A fault in the standard control software supplied with the Ventive heating and ventilation system installed in Sutton homes was identified from monitoring data, and a simple temporary control algorithm was applied. The finding is set out in an independent report on the ECO3 Innovation Demonstration Action Energiesprong whole-house retrofit, covering homes in Sutton, south London, and Nottingham, published on 25 July 20221.
The report states that "analysis of the data identified that the standard control software was not activating the heat pump"1. The Ventive Home heating and ventilation system used in Sutton is 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 those homes1. The report adds that "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"1.
Eleven dwellings underwent deep retrofits including super-insulation of walls, roof and floor, and whole-house ventilation and heat pumps for space and water heating. Six two- or four-in-a-block cottage brick cavity homes were recruited in Sutton, all six undergoing deep retrofit, and six concrete and timber frame terraced houses were recruited in Nottingham, with five retrofitted1. One of the 11 upgraded dwellings was vacated during the work and was not used in the analysis1.
Across the 10 remaining retrofits, the average improvement in Heat Transfer Coefficient was 33 W/K (minimum 5 W/K, maximum 61 W/K), a 21% reduction in heat loss, with p=0.02 for the significance test across the whole cohort1. Including the weak performance of the Sutton heating system, the average bill saving was £201 a year (minimum a cost rise of £6, maximum saving of £408), or 22% (-0.6%/+45%). Had the Sutton heat pumps performed at the same efficiency as those in Nottingham, the average bill saving would have been £426 a year (minimum £292, maximum £555), or 46% (31%/60%)1.
| Measure | As recorded | If Sutton heat pumps matched Nottingham efficiency |
|---|---|---|
| Average annual bill saving | £201 (range: £6 rise to £408 saving) | £426 (range: £292 to £555) |
| Percentage saving | 22% (-0.6%/+45%) | 46% (31%/60%) |
Mean internal temperature rose from 19.2°C before to 20.0°C after the retrofits, which was not statistically significant; adjusting for higher external temperatures in the post-retrofit phase, the report estimates a rise from 19.6°C to 20.0°C1. Lifetime Bill Savings over 15 years for the 10 dwellings are put at between -£90 and £6,110 (central estimate £3,020) based on actual Sutton performance, rising to between £4,390 and £8,320 (central estimate £6,390) if the Ventive system were repaired or replaced with a heat pump running as efficiently as those in Nottingham1.
Why it matters for households
The Sutton case shows how a control fault, rather than the fabric of the building, can determine whether a heat pump delivers. The retrofit itself cut heat loss, but because the standard control software was not activating the heat pump, the homes used more energy than intended and the bill savings were disappointing1. For a household, the practical consequence is that the performance of a heat pump depends on the controls that switch and modulate it, not only on the insulation and the unit itself. The report notes that residents in Sutton were either less comfortable or gave the same rating after the work as before, and that all Sutton households thought energy use was too high after the retrofits, which it links to dissatisfaction with delays and technical problems with the heating system1. In Nottingham, where the heat pumps performed as expected, all residents were more comfortable overall and all were happier with winter temperatures1.
What happens next
The exhaust-air heat pumps have been fully commissioned in one Sutton house, with contractors rolling out the solution to the other five homes imminently, as stated in the July 2022 report1. No completion date for that rollout is given in the report. The report also notes that the HTC calculation had to be finished before Ofgem's 31st March deadline, and that the first units were due in early January but did not actually arrive until late February1.