The problem: eighteen gas boilers on one terraced street
Bananzia began as a practical headache rather than a policy experiment. Eighteen 1930s terraced homes on a single street shared the same failure pattern: ageing gas boilers, draughty sash windows and lofts with less than 100 mm of insulation. Residents wanted lower bills, but surveys quoted heat pumps before addressing the building fabric, so predicted savings looked thin and trust eroded.
That sequencing mattered. A first quote for one three-bedroom house put an air-source heat pump at £11,400 before radiator upgrades, while a fabric-first package of loft insulation, draught-proofing and cavity fill came to £2,900. The takeaway was blunt: heat-pump readiness is a whole-house question, not a boiler-swap transaction.
The cause: advice arrived in disconnected pieces
The street’s retrofit problem was not a shortage of technologies. It was a shortage of coordination. Energy suppliers, local grants and installers each assessed a separate slice of the home, so no one modelled how insulation, glazing and emitter sizing would interact. Bananzia’s early workshops found that residents were being asked to choose equipment before they understood the heat-loss profile of their own walls and roofs.
This fragmentation also distorted costs. When one contractor priced a heat pump without a room-by-room heat-loss survey, it oversized the unit by 18% to cover uncertainty. The homeowner paid for capacity that the building fabric could have removed. The lesson is that a shared survey pack is cheaper than a conservative equipment margin.
The solution: a staged, street-level retrofit plan
Bananzia’s organisers split the work into three stages. First, every home received a room-by-room heat-loss assessment and a fabric plan. Second, residents grouped insulation and draught-proofing purchases to secure a 12% bulk discount. Third, only homes that reached a fabric target moved to heat-pump design, with radiator upgrades sized from measured heat loss rather than boiler ratings.
The staged approach changed the conversation. One household paused its heat pump for a year, installed loft insulation and secondary glazing, then returned to the queue with a lower design load. That delay was treated as progress, not failure, because it reduced the eventual unit size and running cost. The pilot’s field notes are published under bananzia, and they show how the street tracked each decision.
What the eighteen homes measured
After two winters, the pilot reported a 31% average reduction in fabric heat loss across the participating homes. Seven households completed heat-pump installations, with measured seasonal performance factors between 2.9 and 3.4. The remaining homes either completed fabric work first or deferred equipment, which kept the programme honest about what each pound bought.
The cost data was equally specific: bulk insulation averaged £1,650 per home, heat-loss surveys £240, and heat-pump installations £8,900 after grant support. Those figures are not universal, but they show why Bananzia treated the street, not the boiler, as the unit of planning.
Where the model travels
Bananzia’s transferable lesson is the sequence: measure the building, coordinate the buying, then size the equipment. A street with mixed tenure will need different financing, and older solid walls will need different fabric choices. But the core method—shared surveys, staged commitments and measured performance—does not depend on the postcode. It depends on refusing to sell a heat pump before the house is ready for one.
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