The complete guide for homeowners and landlords choosing an electric boiler for underfloor heating and hot water
If you're planning underfloor heating (UFH), hot water for a rental property, or a full central heating system for a home without mains gas, an electric boiler is one of the most practical, low-maintenance and future-proof solutions on the market. There's no flue to install, no gas supply to arrange, no annual gas safety inspection, and every kWh you put in comes back out as heat.
But choosing between an electric combi boiler, an electric heat-only boiler, or an instant electric water heater is not always obvious — and getting the sizing wrong is the single most common cause of buyer's regret. This guide walks homeowners and landlords through every option, gives you an interactive tool to work out the correct kW rating for your property, and covers niche applications from single-room UFH loops to caravan hot water.
Combi vs Heat Only vs Instant Water Heater — which do you need?
Before you look at kW ratings, you need to decide which type of electric appliance actually fits your setup. The three product families overlap in some ways and are completely different in others. Here's the plain-English breakdown.
Type 1
Electric Combi Boiler
- Heats your radiators or UFH loop
- Heats hot water on demand — no cylinder needed
- Sealed pressurised system
- Ideal for flats, apartments, small-to-medium homes with one bathroom
- Best when you want gas-boiler convenience without the gas
Pick this if: you want one wall-mounted box that does everything.
Type 2
Electric Heat Only Boiler
- Heats your radiators or UFH loop only
- Works with a feed-and-expansion tank in the loft (open-vented)
- Does not heat domestic hot water directly
- Needs a cylinder or immersion for showers/taps
- Ideal for like-for-like swaps on older systems, and for UFH-only installations
Pick this if: you're replacing a regular gas boiler on an existing open-vented system, or you only need central heating (e.g. UFH without a shower).
Type 3
Instant Water Heater
- Heats water only — no central heating output
- Point-of-use: mounted near the tap, shower or UFH manifold
- No storage, no standby loss
- Small compact wall unit
- Ideal for caravans, outbuildings, granny annexes, single-room UFH loops, single wash-basin duty
Pick this if: you only need hot water (not heating), or you have a small isolated demand.
How electric boiler sizing works — the maths behind the marketing
The kW rating on an electric boiler does two jobs at once — it determines how much heat it can push into your radiators or UFH, and how much hot water it can produce on demand. Because it's a single element doing both, you cannot get more out than you put in.
For hot water, the calculation is straightforward. Assuming a cold mains temperature of 10°C and a desired output of 45°C (a 35°C rise, which is the practical target for a comfortable shower):
| Boiler Output | DHW Flow Rate (35°C rise) | What This Actually Delivers | Best For |
|---|---|---|---|
| 7kW | ~2.9 L/min | Trickle. Below a comfortable shower flow. | Single-occupancy flat with bath fill only, or low-demand UFH-only heating |
| 11kW | ~4.5 L/min | Weak shower, marginal for comfort | Small studio, low-hot-water-use property |
| 14kW | ~5.7 L/min | Practical minimum for a satisfying single shower | Most 2–3 bed flats and small houses |
| 18kW (3-phase) | ~7.3 L/min | Comfortable shower + basin use | Larger 3-bed homes |
| 21kW (3-phase) | ~8.5 L/min | Full mixer shower comfortably | 4-bed homes with regular hot water demand |
| 24kW+ (3-phase) | ~9.7+ L/min | Bath fill + shower simultaneously (with care) | Multi-bathroom homes on 3-phase supply |
⚠️ Reality check for landlords: a "cheap" 7kW electric combi in a rental property will almost certainly generate complaints and repair callouts within the first winter. If the property has any shower use at all, budget for a 14kW single-phase or 18kW+ three-phase model from the outset. It's much cheaper than a re-installation.
For radiators and UFH, the rating governs how quickly the boiler can raise your circuit water temperature — but this is a less critical constraint because heating demand is spread over hours, not seconds. A well-sized 12kW boiler will happily supply a 100–120m² UFH-only property, whereas the same 12kW will feel restrictive on hot water if that's your primary need.
