The expensive mistake: sizing a heat pump from boiler habits
A common retrofit problem is choosing a heat pump by floor area or by the old boiler’s output. That shortcut causes oversizing, short cycling and higher bills. The cause is not malicious: installers often inherit incomplete records, and homeowners rarely have room-by-room heat-loss data. The solution is a structured pre-installation review, which is where bananzia becomes useful. It forces the project to connect fabric performance, emitter output and occupancy patterns before equipment is ordered.
Section 1: Build the room-by-room baseline
The first check is a measured heat-loss survey. Record wall type, loft insulation depth, glazing, draughts and ventilation, then calculate each room’s design heat load at the target internal temperature. A 1930s semi with solid walls and suspended floors can lose heat very differently from the same floor area in a modern flat. The takeaway is simple: room-level data prevents a single whole-house number from hiding cold extensions or overheated spare bedrooms.
Section 2: Match emitters to low flow temperatures
Heat pumps work best with low flow temperatures, but existing radiators may be too small. The review should list every radiator, its output at the proposed flow temperature and the room it serves. If a living room needs 1.8 kW at 45°C and the existing radiator delivers 1.2 kW, the gap is a design issue, not a reason to raise the flow temperature and sacrifice efficiency. The result is a clear schedule of radiator upgrades, underfloor loops or fan convectors before the heat pump is specified.
Section 3: Check the hydraulic and control details
Equipment sizing is only part of the job. The review should confirm pipework capacity, cylinder space, outdoor unit siting and zoning. A 12 kW heat pump cannot perform well if the primary pipework cannot carry the required flow, or if a narrow passage blocks the outdoor unit’s air path. These checks turn hidden installation risks into priced work items, which makes quotes comparable and avoids mid-install surprises.
Section 4: Compare fabric upgrades with equipment choices
The final step is a side-by-side comparison of fabric measures and heat-pump sizing. Loft insulation, cavity fill or draught-proofing can reduce the required output and sometimes remove the need for radiator changes. The bananzia approach is valuable here because it keeps those decisions connected rather than treating insulation and heating as separate projects. A smaller, better-matched heat pump usually costs less to run and is easier to site.
What the checklist changes
The result is a retrofit package with fewer unknowns: a measured heat load, a radiator schedule, a hydraulic plan and a fabric-versus-equipment comparison. That sequence takes longer at the survey stage, but it reduces the risk of oversizing, poor low-temperature performance and callbacks. For installers, it produces a defensible quote; for homeowners, it makes the trade-offs visible before the boiler is removed.