## 4 Results and Discussion  
### Validation  
Before analysing the output data in depth, validation of the SAP-based model was carried out. Comparing the material and ventilation heat loss values for 2006 from the Domestic Energy Fact File 2008 (DEFF 08) [28] with those from the model, a total variation of around 2% is evident. This indicates an acceptable level of accuracy. The large discrepancy for the heat loss value for the roof could be down to the fact that the assumption for the roof area may be incorrect as well as the assumption that the entire roof area has the same level of insulation. Despite this discrepancy, the other individual values and the total are within acceptable limits of the DEFF 08 values. A heat loss value that is included in the model that is excluded from DEFF 08 is that for thermal bridging. At a mean of 30 W/K, this value does make a reasonable contribution to the mean total heat loss parameter (280.7 W/K). This is an oversight that should perhaps be considered with later publications of DEFF. A second output parameter that bears comparison with DEFF 08 values is that for total annual energy consumption per household for that part of the model that considered a gas boiler installation at a mean efficiency of 76%. As can be seen in Table 3 there is only 1% discrepancy, thus once again indicating the model is producing meaningful results. [PERSON] [21], using an older EHCS dataset from 2001 and a modified version of the BREDEM-8 model called CDEM, showed an average dwelling heat loss value of 261 W/K and an annual energy consumption of 23,989 kWh across all 21,000 dwellings in the dataset. The model developed for this study compares well with this, as properties have seen efficiency improvements over the intervening years between the two EHCS studies. Furthermore, DEFF 08 gives values for 2001 as 268 W/K and 22,472 kWh respectively, both values very close to those from [PERSON], indicating that DEFF 08 is a reasonable benchmark to evaluate the validity of BREDEM based models. A further validation for DEFF being used as a benchmark is found in work by ECI [29] which gives annual household energy values from two different models of 22,606 kWh for 1996 and 22,335 kWh for 2000 compared with DEFF values of 23,139 kWh and 21,964 kWh respectively--a 2% and 3% deviation from DEFF. Whilst not the primary focus of this study, it was considered important to evaluate the total energy consumption per property, including all thermal energy requirements, along with lights and appliances as calculated by the SAP model. When total annual energy consumption is evaluated, the results highlight that even poorly-performing heat pumps contribute to a significant reduction in total consumption per property. This is illustrated in Figure 4, where the maxima for both heat pump technologies is significantly lower than those for electric and gas heating respectively. The cumulative distribution also identifies that around 85% of heat pump-fitted properties will consume less than 10,000 kWh energy compared with only 15% of gas boiler fitted properties and 30% of electrically heated properties. Figure 4 also shows the mean annual energy consumption for heat pump-fitted properties as being between 8200 kWh (GSHP) and 8900 kWh (ASHP), whereas that for gas boiler fitted properties is 20,400 kWh. This is a reduction in annual energy consumption of between 56% and 60% when compared with gas boilers. A further comparison was made with electric resistive heating operating at 100% efficiency, this being representative of properties that are not on the gas distribution network. For these properties, the mean annual energy consumption is 15,650 kWh. This represents a 43% and 48% reduction in consumption for ASHP and GSHP-fitted properties respectively (Table 4). Although non-thermal energy use is included in the total, this on average is around 800 kWh (based upon the floor areas of the properties) and so only has the effect of shifting the curves slightly to the right.