Baxi Amptec S18 18kW Electric System Boiler (Three Phase)
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The Baxi Amptec S18 is an 18 kW three-phase electric system boiler for wet central heating — now sold under the Baxi brand, previously Heatrae Sadia. The largest model in the Amptec S range, it needs no gas supply and no flue, runs both radiators and underfloor heating, and carries its circulation pump, expansion vessel and HMI display inside one compact, UK-built unit that fits a 290 mm deep cupboard.
Applications & suitability
Sealed wet central heating in properties with a three-phase electrical supply and no gas. Runs radiators or underfloor circuits from 30–80 °C, and with pump, expansion vessel and controls built in it drops into a 290 mm deep cupboard with no flue to route.
The biggest off-gas heating demands in the range — large houses, HMOs and light commercial spaces, on new electric installations or refurbishments of existing electric boilers. Baxi state it can also be used in conjunction with a solar PV battery.
Certified OpenTherm, so it pairs with OpenTherm controls including the Baxi uSense 2 for best efficiency. As a system boiler it works with a separate hot water cylinder, and it joins Baxi’s existing Amptec and Electromax electric range.
Requires a three-phase supply — most UK homes are single phase, so check before ordering. Single-phase properties should use the S4–S12 instead. Not a combi: stored hot water needs a separate cylinder.
Key specifications
| General | |
| Brand | Baxi |
| Model | Amptec S18 |
| Boiler type | Electric system boiler |
| Output (central heating) | 18 kW |
| Power supply | Three phase |
| Performance | |
| Flow temperature (max) | 80 °C (adjustable from 30 °C) |
| Modulation ratio | Up to 9:1 |
| Controls compatibility | Certified OpenTherm (incl. Baxi uSense 2) |
| Dimensions & weight | |
| Packaged dimensions (H × W × D) | 889 × 444 × 350 mm |
| Lift weight (dry) | 22.5 kg |
| Installed weight (filled) | 24.5 kg |
| Packaged weight | 25.7 kg |
| Warranty & provenance | |
| Warranty | 2 years |
| Country of manufacture | United Kingdom |
Installation & compatibility
No gas and no flue, so no Gas Safe engineer is needed. The fixed three-phase electrical connection must be made by a qualified electrician — in England and Wales this work falls under Part P of the Building Regulations. A filling loop and rear pipework kit are available separately.
Related & next steps
For filling and pressurising the sealed system at install.
View filling loopRoutes pipework behind the boiler for a tidier first fix.
View pipework kitAlso available in single-phase 4 kW, 6 kW, 8 kW and 12 kW models, and a three-phase 14 kW.
Ideal For
The largest off-gas properties with a three-phase supply — maximum electric heating output from a compact, flueless unit, with next-day national delivery or click & collect from our Reading trade counter.
