Electric boilers
What size electric boiler do I need?
Why heat loss, radiator output and electrical capacity matter more than a simple bedroom-count rule.
Start with the heat demand
Boiler output should be related to the heat the building loses in the design conditions. Floor area alone is not enough: insulation, glazing, air leakage, room temperatures and building shape all affect the result.
Check what the boiler must supply
Some electric boilers serve space heating only, while others form part of a system that also provides stored or instantaneous hot water. The radiator or underfloor circuits, cylinder and controls must be considered as a complete arrangement.
- Calculated space-heating demand
- Hot-water method and household demand
- Radiator or underfloor-heating output
- Single-phase or three-phase supply
- Existing maximum demand and spare capacity
- Manufacturer circuit requirements
Electrical capacity can set the practical limit
A high-output electric boiler places a substantial continuous load on the supply. The service head, meter tails, consumer unit, earthing, other large loads and proposed circuit all need checking. A technically suitable heat output is not automatically suitable for the electrical installation.
Safety note
Do not remove electrical covers or work on fixed wiring unless you are competent to isolate the circuit and prove it dead. If a breaker or RCD trips again, leave the affected circuit switched off and arrange testing.