Electric Boiler Comparison: ECA Arceus vs Trianco vs Strom vs Viessmann
Arda DaneElectric boilers have moved from niche to mainstream. Whether you are replacing a gas combi, heating a flat with no gas supply or specifying a system for a new build, there are now several credible options on the UK market. The trouble is that each manufacturer publishes its figures in a slightly different way, which makes a fair side-by-side comparison harder than it should be.
In this guide we compare four electric boilers that regularly come up in UK enquiries: the ECA Arceus, the Trianco Aztec Mini, Strom and the Viessmann Vitotron 100. Every figure below comes from the manufacturers' current published manuals and product information, and where headline numbers are quoted on different bases we have normalised them so you are comparing like with like.
The short answer
- For most homes with one to three bedrooms and one bathroom, an instantaneous electric combi such as the Arceus 12 kW is a sensible starting point, provided the calculated heat loss is no more than 12 kW.
- For the strongest instantaneous hot water flow in this group, the Arceus 15 leads once figures are normalised, with Strom close behind.
- For multiple bathrooms or simultaneous outlets, a system boiler with a correctly sized cylinder is the better engineering answer. That could be an Arceus ST system boiler or the Viessmann Vitotron 100.
- For the lowest published minimum output, Strom modulates down to 1 kW.
Side-by-side comparison
| Criterion | ECA Arceus | Trianco Aztec Mini | Strom | Viessmann Vitotron 100 |
|---|---|---|---|---|
| Formats | Combi, system and heat only | Combi | Combi and system variants | System and heat only, with a cylinder for hot water |
| Power and supply | 12 and 15 kW, each for 230 V single-phase or 400 V three-phase | 8, 10 and 12 kW single-phase | 7, 11 and 14.4 kW single-phase; 18 to 24 kW three-phase | 4 to 8 kW single-phase; 12 to 24 kW three-phase |
| Published hot water figure | 15 kW: 8 L/min at a 26.9°C rise | 12 kW: 5 L/min at a 35°C rise | 14.4 kW: 7 L/min at a 30°C rise | Not instantaneous - hot water via a cylinder |
| Controls and modulation | 2 kW minimum; OpenTherm compatible; accepts on/off, modulating, timer and outdoor sensor inputs | Constant temperature control; no OpenTherm reference found in the manual | Modulates down to 1 kW; no OpenTherm reference found in the manual | Weather compensation control |
| Installation kit | Valve kit and WRAS approved filling loop supplied; 8 L expansion vessel | Valves, filling and filter or scale protection sourced separately; 5 L vessel | External bypass, valves and filling sourced separately; internal vessel | Pump, safety group and 5 L vessel; separate cylinder required |
| Warranty headline | 3 years parts and labour, plus 10 years element cover (terms apply) | 2 years after registration | 5 years structured: 2 years parts and labour, then parts only | Check current UK terms with Viessmann |
Where a feature is marked as "not found", it means we could not locate it in the reviewed manual - not that no optional solution exists.
A closer look at each boiler
ECA Arceus
The Arceus is available as an MT combination boiler, an ST system boiler and a CH heat only boiler, in 12 and 15 kW outputs. Both outputs are documented for either 230 V single-phase or 400 V three-phase supplies, which gives installers flexibility where a property has three-phase available.
Inside, the layout will feel familiar to anyone who has worked on a gas combi: a circulation pump, a motorised three way valve and a secondary plate heat exchanger for instantaneous hot water, plus an 8 L expansion vessel. What is missing is the combustion side - no burner, gas valve, fan, flue or condensate trap. The combi is supplied with a valve and filling kit including CH flow and return valves, DHW inlet and outlet valves, a WRAS approved filling loop and a manometer, so fewer ancillary parts need to be sourced separately.
Built-in protection functions include an automatic bypass, pump overrun, frost protection, pump and three way valve exercise, an automatic air vent and an annual service reminder. The heating elements can be tested individually using a dedicated test mode, cleaned of limescale and replaced one at a time without removing the main tank. The Arceus combi range holds WRAS approval #2205321.
