Sizing your radiators by heat loss: the 2026 method

Understanding heat loss to choose the right radiator

Heat loss: what exactly are we talking about on a job?

On a job, heat loss is the heat that escapes the home when it's cold outside. It comes from the walls and other surfaces, from thermal bridges, and from air coming in and going out. The higher it is, the more your radiator has to compensate, and the higher the bill climbs.

The factors that weigh the most (walls, windows, roof, ventilation)

In practice, the roof and the walls carry a lot of weight, especially in older houses. Poorly insulated windows add a cold-surface sensation. Ventilation and infiltration can also "suck out" heat if airtightness is poor or flow rates are badly set.

The direct link between heat loss, output and room-by-room comfort

Sizing is done room by room. A little-exposed bedroom doesn't have the same needs as a living room with large glazed bays. The right approach is to estimate the useful output based on floor area, insulation, exposure and ventilation, then choose a suitable radiator to avoid swings, cold spots and overheating.

Gathering the right data before sizing the radiator

Recording areas, volumes and orientations without mistakes

Before choosing a radiator, start from reality. Measure the floor area, the ceiling height and the volume of each room. Also note the orientation of the walls and windows, since a north-facing wall doesn't "lose" heat the way a south-facing bay window does. Keep a simple record, a dimensioned plan or sketch, with reliable measurements.

• Area of surfaces facing outside or an unheated space.
• Glazed area, window type, presence of shutters.
• Room above, room below, and actual use.

Identifying existing insulation and visible thermal bridges

Identify the insulation present — loft, walls, ground floor — and its continuity. The clues show up quickly. Shutter boxes, window reveals, wall-roof junctions, damp marks or cold spots often signal thermal bridges. This picture of the "site reality" avoids oversizing, or installing a radiator that struggles, especially with low-temperature water.

Accounting for the local climate and reference temperatures in 2026

In 2026, sizing is still done with a base outdoor temperature suited to the municipality and altitude, following a recognised heating calculation method (such as NF EN 12831) and local climate data. Also check the target setpoint, often 19°C in living spaces. A well-chosen radiator delivers steady heat, not a burst of it.

Calculating radiator output from heat loss

Step-by-step method: watts needed per room (the calculation logic)

The starting point is the room's heat loss in watts, ideally taken from an audit or a thermal calculation. Keep it simple.

1. Identify the heat loss of each room under the base scenario (target indoor temperature and design outdoor temperature).
2. Add, if needed, the share due to air renewal (mechanical ventilation, air inlets, leaks).
3. The radiator output to cover matches this total in W, room by room.

Adjusting for the heating flow temperature (high or low temperature)

A radiator is rated for a specific water regime. At high temperature, it delivers more watts. At low temperature (often with a heat pump), the output drops. Rely on the manufacturers' tables and the correction coefficients for the relevant ΔT. Keep in mind the ΔT commonly used in catalogues.

Adding a useful margin without oversizing

Plan for a small margin, typically 5 to 10%, for colder days and uncertainties. Avoid massive oversizing. A radiator that's too powerful complicates control and can degrade comfort.

Matching the radiator choice to the heating system and use

Water radiator: compatibility with boiler, heat pump and settings

A water radiator has to match the heat generator's temperature regime. With a boiler, most models are compatible, but the output depends on the flow temperature. With a heat pump, aim for low-temperature emitters (usually larger) and take care with balancing, thermostatic valves and the weather compensation curve to avoid cycling.

Electric radiator: thermal mass, convection, radiant heat, and consumption

On the electric side, a convector radiator heats fast but can dry out the air. Radiant heat gives comfort at a lower temperature. Thermal mass (dry or fluid) smooths out swings. Consumption mostly comes down to insulation and precise control (thermostat, scheduling).

"At-risk" rooms: bathroom, cold corners, large glazed bays

In a bathroom, choose a radiator suited to the safety zones and, if needed, a heated towel rail with an occasional boost. Near large glazed bays or in a corner, increase the output or place a radiator under the window to counter the cold-surface effect. Also address infiltration to limit cold spots.

Avoiding common sizing mistakes and de-risking your quote

Frequent mistakes: underestimated heat loss, forgotten ventilation, unrealistic assumptions

The foundation is a consistent room-by-room heat-loss calculation, based on a recognised method (such as NF EN 12831). Mistakes often come from overestimated insulation, forgotten thermal bridges, or ventilation "left out of the picture". Also watch for overly optimistic assumptions (uniform indoor temperature, perfect control, unchanged emitters).

• Check untreated surfaces, loft, ground floors, joinery.
• Include ventilation and infiltration flow rates, otherwise real-world comfort falls short.

Quick on-site checks: output consistency and balancing

On site, cross-check areas, ceiling heights, and the condition of the pipework and valves. An undersized radiator forces the water temperature up and hurts efficiency. Plan for a simple flow-rate check and a documented hydraulic balancing.

Justifying your sizing to the customer (comfort, savings, compliance)

Explain the "why" with readable figures: output per room, a reasonable safety margin, impact on noise, cycling, and the bill. Attach the calculation note, the assumptions, and the manufacturer data sheets. You de-risk the quote, and you demonstrate a compliant, professional approach.