EPC software seems unable to handle eco improvements
Posted by Martin Hodnett on 2 August 2012 at 10:44 am
We live in an old Devon longhouse constructed of cob. An Esse wood burning range cooker, a solar thermal panel system, and an oil-fired boiler (as backup for very cold weather) heat an 850l thermal store, which runs the central heating (radiators), with weather compensation, and provides mains pressure hot water. There are also two wood burning stoves, a 2kW solar PV system and a 6 kW wind turbine. The attic space is well insulated, the cob walls are very thick and there is some double glazing.
The first run with the EPC software gave the house a rating of 46, which is the middle of band E. This meant that we would not be eligible for the FIT for the new PV system. The EPC assessor and I were more than a bit surprised! The first assessment was based on the house being entirely heated by wood.
We then told the software that the house was heated using oil, which took the score to 58 and band D. It was only then that I realised several things:
- the EPC rating is based on heating cost – not carbon emissions,
- the EPC software costs the wood-fuel based on open market prices
- the EPC software uses an assumed efficiency for a wood-fired boiler
Burning wood for fuel keeps your carbon emissions down, but increases the EPC rating because wood is an expensive fuel, and the software assumes a relatively low efficiency for the boiler. In our case, we use wood for fuel because it comes from the maintenance of our own hedgerows and costs us our own labour, and fuel for the chainsaws and tractor to transport it.
The EPC figure still seemed remarkably low, so the assessor and I went through the various components of the software to ensure that the more unusual “eco” items had been entered correctly. He was at his PC in his office and I was at home on the phone to him. There were various items that the software could not handle. The particular issues that we found were:- the largest size of thermal store in the software was 170 l – some stores (or accumulator tanks) for large properties can be up to 2,000 l in size.
- For the wind turbine, the hub height and the rotor diameter had to be entered, but these were limited to a maximum of 5 m and 2m respectively. The hub height was the height above the roof ridge. A 6kW “domestic” wind turbine has a typical hub height of 15 m and a rotor diameter of 5.7 m, and is also not mounted on the roof of the house! As a result of the assumptions about the mounting height and diameter of a “domestic” turbine, the software cannot correctly assess their contribution to running costs.
- There was no option to select a range cooker as the source of heat to the central heating system, and no efficiency data for it either
- When specifying the solar thermal system, there was no option to select compound parabolic collector (CPC) panels. The only options were flat panels (glazed or unglazed) and evacuated tubes. CPC panels have a higher performance than flat panels. The collector area that could be selected was also limited, and much less than the area of panels that we have. As a result, the incorrect data about the solar thermal system made no difference to the EPC rating.
These limitations meant that it was not possible for the EPC software to correctly assess the EPC rating of this property. In order to get a reasonable figure out of it, we had to pick the highest values that the software would let us enter, but these would have considerably underestimated the contribution of these technologies to reducing heating costs.
The rating did improve, but the factor that made the biggest difference was specifying the oil fired boiler type (a Grant Multipass), rather than a generic oil boiler. The EPC then jumped to 86, which is a much more satisfactory band B, but it would appear that, had the software been able to correctly handle the “eco” technology that we have installed , we might have achieved an A rating!
The EPC software probably works extremely well for the majority of properties, but it is of concern that it cannot handle eco technology properly.
About the author: Martin Hodnett is a retired research scientist, with an interest in green building.
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