## 2 Current Situation
### Heat Pump Performance--The Reality  
There is a lack of published field trial data for heat pump installations in the UK, with specific studies focusing on user perception (e.g., internal warmth, ease of operation) and economic benefits rather than heat pumps performance in terms of efficiency [15; 16; 17]. In 2010 the EST [17] published a field trial report on heat pumps in the UK that identified that heat pumps performed at efficiency levels between 160%-220% and 180%-300% for air and ground source respectively, as illustrated in Figure 1. One other study conducted by [PERSON] [18] had results broadly consistent with EST data. When the published studies are investigated more closely, it is concerning that many of the sites studied are clustered geographically and there is no indication whether this may skew the results in terms of ambient temperature, humidity, exposure, _etc_.  
A study in New Zealand [19], which has a similar climate to the UK, investigated the energy performance of ASHP water heaters (_i.e._, no space heating) both in real-world installations and corresponding models, confirmed that efficiency can drop to as low as 100%-150%, for water heated to 60 \({}^{\circ}\)C and for an ambient temperature of 5 \({}^{\circ}\)C. This observation is consistent with other ASHP water heater studies [20] that compared performance with an increasing difference between ambient air temperature and water output temperature.  
Studies carrying out sensitivity analyses on energy efficiency measures and CO\({}_{2}\) emissions [21] identified that modest changes in boiler efficiency (baseline efficiency 70%) of \(\pm\)10% could result in CO\({}_{2}\) emissions varying by 5%. Furthermore, heating demand temperature and external temperature sensitivity analysis also contributed to variations of CO\({}_{2}\) emissions by up to 15% and 5% respectively, however this was only as an influence on heat loss calculations and not as an influence on the heating source efficiency, as in the case of heat pumps.  
The sensitivity analysis conducted by [PERSON] [21], ASHP studies and modelling by [PERSON] [19] and the early EST field trial [17] evidence is of concern because, although some of these discrepancies are recognised in The Renewable Energy Review [6], the Government models still appear to be very  
Figure 1: Energy Saving Trust (EST) field trial results. Data derived from [17].  
optimistic with regards future performance and do not recognise the potential significant impact that poor performance of heat pumps could have on Government renewable and CO\({}_{2}\) abatement targets. Furthermore, the current modelling methods and Government strategies tend to focus more on absolute values rather than looking at the probability distribution of emission levels or market penetration, for example, which is supported in work by [PERSON] [22].  
In order to address these shortcomings, this study will use field trial data along with Monte Carlo analysis of heat pump efficiency and data from the English Housing Condition Survey (EHCS) [12] to ascertain the potential scale of the discrepancies of heat pump performance and introduce CO\({}_{2}\) emissions and energy consumption probability distributions in to the modelling outputs.