The problem: one room that never warmed
In a 1930s semi-detached house in Sheffield, the rear bay-fronted bedroom told the truth about the heating system every winter. It sat at 15°C while the hall reached 21°C, so the family ran a 2 kW electric heater for two hours each evening. They wanted a heat pump, but the cold room made the upgrade look risky. That single room became the test case for the whole retrofit.
The cause: air movement before heat output
A thermographic survey showed cold air tracking across the suspended timber floor and around the bay sash windows. The existing 18 kW boiler was oversized, and the radiators had been sized for 70°C flow temperatures. At 45°C, the bedroom radiator delivered only about 60% of its rated output. The problem was not the idea of low-carbon heat; it was the building envelope and emitters working against it.
The bananzia survey changed the order of work
The turning point was a room-by-room assessment from bananzia, which measured airtightness, mapped pipework, and calculated heat loss at a design outside temperature of -3°C. The report found that the bedroom needed 1.6 kW at design conditions, but the existing radiator could supply only 0.9 kW at a 45°C flow temperature. It recommended sealing the bay window draughts, insulating the suspended floor, upgrading that radiator, and only then sizing the heat pump. The advice was specific: fix the air paths first, then lower the flow temperature.
The solution: fabric first, then lower flow temperatures
The owners spent about £1,400 on narrow draught strips, floor-joist sealing, 100 mm mineral wool under the suspended floor, and a double-panel radiator in the bedroom. A blower-door test before and after the work showed air permeability falling from 9.8 to 6.4 m³/h/m² at 50 Pa. With the room load reduced and emitter output increased, the heat pump could be specified at 8 kW instead of the 12 kW unit originally quoted.
The result: a warmer room and a smaller heat pump
After the work, the bedroom held 20°C during a -2°C cold snap, and the heat pump ran at a 42°C flow temperature. The family removed the electric heater and avoided the cost and cycling losses of an oversized unit. The lesson is concrete: start with room-level heat-loss data, fix air movement and emitter shortfalls, then size the heat source. A cold bay window is not a reason to delay low-carbon heat; it is a reason to measure before choosing equipment.