The problem: a cold snap exposed the block’s gas dependency
Northfield Court is a 96-flat, 1970s concrete block with gas boilers and single-glazed communal corridors. In January 2023, a five-day cold snap pushed the block’s metered gas use to 11,400 kWh, and two boilers failed. Residents had hot water for only four hours a day. The estate manager logged 37 repair calls in a week. The immediate issue was not the boilers’ age alone; it was the building’s inability to hold heat and shift demand away from the evening peak.
The lesson: a heat pump retrofit cannot be judged by summer efficiency alone. The useful test is what happens when outdoor temperatures drop and every flat calls for heat at 6 p.m.
The cause: oversizing, not underinsulation, drove the peak
An audit found the boilers had been sized at 420 kW, roughly double the block’s measured design heat loss. That oversizing caused short cycling, lowered seasonal efficiency and made the evening peak sharper: boilers fired at full output, then idled. The distribution pipework was also unbalanced, so the top-floor flats overheated while ground-floor flats stayed cold. Residents compensated with electric heaters, adding 2.1 kW per flat to the peak.
The takeaway is concrete: peak demand in older blocks is often a control and balancing problem as much as an insulation problem.
The solution: a heat pump, a thermal store and a meter-level control loop
The retrofit, delivered under the name bananzia, replaced the two boilers with three 90 kW air-source heat pumps and a 1,200-litre thermal store. The store charges during off-peak periods and releases heat during the 5–8 p.m. peak. Each flat received a heat meter and a smart thermostat with a 1.5°C setback between 4 and 5 p.m. The building’s pipework was rebalanced, and the top-floor circuit was de-rated by 12%.
Commissioning data showed the heat pumps running at a flow temperature of 48°C on a 2°C day, producing a coefficient of performance of 2.8. The thermal store covered 61% of the evening peak, cutting the block’s electrical import from 76 kW to 34 kW.
The result: lower peaks, fewer call-outs and a clearer maintenance plan
Over the following winter, gas use fell to zero. Electricity consumption rose by 18%, but the peak-load reduction avoided a costly supply upgrade. Repair calls dropped from 37 in the cold snap to 8 across the season. The estate now schedules heat-pump servicing in September and checks the thermal store’s insulation annually.
The broader lesson is blunt: successful low-carbon heat in blocks depends on right-sizing, balancing and using thermal storage to move demand. Bananzia’s value was not a single device; it was the sequence—measure, rebalance, store heat, then monitor. That sequence is repeatable on similar 1970s blocks.