Scotland has an ambitious target to reduce all greenhouse gas emissions to net-zero by 2045. This cannot be achieved without major cuts in emissions from heating buildings, and heat pumps are the main contender for reducing emissions from heating.

We carried out detailed modelling for WWF-Scotland and the Scottish Government, examining the most cost-effective ways to convert Scottish homes to zero-carbon heat pumps. We used dynamic simulations, with hourly calculations through the year, to explore cost-optimal combinations of different heat pump configurations and energy efficiency upgrades. We used 12 different house types, developed to be representative of 93% of Scottish homes. We also examined five different alternative heating systems: air-source heat pumps, air-to-air heat pumps, ground-source heat pumps, electric storage heaters, and radiant electric heaters.

The modelling showed that for most gas or oil-fired homes in Scotland, the most cost-effective low-carbon alternative over 15 years was using an air-source heat pump, with improved air tightness in nearly all cases, and top-up loft insulation for homes with less than 300mm of loft insulation. We also found that it was economically viable to install cavity wall insulation in dwellings with cavity walls that do not already have this, along with double-glazing for homes without double-glazing.

Our work showed that the most cost-effective way to control and configure heat pumps was with high set-back temperatures (target temperature when occupants are out or asleep) – typically 18C. This avoids the problem of high demand for heat for homes to reach comfort temperatures in the morning or when people return home, which means that lower, more efficient, flow temperatures are viable, and also that smaller heat pumps can be installed.

We found that running costs with heat pumps (and parallel improvements in insulation/airtightness) were often lower than with electric storage heaters, oil or gas heating, but that up-front capital costs are higher than simply replacing boilers. This is why the Scottish Government should continue with grants to help households meet capital costs.

We also found that around 14% of the Scottish housing stock has space constraints that would make it hard to install an air-source heat pump. For these homes we found that air-to-air heat pumps (with internal units in each room, and no radiators) were a better solution.

We recommended the Scottish Government adopts a new metric for homes in terms of annual energy demand for space heating demand per m2, and the Government should promote a suggested target range of 65-85kWh/m2, with exemptions for heritage buildings with solid walls where limited space means that internal wall insulation is not possible.

Our report is available here.

WWF’s interpretation of our work, and their suggestions for Scotland’s next steps in this area, are here:

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