🔧 Electric Boiler Sizing Calculator
Tell us about your property and we'll suggest the right kW range and boiler type. This is a guide — always confirm with a Part P electrician before ordering.
These figures are indicative. A qualified installer must perform a full heat-loss calculation (typically to MCS 021 or CIBSE guidance) and confirm your electrical supply can safely support the selected model before installation.
Underfloor heating: matching floor area to boiler capacity
Underfloor heating is the most efficient way to use an electric boiler because it runs at low flow temperatures (typically 35–50°C, versus 60–80°C for radiators). Lower flow temperature means the boiler cycles less, runs quieter, and — critically — pairs beautifully with solar PV or off-peak electricity tariffs.
Water-based (hydronic) UFH is the type discussed here — it works with a boiler and manifold. The typical output of a well-installed UFH system in a well-insulated property is around 60–80 W/m². In older or poorly insulated properties this can rise to 100 W/m² or more, and the boiler needs to be sized to that peak load.
| Floor Area | New Build / Passivhaus | Well Insulated (post-2010) | Older / Standard Property | Suggested Boiler kW |
|---|---|---|---|---|
| 30–50m² (single room / annex) | ~2–3kW | ~3–4kW | ~4–5kW | 4–7kW heat only |
| 60–80m² (small flat) | ~4–5kW | ~5–6kW | ~6–8kW | 6–9kW heat only, or 11–14kW combi if hot water also required |
| 100–120m² (2–3 bed home) | ~6–8kW | ~8–10kW | ~10–13kW | 9–12kW heat only, or 14kW combi |
| 150–180m² (larger home) | ~10–12kW | ~12–14kW | ~15–18kW | 12–15kW heat only, or 14–18kW combi |
| 200m²+ (large detached) | ~13–15kW | ~16–18kW | ~20kW+ | 18–24kW+, three-phase supply usually required |
💡 Design tip: UFH manifolds have their own blending valve that mixes the boiler flow with cooler return water to hit the target loop temperature. This means the boiler doesn't need to modulate down to UFH temperature itself — modern electric boilers with a wide 30–80°C flow range (like the EHC Fusion Astro V2 or Elterm Ensign) work well with UFH manifolds out of the box.
Just want underfloor heating? Can you skip the cylinder entirely?
Yes — and this is one of the most cost-effective setups in the electric boiler catalogue. If you have no requirement for hot water from the boiler (for example, you have a separate electric shower, an instant water heater at the kitchen sink, or the UFH loop is heating an outbuilding or garden room), an electric heat-only boiler paired directly with a UFH manifold — with no cylinder, no hot water circuit, no combi complexity — is the simplest and cheapest solution.
The setup is minimal: heat-only boiler on the wall, flow and return pipes down to the UFH manifold, thermostats in each zone, and a feed-and-expansion tank (or a small sealed expansion vessel, depending on the model). That's it. No cylinder, no unvented safety kit, no G3 registration, no annual servicing overheads on the DHW side.
Small UFH-only property (30–80m²)
A 4–7kW heat-only boiler like the Strom 6kW SBSP6H or Elterm Capitan 6kW is enough for a well-insulated single-storey space. Pair with hot water from an instant water heater at the sink and you have a fully electric, cylinder-free property.
Medium UFH-only property (80–150m²)
A 9–12kW heat-only boiler like the Strom 11kW SBSP11H, EHS Flex 2–12kW or ThermoSphere TSEFB 4.5–15kW covers most 2–3 bed homes with wet UFH throughout. Range-rateable models let the installer dial output to match the actual heat loss.
Larger UFH-only property (150m²+)
The Strom 14.4kW SBSP15H is the largest single-phase heat-only option. For anything beyond, three-phase heat-only models are required — commercial units like the Elterm Battalion 30–48kW handle large developments or multi-house setups.
Existing gas heat-only system
If you're swapping out an old regular gas boiler on an existing open-vented system with a loft tank, the EHS Flex 12kW or Strom 11kW SBSP11H connects like-for-like. No pipework redesign, no unvented conversion required.
