Electric Boiler Replacement — Marylebone Mansion Flat

Electric boilers have a particular niche in London's heating market that gas boilers cannot fill: flats where flue routes are impossible, listed buildings where external wall penetrations are prohibited, and properties where the gas supply has been capped or was never installed. This Marylebone mansion flat was all three.

The flat — a two-bedroom Grade II listed conversion on Devonshire Place, part of a Nash terrace — had been heated since 2003 by a Heatrae Sadia electric boiler installed in a utility cupboard off the kitchen. The boiler had served the flat well for over two decades, but the last 18 months had told a clear story: three call-outs for intermittent heating failures, a heating element replacement that took three weeks to source, and energy bills that had crept up steadily as the unit's efficiency degraded with age.

When the boiler failed again in early June — this time with a tripped overload that wouldn't reset — the client called RARE. The diagnostic was unambiguous: the internal heating element had failed, the replacement part was discontinued, and the control board was showing signs of thermal fatigue. The boiler was beyond economical repair. Replacement was the only viable path.

The constraints were significant. The flat is Grade II listed, which meant no new external wall penetrations for a flue — ruling out a gas boiler entirely, even if a gas supply had been available, which it wasn't. The utility cupboard was compact, with limited clearance on all sides. And the electrical supply to the cupboard, while adequate for the old boiler, would need assessment to confirm it could handle a modern 12kW electric combi without overloading the circuit.

RARE's engineer carried out a full electrical load assessment on the same visit. The existing supply to the utility cupboard was a 32A radial circuit on a 40A consumer unit — sufficient for the old boiler but marginal for a modern 12kW unit drawing close to 50A at full load. The recommendation was to upgrade the circuit to a dedicated 40A radial with a new RCBO at the consumer unit, giving comfortable headroom for the new boiler and any future electrical additions.

The boiler specification was an EHC Comet 12kW electric combi — a compact, wall-mounted unit designed specifically for flats and small properties without gas. At 380mm wide and 610mm tall, it fit the utility cupboard with clearance on all sides, requiring no modification to the surrounding joinery. The EHC Comet is close to 100% efficient at point of use, requires no flue, no gas safety certification, and carries a 5-year manufacturer warranty — ideal for a listed building where ongoing gas compliance would add complexity.

The installation was completed in a single day. The morning was spent on the electrical upgrade: a new 6mm² radial circuit run from the consumer unit to the utility cupboard, a dedicated 40A RCBO installed, and the circuit tested and certified. The old Heatrae Sadia was disconnected, drained and removed by early afternoon. The EHC Comet was positioned, connected to the heating circuit, cold water supply and electrical supply, and commissioned by 4pm.

A Heatmiser NeoStat thermostat replaced the old dial controller, giving the client proper programmable temperature control for the first time. The Part P electrical certification was completed and issued from site. The 5-year EHC warranty was registered online before the engineer left.