
Can the Flex Combi fill a bath?
Do electric boilers need an annual service like gas boilers?
Can I use the Micro Boiler for underfloor heating?
Can I run a ThermoSphere boiler from solar PV?
How does the ThermoSphere warranty work?
Not sure which ThermoSphere model is right for you?
Our team can size the right output, confirm your electrical supply is adequate, and put together a full boiler package with cylinder, filter and controls where needed. Free UK Mainland delivery on most orders and Klarna finance available at checkout.