## 4 Results and Discussion
### Discussion  
Heat pumps operating at the levels observed in recent UK field trials are disappointing and demonstrate that with the current emissions intensity of the electricity grid, heat pumps aren't capable of contributing significantly to CO\({}_{2}\) reductions when considered as a replacement for gas boilers. Where heat pumps do demonstrate benefits, even operating at these disappointing levels of performance, are as a replacement for electric resistive heating. This is clearly illustrated in Figures 4 and 5 by the fact that the heat pump performance distributions are significantly above those for electric heating in comparable properties.  
By evaluating UK Government strategy in depth [4,5], it is found that the probability of energy consumption and emissions for a property fitted with ASHP or GSHP is around half that of the mean of the resistive heating alternative, _i.e._, 7500 kWh and 4000 kg CO\({}_{2}\) respectively, is shown in Table 8.  
It is relevant to evaluate this subset of heating type, as electric non-central heating falls into the Government classification of a Hard to Treat (HTT) or Hard to Heat home [30]. Furthermore, as identified in Section 4.2, a greater than 40% reduction in energy consumption is indicated with a heat pump, even when operating at poor levels of performance, which for a household living in fuel poverty would be a welcome saving. From the EHCS it is possible to identify 451 households (3% of the EHCS dataset) which have electric heating and are in fuel poverty, and a further 4.5% [30] are not classed as fuel poor but still have electric heating. Extrapolating these values suggests that 639,000 of the total English housing stock fall into the fuel poverty subset with a further 1 million utilising electric resistive heating.  
By considering hard-to-heat and fuel poor households, it is possible to develop a prioritisation plan for heat pump roll-out that does not necessarily require the highest performance efficiencies in order to  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Heating Technology** & **Probability of Annual Energy** & **Probability of Annual** \\
**and Efficiency** & **Consumption Below 7500 kWh** & **Emissions Below 4000 kg CO\({}_{2}\)** \\ \hline Resistive Heating (100\%) & 12\% & 10\% \\ ASHP (215\%) & 64\% & 54\% \\ GSHP (234\%) & 72\% & 60\% \\ \hline \hline \end{tabular}
\end{table}
Table 8: Probability of heat pump energy consumption and CO\({}_{2}\) emissions being half of that of resistive heating alternative.  
deliver savings in both energy consumption and CO\({}_{2}\) emissions. Phase 1 would begin with the 639,000 homes electrically heated homes in fuel poverty. The followed phase would include the remaining 1 million electrically heated homes. The net and relative emission reductions across the English housing stock are shown in Table 9; this shows that even heat pumps performing at or below efficiency levels recorded in UK field trials could deliver real emissions savings.  
It should be noted that in the wider international context, domestic air conditioning cooling loads can be significant for regions with warmer climates than the UK. For these regions, the potential for a reduction in electricity consumption via the replacement by GSHPs of both resistive heaters and electric chillers may be significant where HPs provide heating in the winter and also cooling in the summer. Although outside the scope of this paper, future analysis for relevant regions may prove fruitful.