Trianco Aztec Mini
The Trianco Aztec Mini is a compact single-phase electric combi available in 8, 10 and 12 kW outputs. Its manual describes constant temperature control, and installation calls for external valves, a filling connection and filtration or scale protection, alongside a 5 L expansion vessel. The headline warranty is two years after registration. Availability of Trianco's combi models can vary, so it is worth confirming current supply before specifying.
Strom
Strom offers combi and system variants from 7 to 14.4 kW on a single-phase supply, with 18 to 24 kW three-phase models. Its standout published feature is modulation down to 1 kW, the lowest minimum output in this comparison, along with a patented single unit exchanger concept. Installation requires an external bypass, valves and filling connection. Strom's warranty is structured over five years: two years parts and labour, followed by parts only for the balance.
Viessmann Vitotron 100
The Vitotron 100 is a well established system and heat only electric boiler with weather compensation control, covering 4 to 8 kW on single-phase and 12 to 24 kW on three-phase. It is not an instantaneous combi - hot water comes from a separate cylinder - so it is a strong option where stored hot water is the right answer, rather than a direct alternative to a combi. Check current UK warranty terms with Viessmann for your project.
Hot water performance: comparing like with like
Manufacturers quote hot water flow at different temperature rises, which makes raw headline figures misleading. A "30°C rise" is not a 30°C tap temperature - with a typical 10°C incoming mains supply, a 30°C rise gives 40°C water at the outlet.
To compare fairly, we converted each combi's published figure to the same 10°C inlet and two realistic outlet targets: 40°C (a comfortable shower) and 44°C (a hotter shower or bath fill). We have also shown roughly how long each would take to deliver 120 litres.
| Model | Published figure | 40°C outlet (30°C rise) | 44°C outlet (34°C rise) |
|---|---|---|---|
| ECA Arceus 15 | 8.0 L/min at 26.9°C rise | About 7.2 L/min (120 L in about 16.7 min) | About 6.3 L/min (120 L in about 19.0 min) |
| Strom 14.4 | 7.0 L/min at 30°C rise | 7.0 L/min (120 L in about 17.1 min) | About 6.2 L/min (120 L in about 19.4 min) |
| Trianco Aztec 12 | 5.0 L/min at 35°C rise | About 5.8 L/min (120 L in about 20.6 min) | About 5.1 L/min (120 L in about 23.3 min) |
| ECA Arceus 12 | 7.0 L/min at 24.6°C rise | About 5.7 L/min (120 L in about 20.9 min) | About 5.1 L/min (120 L in about 23.7 min) |
The Arceus 15 leads this set, although its margin over Strom is narrow. At 12 kW, the Arceus and Trianco are effectively level. Raising the target from 40°C to 44°C reduces every boiler's flow by around 12% - a real difference, but nowhere near the 3 L/min figures sometimes quoted for electric combis.
Method: nominal calculation using Q2 = Q1 x ΔT1 / ΔT2, 10°C inlet, no allowance for pipework losses. The Arceus manual states a ±15% flow tolerance. These are calculations from published figures, not independent tests.
Whichever combi you choose, an instantaneous electric combi supplies one outlet at a time. If your household regularly runs a shower and a bath together, or has more than one bathroom, a system boiler with a cylinder will serve you better.
Electrical supply: check this before you order
An electric boiler draws significant current, so the incoming supply needs to be confirmed at the design stage. Here are the Arceus published values from the current operating manual:
| Model | Supply | Nominal current | Supply cord | Minimum MCB |
|---|---|---|---|---|
| Arceus 12 | 230 V single-phase | 52 A | 3 x 10 mm² | B63, 1P |
| Arceus 12 | 400 V three-phase | 17 A | 5 x 4 mm² | B20, 3P |
| Arceus 15 | 230 V single-phase | 65 A | 3 x 16 mm² | B80, 1P |
| Arceus 15 | 400 V three-phase | 21 A | 5 x 6 mm² | B25, 3P |
The manual also specifies a 30 mA RCD and a dedicated circuit. Note that the 15 kW model on a single-phase supply needs a B80 minimum - a 63 A supply should not be assumed. These values are an input to a competent electrical design to BS 7671, not a substitute for one. Your installer should confirm maximum demand and incoming supply capacity, including with the DNO where needed.
