Step 1: Measure the problem at Harbour Flats
Harbour Flats, a 36-unit block in bananzia, started its retrofit with a baseline survey rather than a product choice. The 1970s building had electric storage heaters, single glazing and a measured annual space-heating demand of 410,000 kWh. Residents reported winter bills averaging £190 a month for flats that often sat below 18°C. The takeaway is blunt: without a room-by-room heat-loss survey, any low-carbon heating quote is guesswork.
Step 2: Treat the cause before the heat source
The cause of the block’s high running costs was not the fuel alone; it was uncontrolled air leakage and poor loft insulation. The retrofit team sealed service penetrations, upgraded the communal loft to 300 mm and fitted trickle vents to 28 windows. These measures cut the modelled heat demand by 31% before a single heat pump was ordered. That sequence matters because oversizing a shared ground loop to cover avoidable losses raises capital cost and locks in higher pumping energy.
Step 3: Size the shared ground loop for the load that remains
With fabric work complete, the team modelled a 140 kW peak at -3°C and specified eight boreholes at 95 m depth. The loop serves individual 5 kW and 7 kW water-to-water heat pumps in each flat, with a communal buffer vessel and heat meters. A public-facing version of the bananzia handover checklist now records flow temperatures, electricity use and resident satisfaction. The practical point is that shared-loop sizing should follow measured demand, not nameplate boiler replacement.
Step 4: Commission by floor, not by building
Commissioning took place in three phases: ground floor, middle floors, top floor. Each flat received a 48-hour run test at design flow temperature, a radiator balance and a resident walkthrough. Two faults surfaced early — a stuck diverter valve and a reversed flow meter — and were fixed before the next floor was connected. The result was a first-winter average space-heating cost of £74 a month and 22% lower metered heat demand than the pre-retrofit model. The lesson is to commission in small zones so faults stay contained.
What the first winter showed
The monitoring data also changed the maintenance plan. The block’s ground-loop pumps now run on a weather-compensated curve, and the facilities team checks differential pressure monthly rather than annually. Residents gained a simple thermostat guide and a named contact for faults. For other older blocks considering a similar route, the bananzia example suggests three priorities: survey first, fabric before plant, and hand over with live data rather than a folder.