London Radiator Sizing Guide: How to Calculate the Right Radiator Size

Why Radiator Sizing Matters

An undersized radiator cannot heat a room to the set temperature — the boiler runs continuously without reaching the thermostat setpoint, wasting energy and accelerating boiler wear. An oversized radiator heats the room too quickly and causes the heating system to cycle on and off more frequently than designed, which also reduces efficiency and increases component wear. In London, where many properties have been through multiple heating system changes and room layouts may have altered significantly from the original Victorian or Edwardian configuration, it is common to find radiators that were sized for a different room layout or a different heating system type. Radiator sizing is not simply choosing the largest unit that fits the available wall space.

The BTU Calculation

Radiator output is measured in BTU (British Thermal Units) per hour, or in watts (1 kW = 3,412 BTU/h). The required output for a room is calculated from its heat loss, which depends on: room volume (length x width x height), external wall area and insulation level, window area and glazing type, ceiling and floor construction, and the desired room temperature. For a quick estimate, London properties of Victorian or Edwardian construction with standard cavity or solid wall construction, double glazing, and a target room temperature of 21 degrees Celsius typically require 100 to 120 watts per square metre of floor area. A modern well-insulated flat may require as little as 60 to 80 watts per square metre. A poorly insulated older property with single glazing may require 130 to 150 watts per square metre.

Room-by-Room Guidance for London Properties

A typical Victorian London terraced house room-by-room guide at standard construction: living room (4m x 4m, 16 square metres, two external walls, one bay window, first floor) — approximately 2,000 to 2,400 watts required; master bedroom (4m x 3.5m, one external wall, standard double window) — approximately 1,200 to 1,500 watts; bathroom (2.5m x 2m, one external wall, frosted window) — approximately 600 to 800 watts; kitchen (4m x 3m, external rear wall, door to garden) — approximately 1,200 to 1,500 watts; hallway and stairs (variable) — often no radiator in period properties, or a single 600 to 800 watt unit. These figures are indicative starting points — a heating engineer carrying out a full heat loss survey will calculate precisely, taking into account the specific construction type, insulation levels, and local climate data.

Choosing Radiator Dimensions

Once the required wattage is known, the next step is selecting a radiator that achieves that output within the available wall space. Radiator output at a given temperature difference (Delta T) varies by surface area, panel count, and fin configuration. A double-panel double-convector (Type 22) radiator delivers more heat per unit of wall space than a single-panel (Type 11). For the same wattage output, a Type 22 radiator can be significantly shorter and narrower than a Type 11, which matters in London where wall space is often limited by bay windows, chimney breasts, and doorways. Online radiator calculators and manufacturer datasheets publish output figures at Delta T 50 (the standard test condition) and at Delta T 60 — ensure you are comparing like for like and that the system operating temperature matches the test condition used.

Adjusting for Modern Heating System Temperatures

Traditional high-temperature heating systems run flow temperatures of 75 to 80 degrees Celsius — the Delta T 50 test condition. Modern condensing boilers running at high efficiency operate at 55 to 70 degrees Celsius flow temperature, and heat pump systems typically run at 35 to 45 degrees Celsius. At lower flow temperatures, the same radiator delivers less heat. If you are replacing a traditional boiler with a modern condensing boiler or a heat pump, radiators sized for the old high-temperature system may need to be upsized to compensate for the lower flow temperature. A heating engineer can recalculate the required radiator output at the new design flow temperature and advise whether existing radiators are adequate or require upgrading.

Radiator Placement and Circulation

The traditional position for a radiator in a London property is below the window — the cold surface of the window creates a downward convection current that draws room air down past the radiator, improving circulation. This placement also reduces the cold draught effect of the window. Where below-window placement is not possible (fitted furniture, radiator access constraints), placing the radiator on the coldest external wall is the next best option. Radiators should not be positioned behind curtains or furniture that block the convective airflow, as this significantly reduces effective output. TRVs should be installed on all radiators in rooms with thermostats, and should not be installed in the same room as the main room thermostat.

Getting Professional Sizing

For a new central heating installation or a significant system upgrade, professional radiator sizing carried out by a Gas Safe heating engineer using heat loss software (typically CIBSE approved) is the correct approach. A proper heat loss survey for a three-bedroom London terraced house takes approximately one hour and ensures the system is correctly specified. The cost of this survey is typically included in a heating installation quote. For a like-for-like radiator replacement (replacing a failed radiator with the same size), professional sizing is less critical, though it is worth checking that the existing radiator was itself correctly sized for the room.