12 kW or 15 kW? Getting the size right
Marco Rich, Managing Director of ECA UK, puts it simply:
"From overseeing Arceus sales, technical support and aftercare across the UK, my experience is that the 12 kW MT combination boiler is the right starting point for most homes with one to three bedrooms and one bathroom. Move to 15 kW only when the calculated heating load genuinely requires it."
A common rule of thumb is 1.5 kW per radiator, so around eight radiators suggests a 12 kW conversation and nine or more should prompt a closer look at 15 kW. But radiator count is only a screening tool. A Stelrad Type 22 panel at 600 x 1000 mm, for example, is rated at 1,778 W at ΔT50, but that drops to roughly 1,330 W at ΔT40 and about 916 W at ΔT30. The right answer always comes from a room by room heat loss calculation, followed by a check of emitter output, electrical supply and hot water demand.
What comes in the box matters
A lower box price can turn out to be a false economy if valves, a bypass, a filling loop or other parts have to be added on the day. When comparing quotes, ask: what is included to make this a commissionable system?
The Arceus combi ships with its own valve and filling kit and has an automatic bypass built in. The Trianco and Strom installation documentation expects several of these parts to be sourced separately. Either approach works - but it should be reflected in the total installed cost, not just the appliance price. Correct system cleaning, water quality and filtration remain part of good practice whichever boiler you choose.
Warranty and aftercare
Warranty headlines are not all structured the same way, so read the detail:
- ECA Arceus: 3 years parts and labour on the appliance, plus a separate 10 years of cover on the heating elements. Registration, annual servicing and current written terms apply.
- Trianco Aztec Mini: 2 years after registration.
- Strom: 5 years in total, made up of 2 years parts and labour and the balance on parts only.
- Viessmann Vitotron 100: check current UK terms for your project.
Beyond the warranty itself, look at how easily the boiler can be serviced. The Arceus service manual documents individual element testing, limescale cleaning and single element replacement, with a published spare parts schedule.
Which electric boiler should you choose?
- Smaller home, one bathroom, heat loss up to 12 kW: Arceus 12 MT combi.
- Higher calculated heating load, still one bathroom: Arceus 15 MT combi, or Strom 14.4 as a close alternative on hot water flow.
- Three-phase supply available: Arceus 12 or 15, both documented for 400 V, or the three-phase Strom and Viessmann models.
- Multiple bathrooms or simultaneous outlets: a system boiler with an unvented cylinder, such as an Arceus ST or the Viessmann Vitotron 100.
- Heating only, with hot water provided separately: Arceus CH heat only or Viessmann Vitotron 100.
Every competitor here has genuine strengths. Where the Arceus stands out is in combining a supplied installation kit, an 8 L expansion vessel, dual supply options, OpenTherm compatibility, familiar plate heat exchanger and three way valve hydraulics, and element-level serviceability in one product.
Frequently asked questions
Do I need a Gas Safe engineer to fit an electric boiler?
No, an electric boiler is not gas work. It does still need competent plumbing and electrical installation in line with Building Regulations and BS 7671. If an existing gas boiler is being removed, disconnecting or capping the gas supply must be done by a Gas Safe registered engineer.
Can an electric combi run two showers at once?
Instantaneous electric combis like the Arceus MT, Trianco Aztec Mini and Strom combi are designed to supply one outlet at a time. For simultaneous use, choose a system boiler with a cylinder.
Can I fit a 15 kW electric boiler on a 63 A supply?
For the Arceus 15 on single-phase, the manufacturer specifies a B80 minimum, so a 63 A supply should not be assumed. The 12 kW model on single-phase specifies B63. Always have an electrician confirm your supply and maximum demand.
Will a smart thermostat work with an electric boiler?
The Arceus accepts on/off and modulating room controls, timers and an outdoor sensor, and is OpenTherm compatible. Any savings depend on your home, radiators, settings and how you use the heating, so treat claimed gas boiler savings with caution.
Need help choosing?
Our UK technical team can help you or your installer select the right Arceus model based on heat loss, electrical supply and hot water needs. Call 0800 640 9988 or explore the range at ecaonline.co.uk.
Figures in this article are taken from manufacturers' published manuals and product information reviewed in August 2026. Specifications, availability and warranty terms can change, so always confirm the current details and the latest manual revision before ordering.
