The Proof of the Pudding is in the Heating: A Field Experiment on Household Engagement with Heat Pump Flexibility

As renewable energy grows, flexible electricity demand becomes essential. We conducted a field experiment with nine heat pumps in well-insulated homes near Ghent, Belgium. During 287 flexibility interventions, we remotely deactivated heating until indoor temperatures reached predefined thresholds or households manually overruled the intervention. After initiating a flexibility event, the heat pump power is initially lowered by 250Won average per unit in the fleet. As some heat pumps in the fleet reactivate, they consume more power to restore their threshold temperatures, triggering a rebound effect that gradually reduces net power savings achieved. On average, net power savings become zero after 18 hours, followed by a rebound period. Overall heat pump consumption was reduced by 1 kWh per event, stabilizing 36 hours after the event start. If flexibility activation is timed strategically, up to €1.1 can be saved through price arbitrage, assuming energy-crisis-level wholesale prices. Colder weather significantly influences savings, by increasing heat pump power available for flexibility but also amplifying rebound effects. This flexibility came with moderate comfort impacts: on average, indoor temperatures were 0.38°C lower during interventions. However, 19% of interventions were manually overruled when larger temperature drops occurred, with households citing discomfort, illness, or occupancy as factors on an online dashboard. These findings suggest that flexible residential heating can support renewable energy integration with moderate comfort impacts.