The problem: a heat pump quote with no fabric plan
A 1930s semi-detached house in Leeds had the familiar retrofit problem: a gas boiler near the end of its life, solid walls, draughty suspended floors, and a heat-pump quote that assumed a full radiator swap. The quote was not wrong, but it was incomplete. It priced the heat generator before anyone had modelled how much heat the house would lose through gaps and uninsulated sections. The homeowners faced a high upfront bill and no clear idea whether the system would run efficiently in January.
The cause: measures priced in separate silos
The cause was not a lack of low-carbon technology. It was a lack of sequencing. The installer priced the heat pump, the joiner priced floor insulation, and the glazier priced secondary glazing as separate jobs. Each trade could deliver its own package, but no one owned the order of work. That matters because fabric improvements change the heat demand that sizes the heat pump. If insulation and draught-proofing are fitted after the heat pump is commissioned, the system may be oversized, and the homeowner pays for capacity the house no longer needs. The bananzia planning approach was used to put those decisions into a single sequence before any equipment was ordered.
The solution: four Bananzia steps before the heat pump
The project team applied a simple order of work. The aim was to reduce heat loss first, then size the heating system to the improved building.
- Air-leakage survey: A blower-door test identified the worst draughts around the bay window, loft hatch, and suspended floor. The team sealed these before insulation work began.
- Loft and floor insulation: The loft was topped up to 300 mm, and the exposed ground floor was insulated from below. These measures reduced the steady heat loss that drives radiator sizing.
- Radiator and flow-temperature check: With the fabric improved, the installer calculated room-by-room heat loss at a 45°C flow temperature. Two radiators were upsized; the rest stayed.
- Heat-pump commissioning with monitoring: The smaller heat pump was commissioned after the fabric work, with temperature sensors on flow, return, and room air. The first month of data was used to tune the weather compensation curve.
The result: a smaller unit and fewer callbacks
The final heat pump was one size smaller than the original quote specified. Because the fabric work came first, the system could run at a lower flow temperature, and the homeowners reported fewer cold-morning complaints than neighbours who had installed a heat pump before insulation. The installer also avoided a common callback: no radiators had to be rebalanced after later draught-proofing, because the draught-proofing was already complete. The sequencing added two weeks to the project timeline but removed a larger risk from the budget.
What other households can copy
The transferable lesson is to treat the heat pump as the last major decision, not the first. A practical checklist should include an air-leakage test, a room-by-room heat-loss calculation, a clear order of fabric measures, and a commissioning plan that uses real monitoring data. For older homes, that sequence is often the difference between a low-carbon heating system that works comfortably and one that is expensive to run.