One of the six Sutton homes in the Energiesprong whole-house retrofit trial has had its exhaust-air heat pump fully commissioned, and contractors were rolling out the solution to the other five homes, according to an independent report published on 25 July 20221. Residents were reimbursed for extra electricity used during the trial1.
The report, covering ECO3 Innovation Demonstration Action retrofits in Sutton and Nottingham, found that analysis of monitoring data identified that the standard control software was not activating the heat pump1. The Ventive Home heating and ventilation system used in Sutton was described as innovative and, in retrospect, perhaps not sufficiently tested in the field, which led to problems with controls and meant the heat pump was not used, increasing energy use in the Sutton homes1.
Eleven dwellings underwent deep retrofits including super-insulation of walls, roof and floor, and whole-house ventilation and heat pumps for space and water heating1. Six homes were recruited in Nottingham, with five retrofitted, and six in Sutton, all retrofitted1. One upgraded dwelling was vacated during the work and was not used in the analysis1.
Across the ten remaining retrofits, the average improvement in Heat Transfer Coefficient was 33 W/K, a 21% reduction in heat loss1. Including the weak performance of the Sutton heating system, the average bill saving was £201 a year, or 22%, with a minimum cost rise of £6 and a maximum saving of £4081. Had the Sutton heat pumps performed at the same efficiency as those in Nottingham, the average bill saving would have been £426 a year, or 46%1.
"The exhaust-air heat pumps have now been fully commissioned in one house and contractors are rolling out the solution to the other five homes imminently."
| Measure | Before | After |
|---|---|---|
| Mean internal temperature | 19.2°C | 20.0°C |
| Adjusted internal temperature | 19.6°C | 20.0°C |
| Average bill saving (actual) | £201 a year (22%) | |
| Average bill saving (if Sutton matched Nottingham) | £426 a year (46%) |
The report notes that the Ofgem price caps introduced in April 2022 increased gas prices by 103% compared with the SAP prices, and electricity prices by 78%1. Lifetime Bill Savings for the ten dwellings were estimated at between -£90 and £6,110, with a central estimate of £3,020; if the Sutton system were repaired or replaced with a heat pump running as efficiently as those in Nottingham, this would rise to between £4,390 and £8,320, central estimate £6,3901.
Why it matters for households
The Sutton case shows how much of a home's energy independence rests on the heating system working as designed, not just on the fabric of the building. The insulation and ventilation measures delivered a statistically significant reduction in heat loss, but the heat pump controls fault meant residents used more electricity than expected and saw little or no bill benefit1. For a household, the difference between a working and a non-working exhaust air heat pump was £225 a year on average across the trial1.
The report also records that the Comfort Plan, which normally gives tenants a fixed monthly charge for energy services, did not apply in these retrofits at Ofgem's request, and that Energiesprong compensated tenants for any change to energy bills1. The standard Energiesprong warranty guarantees heating to 21°C, 140 litres of hot water per home per day, and 2,300 kWh a year for appliances, with a bill cap typically of 2,000 kWh of electricity a year1. None of these guarantees applied in the same form here1.
What happens next
The report says the fix was being rolled out to the other five Sutton homes imminently as of 25 July 20221. No completion date for that rollout has been reported.