How the boiler escape plan works

Heat demand is the heat the home needs each year. Without an address, the widget starts from typical fuel use and multiplies it by the boiler’s efficiency. With an address, it uses the home’s EPC instead: floor area times the median gas use per square metre of houses with the same rating (from meter readings), converted to heat in the same way.

Keeping the boiler: heat demand divided by boiler efficiency gives the fuel burnt, which is multiplied by the fuel’s carbon factor. This is the same every year.

Switching: heat demand divided by the heat pump’s seasonal performance gives the electricity it uses. That is multiplied by each year’s grid carbon factor, which falls as the grid decarbonises, and the carbon from making the heat pump is added.

The lock-in figure is the difference between the two over 12 years. It is roughly what replacing a broken boiler with another one commits a household to.

The breakdown chance comes from an assumed curve, because no UK data on boiler lifetimes is published. It is a Weibull curve with shape 2.5 and scale 14 years, giving an average life of about 12 years. On it, about 11% of 10-year-old boilers fail within a year, 14% at 12 and 19% at 15.

The readiness score adds up the answers to five questions about insulation, radiators, hot water, outdoor space and the fuse board. It is a guide to next steps.