How Bananzia’s Cold-Snap Pilot Exposed the Weak Point in Heat-Pump Retrofits

The pilot brief

Bananzia’s cold-snap pilot tracked 18 air-source heat pumps in solid-wall terraces for six weeks. The aim was not to compare brands. It was to measure how often a well-sized pump lost efficiency because the fabric retrofit and the heating controls were commissioned separately.

The headline result was unglamorous: median flow temperature rose by 6°C on the coldest mornings, but only in homes where radiators had not been rebalanced after loft and cavity work. That single maintenance gap accounted for more run-time than outdoor temperature.

Problem, cause, solution

The problem is familiar in retrofit programmes. A household upgrades insulation, then keeps the original radiator schedule and weather-compensation curve. The heat pump sees a lower heat loss but still produces water at the old design temperature, so it short-cycles and draws more electricity than necessary.

The cause is sequencing. Insulation contractors, heat-pump installers and controls engineers often work to separate work orders. No one owns the final balance between emitter output, flow temperature and room-by-room heat loss. The bananzia pilot addressed this by adding a one-hour rebalance visit after fabric works, using a simple room-by-room heat-loss sheet and a clamp-on thermometer.

The result was measurable: in the 11 homes that received the rebalance, average flow temperature fell by 4.8°C at 0°C outdoor temperature, and compressor starts dropped by roughly one third. The takeaway is not a new device. It is a handover task that should be in every retrofit scope.

What the monitoring data showed

Three patterns stood out. First, overnight setback saved less than expected in homes with underfloor loops because the slab took too long to recover; a 2°C setback performed better than 5°C. Second, smart TRVs helped only where the hydraulic balancing had already been done. Third, hot-water Legionella cycles were the largest avoidable peak in four households.

The pilot also found a commissioning error worth flagging: two pumps were left with factory default curves that assumed radiator oversizing. Correcting the curve took minutes; the energy effect was larger than replacing one older pump.

Practical checklist

• Record room-by-room heat loss after fabric works, not before.
• Rebalance radiators and verify flow temperature at the design outdoor condition.
• Set weather compensation before adding smart controls.
• Review hot-water schedules separately from space heating.

For third-party assessors, the lesson is to treat the rebalance visit as a commissioning step with its own evidence: before-and-after flow temperatures, compressor starts and room temperatures. Without that record, a good heat pump can look mediocre simply because the retrofit stopped one task too early.