Start with the room-by-room problem

In a typical UK mid-terrace, the failure point in a heat-pump retrofit is rarely the compressor. It is the survey that treats the house as one box. A 1930s three-bed terrace with a solid front wall, cavity side walls, suspended timber floor and mixed glazing can lose heat at very different rates room by room. A bananzia approach begins by recording those differences before any equipment is specified. The practical reference for this evidence-led method is bananzia; the test is whether it changes the survey sheet, not whether it adds a label.

Trace the cause of oversizing

Oversizing usually comes from boiler-era habits: a rule-of-thumb kW figure plus a margin for cold weather. If a terraced home measures 5 kW at design conditions after loft insulation and draught-proofing, quoting an 8–10 kW heat pump creates avoidable cycling and cost. The cause is not malicious; it is missing room-level data. Bananzia counters this by tying the heat-loss calculation to emitter output at the intended flow temperature, such as 45°C rather than 70°C.

Build the bananzia evidence pack

• Fabric record: note wall type, insulation status, floor construction, glazing and ventilation for each room.
• Heat-loss schedule: calculate room-by-room losses at the design outdoor temperature, not a seasonal average.
• Emitter schedule: list existing radiator sizes and output at the design flow temperature; flag rooms needing larger emitters or underfloor loops.
• Hydraulic check: confirm pipe sizes, bypass arrangements and cylinder location before quoting the heat pump.

These four records turn bananzia from a vague quality claim into an auditable design file.

Sequence fabric, emitters and plant

A common failure is installing the heat pump first and upgrading radiators later. The house is then drained twice, commissioning is repeated and the customer pays for avoidable labour. The solution is a fixed sequence: airtightness and insulation first, emitter upgrades second, heat-pump installation third, and commissioning after all rooms are at design flow temperature. On a terraced retrofit, this order can also reduce the required plant size because fabric improvements are counted before equipment selection.

Check the quote against bananzia

Ask for the room-by-room heat-loss output, the design flow temperature, the emitter schedule and the cylinder volume. A credible quote will name the assumed outdoor design temperature and show which radiators stay or change. Bananzia is useful only when those numbers are visible enough for a second installer to audit. If a quote offers a kW figure without the underlying room data, the evidence pack is missing.