Heating Sizing Pack

Why the radiators that were fine on a boiler leave the house cold — and whether the pipe can carry it at all

Two things decide whether a heat pump retrofit works, and a calculator gives you neither. The radiators: a 1000 W panel gives about 350 W at heat-pump temperatures, so the pack has the correction at six real system temperatures and the badge figure you must actually buy. The pipework: a heat pump moves four times a boiler's water for the same heat, so the pack works out whether what is already in your floor carries it, what it does to the pump head, and what it costs when it does not. 10 pages, printable, with a fill-in room-by-room schedule to 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
• Whether your existing pipework carries a heat pump — by load and flow temperature
• Why a modern condensing boiler is KINDER to pipework than the old one it replaced
• The 22 mm primary that runs at 1.49 m/s on a 10 kW heat pump, against a limit of 1.5
• Microbore tails: fine on a boiler, hopeless on a heat pump, and buried in your floor
• The pump head the circuit demands, to hold a heat pump's own curve against
• 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.