Heating Sizing Pack

Why the radiators that were fine on a boiler leave the house cold on a heat pump

Every radiator on sale is advertised at one output, and that figure is the EN 442 laboratory condition: 75/65°C into a 20°C room. Output falls with temperature difference to the power of 1.3, so at a heat pump's 45/40°C the same radiator gives about a THIRD of its badge figure. That is the entire failure mode of a heat pump retrofit — the heat loss calculated correctly, the pump sized correctly, and the radiators never re-checked. This is the correction for every system temperature, the radiator you must actually buy for a given room load, the ways to fix a short room ranked cheapest first, the fabric measures that come before any of it, and where the work stops being yours by law. 8 pages, printable, ending in a room-by-room schedule you hold a quote against.

Look inside

Two full pages from the pack, exactly as printed. Click either to read it at full size before you decide.

What is in it

• Radiator output correction for six real system temperatures
• Why a 1000 W radiator gives about 350 W on a heat pump
• The radiator you must BUY for a given room load, at every flow temperature
• The same room on a boiler and on a heat pump, side by side
• Five ways to fix a short room, cheapest and least disruptive first
• The honest fallback nobody sells you: raise the flow temperature
• Fabric measures that come before sizing anything, and which are poor value
• Seven sizing rules, including why a boiler's size tells you nothing
• Where heating work stops being a homeowner's job — and what is yours
• The question that tells you whether an installer has actually sized the job
• The order of work, and three things to settle before getting quotes
• Fill-in room-by-room radiator schedule

The pack complements the free calculators — they do the arithmetic, it carries the correction they cannot apply.