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Extension Heating Calculator — Heat Loss & Radiator Sizing
Calculate heat loss for new extension rooms, size radiators or UFH, and check whether your existing boiler has sufficient capacity.
Finished floor to ceiling. Default 2.4m.
Enter the CENTRAL-HEATING output, not the hot-water (DHW) nameplate. A 24kW combi's heating side is typically ~18kW; DHW demand usually governs combi sizing.
Approximate current peak heat demand of the existing dwelling.
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How We Calculate This
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.
Frequently Asked Questions
Heat loss depends on room size, ceiling height, number of external walls, window area, insulation levels and room type. A well-insulated modern room loses roughly 75–110 W/m² of floor area (bathrooms highest because of their 22°C design temperature, bedrooms lowest). This calculator applies a deliberately conservative W/m² rate by room type as a quick starting estimate — it is not a substitute for a full room-by-room heat-load calculation to BS EN 12831-1:2017, which is mandatory under MCS for heat pumps and biomass.
A typical extension room adds 1–4 kW to the heating load. Enter your boiler's central-heating output and estimated current load to check. Note that a combi boiler's headline kW (e.g. 24kW) is its hot-water (DHW) figure; its central-heating side is usually lower (often around 18kW), so use the heating output here. Combi sizing is normally driven by hot-water demand rather than heating load, so a small extension rarely outgrows the boiler. If capacity is tight, consider upgrading or using underfloor heating, which runs at a lower flow temperature.
UFH is popular in extensions because the new floor slab or buildup can easily accommodate UFH pipes. It provides even heat, frees up wall space and works well with heat pumps (lower flow temperature). Radiators are simpler and cheaper to install. UFH can deliver up to 100 W/m², which is usually sufficient for a well-insulated extension.
As an indicative rule of thumb (not a fixed standard), a branch carrying up to about 3 kW is fine on 15mm copper or 16mm plastic; above that, step up to 22mm. Actual capacity depends on the system temperature drop (ΔT) — a modern 20°C design ΔT lets 15mm carry well over 3 kW — plus run length and velocity, so size longer or shared runs from CIBSE pipe-sizing tables. Keep the run from the existing system as short and direct as possible.
Since the June 2023 update to Approved Document L, new or fully replaced wet central-heating systems must be designed for a maximum flow temperature of 55°C wherever practicable. 55°C keeps a condensing boiler in condensing mode (return below its ~54°C dew point) and improves efficiency. Underfloor heating runs cooler still, typically 35–55°C via a blending valve. Lower flow temperatures are also essential for heat-pump compatibility. The older 70–80°C flow temperatures apply to legacy systems, not new work.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.