Instant water heaters as mini UFH boilers (and DHW-only solutions)
Instant electric water heaters aren't just for taps. Compact 4–14kW instant units can power a single small UFH loop, a single tap, a caravan shower, or an outbuilding — all without a cylinder, without a boiler, and often without needing an upgrade to your consumer unit at the lower end of the range.
The ThermoSphere Micro Electric Boiler (4.8–9.6kW) is a standout example — it's marketed as an instant water heater but functions as a compact electric boiler for a single small heating zone or DHW point. At 4.8kW it draws around 21A, which most modern consumer units can accommodate as a dedicated circuit.
Single-zone UFH (bathroom, wet room, kitchen)
A 4.8–9.6kW instant unit connected directly to a small UFH loop — no cylinder, no separate boiler. Ideal for a single bathroom retrofit or a garden room with heated floor.
Hot water only at a single point
Undersink or oversink water heaters (2–3kW) for a single tap. No plumbing to a boiler, no long hot water runs, no standby loss. Ariston Andris Lux and Ferroli MINI5 are entry-level go-to's.
Caravans, mobile homes, statics
Compact 6–8kW single-phase instant heaters like the Veito Speed E fit into a caravan wall and deliver shower-ready hot water on demand — no gas bottle, no external LPG heater, no flue.
Annexes, garden rooms, granny flats
A single instant water heater at the sink plus a small heat-only boiler (or a mini-boiler like the ThermoSphere Micro) covers a self-contained outbuilding with UFH, without duplicating an entire cylinder-based system.
Workshops, gyms, garages
Handwash unit at 3kW, plus a UFH loop powered by a 6–9kW instant unit or heat-only boiler. Minimal running cost, minimal install complexity.
Single-bathroom flat, no central heating
A 9.5kW multipoint instant heater (Ariston Aures Multi, Redring Powerstream) handles the whole flat's hot water — pair with electric radiators or ThermoSphere electric UFH for a completely gas-free small home.
Why choose an electric combi boiler?
Electric combi boilers are the most complete single-unit solution — they replace a gas combi like-for-like in most flats and smaller homes, without any of the flue, gas, or servicing overhead.
No flue, no gas, no combustion
Site it anywhere with a suitable electrical supply — kitchen cupboards, utility rooms, hallway airing cupboards, garages, loft spaces. No wall penetration for a flue, no external terminal, no gas safety inspection. Perfect for listed buildings, flats without external walls, and properties where a gas connection isn't practical.
100% efficient at point of use
Every kWh of electricity in becomes a kWh of heat out — there's no flue loss, no combustion inefficiency, no standby heat loss. When paired with solar PV or a green tariff, the boiler operates with zero direct carbon emissions.
Silent operation
No burner roar, no flue fan, no firing sequence. Most models run at 32 dB(A) or less — quiet enough for installation adjacent to bedrooms, home offices, or living spaces. Modulating ErP pumps replace fixed-speed circulation.
Almost zero maintenance
No burners to clean, no heat exchanger to descale annually, no gas engineer callouts. A magnetic filter check and occasional system inhibitor top-up are typically the only service items.
Solar PV & off-peak ready
The built-in programmer can schedule heating to run during cheap-rate hours (Economy 7, Octopus Agile, Octopus Go). Connect a solar diverter (myenergi eddi and similar) to run the boiler on excess PV generation.
Wide flow temperature range
Modern electric combis modulate flow from 30°C (UFH-compatible) up to 80°C (traditional radiators). One boiler handles both applications without hardware changes — set for UFH in the extension, higher for existing rads.
⚠️ Before you order — check your electrical supply. A 14kW single-phase electric boiler draws around 61 amps. Your installer must confirm your consumer unit can accept a dedicated 10mm² circuit with a 63A or 80A MCB alongside your other loads. Most UK homes have a 100A main fuse, which is enough — but this is not guaranteed and must be verified. For 18kW and above, a three-phase supply is usually required.
Electric combi boilers — one standout from each brand
Our combi boiler range covers seven brands, each with a distinct engineering approach. Here are our recommended flagship models — one per brand — for a typical 2–3 bed UK home with a single bathroom.
