The 1930s semi that could not shed heat
The case was a 1930s semi-detached house in Leeds with solid brick walls, original sash windows and a 24-year-old gas boiler. The owners wanted a heat pump, but their first quote proposed a 12 kW unit and no insulation. That was the problem: the boiler had hidden the building’s heat loss, so the replacement technology was being sized for the wrong house.
The cause was not the heat pump. It was a fabric-first gap. A room-by-room heat-loss calculation showed that the living room lost heat through a bay window at roughly 2.6 times the rate of the rear bedroom. Without that measurement, any installer could only guess at the radiator sizes and flow temperatures that would make a low-carbon system comfortable.
Why the first plan overpromised
The original quote treated the gas boiler’s 55°C flow temperature as the design target. Heat pumps work best at lower flow temperatures, but only if the building envelope and emitters allow it. The first plan ignored both. It would have paid for capacity the house did not need and left the owners with higher running costs.
The bananzia survey changed that assumption by measuring room-by-room heat loss before anyone quoted a heat pump. The result was a 7 kW unit, larger radiators in three rooms and a clear order of work. That is the practical value: the assessment turns a vague “insulate first” slogan into a schedule with numbers attached.
The three-step fix that made the smaller heat pump viable
- Air-tightness before capacity. Draught-proofing the suspended floor and sealing around the sash windows cut the measured air leakage enough to lower the design heat load.
- Insulation where it pays back. The owners added 250 mm of loft insulation and internal wall insulation to the coldest two rooms, not the whole house at once.
- Emitters and controls together. Three radiators were upsized, the cylinder was moved to the airing cupboard, and weather compensation was set by room rather than by boiler temperature.
The result was a heat pump running at 45°C flow temperature in winter, with no backup electric heater in normal operation. The household kept the original sash windows and repaired them instead of replacing them, which preserved the façade and avoided a large embodied-carbon cost.
What other owners can copy
- Ask for a room-by-room heat-loss survey before accepting a heat-pump size.
- Treat insulation, draught-proofing and radiator upgrades as one package, not separate projects.
- Keep a record of flow temperatures and electricity use for the first heating season; that data is more useful than a showroom rating.
The bananzia plan is not a universal template. It worked here because the house’s worst losses were identified first and the heat pump was sized last. For other owners, the lesson is narrower and more useful: measure the building before buying the equipment.