## 4 Results and Discussion
### CO\({}_{2}\) Emissions for Individual Properties  
When CO\({}_{2}\) emissions are considered, by contrast the performance of the heat pumps has a very significant influence on the overall performance of the properties. Using CO\({}_{2}\) emission factors of 0.545 and 0.185 kg CO\({}_{2}\) per kWh of grid electricity and natural gas respectively [24], the distributions shown in Figure 5 are produced. These distributions illustrate that with heat pumps operating at the levels of efficiency indicated by the field trials, total property CO\({}_{2}\) emissions are very similar to that of a gas boiler fitted property. Looking closer at the results in Table 5, both heat pump technologies show higher CO\({}_{2}\) emissions than gas boiler fitted properties, with mean annual emissions of 4485 kg CO\({}_{2}\) per year compared with 4858 kg CO\({}_{2}\) for ASHP and 4512 kg CO\({}_{2}\), an 8% and 1% increase in emissions respectively. This similarity between gas boiler and heat pump emissions is due to the fact that the emissions factor for the electricity grid is around 3 times that of natural gas; therefore with heat pumps operating at 3 times the efficiency of gas boilers the distributions are similar. Referring to the cumulative distribution curve, it can be seen that 70% of properties have emissions at or below the headline mean values which illustrates that focusing solely on mean values is not sufficient for an accurate analysis. Further studies of the data may be able to identify the characteristics of the 30% of properties that are above the mean in order to better target efficiency and technology measures that may have greatest impact. When the distributions are compared with electric resistive \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Heating Technology**} & \multirow{2}{*}{**Mean Efficiency**} & \multirow{2}{*}{**Mean Annual CO\({}_{2}\) Emissions (kg)**} & \multicolumn{2}{c}{**Percentage Increase**} & \multicolumn{1}{c}{**Percentage Decrease**} \\  & & & **Increase Compared with Gas** & \multicolumn{1}{c}{**Compared with Resistive Electric**} \\ \hline Gas Boiler & 76\% & 4485 & - & - \\ Resistive Electric & 100\% & 8533 & 90\% & - \\ Air Source HP & 215\% & 4858 & 8\% & 43\% \\ Ground Source HP & 234\% & 4512 & 1\% & 47\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Annual CO\({}_{2}\) emissions for properties fitted with different heating technologies. Figure 5: Annual CO\({}_{2}\) emissions for heat pumps operating at field study levels of performance. heating, there are significant reductions in emissions of between 43%-47% with heat pump fitted properties as shown in Table 5.  
In order to evaluate the impact of heat pump operational efficiency on property CO\({}_{2}\) emissions, further analysis was conducted by adjusting the input mean of the MC distribution as described in Section 3.2 with the values shown in Table 1. The outputs of this sensitivity analysis are illustrated in Figure 6. As observed previously for ASHPs operating at performance levels recorded in field trials, CO\({}_{2}\) emissions are similar to or worse than a gas boiler fitted property. When performance is assumed to be at the level assumed by BREDEM and SAP models (250%), emissions are still only comparable with a gas boiler operating at current mean levels of performance, _i.e._, 76% efficient. If performance levels are assumed to be those expected in 2030, _i.e._, 450%, there is a much greater reduction in emissions. Operating at this level of performance, mean annual emissions reduce to around 2840 kg CO\({}_{2}\), a fall of 40% when compared with a gas alternative. In addition, the cumulative distribution indicates that 70% of properties would emit less than 3000 kg CO\({}_{2}\). From a probabilistic perspective, at current grid emissions intensity, the cumulative distributions indicate a 70% probability that a given property will emit less than 2800 kg CO\({}_{2}\) (the mean value for a 450% efficient heat pump property) if they are fitted with a heat pump operating at 450% efficiency.