EHC
EHC Fusion Astro V2 — 14.4kW Single Phase
The highest single-phase output in EHC's Astro V2 range. Delivers up to 6 L/min DHW, has weather compensation, built-in programmer, LCD display, and a 30–80°C flow range that works equally well on radiators and UFH. 5-year warranty. Includes valve kit.
£1,430.00 Inc. VAT
Strom
Strom AuraHeat 13.5kW Stored Combi — SBSP13SC
A stored combi with a 35L integrated stainless steel buffer tank and patented Recirc Technology. Delivers a gas-competitive 12 L/min at 30°C rise on a single-phase supply — the best hot water performance of any single-phase electric combi on the market. Ideal for homes that want gas-combi hot water flow without the gas.
£2,770.00 Inc. VAT
Biasi
Biasi Pro-Electric 14kW Single Phase Combi
The most cost-effective way into 14kW single-phase electric combi ownership. Italian-designed, quiet operation, straightforward controls. Compatible with UFH via wide flow temperature range. Comes as a combi fittings pack.
£995.00 Inc. VAT
ThermoSphere
ThermoSphere Flex Electric Combi — TSFLEX14 (4–14kW)
Range-rateable from 4kW up to 14kW — a single unit that adapts to properties from small flats up to typical 3-bed homes. 7-year warranty, instant hot water, no flue, no cylinder required. The most flexible sizing option in our combi range.
£1,170.00 Inc. VAT · was £1,370
EHS
EHS Primus 14000C Flex — 14kW with Installer Kit
Range-rateable 14kW single-phase combi supplied with a full installer kit including magnetic filter, valve set, and cleaner. Solar PV compatible, UFH-ready, no flue. One-box installation package designed to be fitted in an afternoon.
£1,350.00 Inc. VAT · was £1,595
Elterm
Elterm Ensign 9kW CH / 14kW DHW Combi
Unique split-rating design — 9kW dedicated to central heating (perfect for UFH-focused systems), 14kW dedicated to instant hot water. Delivers proper shower flow without needing a bigger overall electrical draw. Ideal for smaller, well-insulated homes with modern showers.
£1,870.00 Inc. VAT
E.C.A
E.C.A Arceus EK 15 MT — 15kW Electric Combi
Turkish-engineered 15kW single-phase combi at a competitive price point. Digital display, integrated pump, wide flow temperature range for both radiator and UFH applications. Solid mid-market alternative to premium brands.
£1,495.00 Inc. VAT
Browse All Electric Combi Boilers →
Electric heat-only boilers — for UFH-only and open-vented systems
If you don't need instant hot water from the same unit — because you have a separate cylinder, immersion, instant water heater, or the boiler is purely for underfloor heating — a heat-only boiler is significantly cheaper, simpler and often more efficient for that specific role.
EHS
EHS Flex Heat Only Electric Boiler — 12kW (Range-Rateable 2–12kW)
EHS's range-rateable heat-only flagship. Dial output from 2kW to 12kW to match the property's actual heat loss — no oversizing, no cycling. Ideal for UFH-only installations up to 120m² well-insulated. No flue, no gas, no annual service. Straightforward like-for-like swap for older gas heat-only boilers.
£670.00 Inc. VAT
ThermoSphere
ThermoSphere Electric Heat Only Boiler (4.5–15kW)
Five-model range from 4.5kW to 15kW — the widest sizing spread of any brand we carry. Pick the exact output for your property's heat loss, from a single-room garden studio (4.5kW) up to a larger 3-bed home (15kW). Compatible with UFH and standard radiators.
From £730.00 Inc. VAT
Strom
Strom 9kW Single Phase Heat Only — SBSP9H
The sweet-spot heat-only choice for a typical 2-bed flat or small house with UFH. Simple wall-mount install, low current draw compared to combi models, works with existing open-vented systems or with a small expansion vessel for sealed setups.
£570.00 Inc. VAT
Elterm
Elterm Capitan Heat Only Electric Boiler
Polish-manufactured heat-only boiler in a compact wall-mount format, available in 6kW upwards. Premium build quality, quiet operation, precision temperature control — a step up from the entry-level options for buyers who want long-term reliability on a system boiler.
