Extension Heating Calculator — Heat Loss & Radiator Sizing

This calculator gives a quick, deliberately conservative estimate of room heat loss from floor area, room type and exposure, then sizes the heating output. It is a starting estimate, not a substitute for a full room-by-room calculation to BS EN 12831-1:2017 (mandatory under MCS for heat pumps and biomass).

Heat loss method

A conservative whole-room heat-loss density (W/m² of floor area) is chosen by room type — ranked by CIBSE Guide A internal design temperatures (bathroom 22°C, living 21°C, kitchen and bedroom ~18°C) — and applied to floor area. Approximate uplifts are then added for external-wall exposure (≈5 W per m² of external wall) and windows (≈50 W each). These adders are rules of thumb; a full fabric calculation uses Σ(U·A)·ΔT instead.

Heating output

Radiator demand is heat loss × 1.1 (10% design margin). The radiator count assumes a Type 22 600×1000mm panel, which is rated ~1869 W at ΔT50 (EN 442, mean water 70°C) but delivers far less at the cooler temperatures of a modern condensing or heat-pump-ready system. We size against a conservative working output of ~1200 W (about ΔT35, correction factor ≈0.63, i.e. mean water ~55°C). Be aware that a strict 55°C-flow / 20°C-drop design is only ΔT30 (mean water 50°C, factor 0.51), de-rating the same panel to roughly 950 W — so always apply the manufacturer's ΔT correction factor to your actual flow and return temperatures, and run a full BS EN 12831 calc for a precise radiator schedule. UFH output is capped at 100 W/m² (BS EN 1264-2, 29°C maximum floor surface temperature). Boiler headroom is checked by adding the new load to existing demand and comparing against the boiler's central-heating output.