Air Source Heat Pumps (ASHPs) are part of the solution to decarbonise the residential heating sector. The coefficient of performance (COP) is a measure of the instantaneous efficiency of a heat pump. The heat energy produced by an ASHP is deemed renewable if it meets a specified sustained COP over a period of time, e.g., a seasonal performance factor (SPF). Heat pump performance in situ often differs from laboratory test conditions. This paper explores the performance of ASHPs in a field trial of deeply retrofitted Irish houses. Analysis shows that all houses in the trial qualify as producing renewable heat but vary from the manufacturer’s laboratory test performance. ASHPs with ratings of 8.5 kW (11.2 kW) underperformed against the manufacturers COP values on average by 16 (24%) at outside temperatures of 7 °C, and 3 (11%) at outside temperatures of 2 °C. The paper evaluates statistical and machine learning models, including a random forest (RF) model, to predict COPs and compare the difference between the manufacturer’s laboratory COPs and the trial results.