From £1,030.00 Inc. VAT
Browse All Heat Only Boilers →
Instant water heaters — for UFH-only, DHW-only, caravans and off-grid
Compact, wall-mount instant water heaters cover the applications where a full combi or heat-only boiler is overkill. They range from tiny 1.5kW undersink units for a hand-wash basin, right up to 21kW+ three-phase whole-flat solutions.
ThermoSphere
ThermoSphere Micro Electric Boiler (4.8–9.6kW) — TSMEB Series
The dual-purpose star. Marketed as an instant water heater but functions as a mini electric boiler — power a single small UFH loop, a garden room, or a caravan's hot water. Five models across a 5kW range. No cylinder required.
From £570.00 Inc. VAT · was £890
Stiebel Eltron
Stiebel Eltron DCE-X 10/12 Premium — 11/13kW
German-engineered premium instantaneous water heater with precise electronic temperature control. Perfect for whole-flat hot water without a boiler, or as a very compact instant hot water solution for a single bathroom + kitchen combination.
£430.00 Inc. VAT
Ariston
Ariston Aures Multi 9.5kW Multipoint
The most budget-friendly whole-flat hot water solution in our range. ErP Class A efficiency, feeds multiple outlets simultaneously (kitchen + bathroom), simple to install. Ideal for landlords upgrading a rental flat's hot water without a full boiler install.
£130.00 Inc. VAT
Veito
Veito Blue S 21kW Three Phase Instant Water Heater
Top-of-range three-phase compact tankless heater — LCD display, ECO panel, solar compatible. Delivers enough flow for multi-bathroom use without a boiler and without a cylinder. Ideal for whole-property hot water in a larger home or commercial property where a three-phase supply is available.
£330.00 Inc. VAT
Browse All Instant Water Heaters →
🎯 The bottom line for homeowners & landlords
- If you have any hot water demand at all — even a single shower — start at 14kW single-phase. 7kW combis are a false economy that generate complaints.
- If you only need underfloor heating — pair a heat-only boiler with a UFH manifold. No cylinder, no combi complexity, cheapest hardware.
- If you want the best of both worlds — the Strom AuraHeat's 35L stored-combi design delivers 12 L/min hot water on a single-phase supply, matching gas-boiler performance.
- If it's a small isolated space — caravan, garden room, granny annex — a ThermoSphere Micro or compact instant water heater does everything without a boiler at all.
- Check your electrical supply first. A 14kW boiler needs a dedicated 10mm² circuit and 63A MCB — your installer must confirm your consumer unit can accommodate this.
At-a-glance comparison — combi vs heat-only vs instant
| Feature | Electric Combi | Electric Heat Only | Instant Water Heater |
|---|---|---|---|
| Central heating output | Yes (radiators + UFH) | Yes (radiators + UFH) | Limited (single small UFH loop only, on higher-kW models) |
| Domestic hot water | Yes, on demand (no cylinder needed) | No — requires a separate cylinder or immersion | Yes, point-of-use only |
| Cylinder required? | No | Yes (for hot water) | No |
| System type | Sealed pressurised | Open-vented (loft tank) | Sealed, point-of-use |
| Flue required | No | No | No |
| Typical kW range | 7–24kW | 4.5–15kW single phase, up to 48kW three-phase | 1.5–27kW |
| Typical price range | £995 – £2,770 | £495 – £2,100 | £90 – £690 |
| Best for | Flats, small-to-medium homes with one bathroom, all-in-one solution | UFH-only setups, like-for-like gas replacements on open-vented systems, larger properties with a separate cylinder | Caravans, annexes, single-room UFH, DHW-only, off-grid |
| Solar PV compatible | Yes | Yes | Yes (with immersion controller for storage models) |
| Annual servicing | Minimal (magnetic filter check) | Minimal | Minimal |
Need help sizing your electric boiler?
Our team can review your property's heat loss, electrical supply capacity, and hot water demand — and recommend the exact boiler that fits. Free, fast, no obligation.