Your recent article on the Code for Sustainable Homes (18 April, page 52) makes a good point. There is indeed much confusion around the code and SAP ratings, mostly because, in important areas, the formulae used in SAP are based on historic data and are not up to date with advances in technology.

For those interested in carbon reductions it is important not to take the output from SAP too literally. This is certainly the case with regard to the use of air-source heat pumps.

We have developed a package with Mitsubishi Electric that combines its air-source heat pump and our water storage cylinder. The package has been independently tested by BRE, which measured its performance over a wide range of operating conditions. These show an annualised coefficient of performance (COP) of about 3.5. However, in SAP, the COP of an air-source heat pump is fixed at a historical maximum of 2.5 and cannot be changed. Furthermore, SAP makes a number of assumptions about the operation of the heat pump that do not apply when connected to an intelligent water storage cylinder.

A COP of 3.5 means that for every 1kW of electrical energy used to power the heat pump, it will generate 3.5kW of heating energy when averaged over the year. This makes it far more efficient than a gas boiler, which at its very best will operate at a maximum efficiency of 93%.

Things are certainly not always as they seem and until SAP is updated it is necessary to look at fundamentals to understand how to achieve genuine carbon reductions. Our advice is to talk to as many experts and suppliers as possible and to use tests and studies carried out under real world conditions.

Mark Foster, managing director, Gledhill Water Storage