Monitoring the energy performance of a Code 6 homeJuly 2013

Just how zero carbon is Code 6? James Parker evaluates the energy performance of a parsonage built to the Church of England’s parsonage design guide.

What really is zero zarbon? A question that many have been asking over the last few years as the debate over the definition rages on. But this is also a question that many clients ask when procuring new buildings with the zero carbon tag. On the domestic level, zero carbon is often accredited using the environmental rating scheme the Code for Sustainable Homes, where dwellings reaching Code Level 6 reach zero carbon on the energy front.

Just over one year ago BSRIA embarked on a project with the Diocese of London to see just how zero carbon Code 6 is. The project was to monitor their latest vicarage, a Code 6 house at St John’s Church in Wembley. The aim was to put Code 6 under the looking glass and examine whether it is an effective strategy for the Diocese on future developments. The church in London has ambitious targets of 42 per cent reduction of CO2 emissions by 2020, and 80 per cent by 2050: ambitious as their building stock contains some of the capitals oldest and most treasured buildings, such as St Paul’s Cathedral.

Background

BSRIA’s initial involvement with the vicarage was as a Code for Sustainable Homes advisor for the diocese through their agent – Wilson Stephen Associates, assisting with the initial design requirements and monitoring the construction process. The vicarage was part of a wider development on church land constructed by Galliford Try Partnerships Ltd, including a new community hall and 20 affordable homes.

The new vicarage is miles ahead, in terms of energy performance, from the previous 1970s vicarage on the same site. The most obvious feature highlighting the green credentials of this otherwise traditional looking house is the 62 m² array of photovoltaic panels generating 8.5 kW peak, although, even these panels blend into the roofline, not like the eco-bling expected of trailblazing eco-houses. Perhaps this is one of the greatest achievements of this development: building a Code 6 house that looks very traditional. This may be because the Church of England’s parsonage design guide (written in 1998 and confusingly referred to as the green guide) which requires many traditional build techniques such as masonry walls (internal and external) together with an expected lifespan of 200 years.

While the walls are of traditional masonry construction materials, the construction is a little different. A wall thickness of over 400 mm is the first clue, with double layers of insulation in the cavity and carbon fibre wall ties used to reduce the thermal bridging. Triple-glazed windows help to further reduce heat loss, and rainwater harvesting helps to reduce the use of potable water. Heating is provided by a ground source heat pump via underfloor heating throughout, together with a mechanical ventilation heat recovery unit to provide useful ventilation while minimising heat loss.

The building was assessed under the May 2009 version of the Code and the 2005 version of SAP for the energy assessment. It scored a total of 93 per cent in the Code assessment, which led to the project receiving a BREEAM award from BRE. The SAP was an A rating with a 118 per cent CO2 reduction (ie dwelling emission rate over target emission rate).

Monitoring

In order to look at the situation in detail, BSRIA added monitoring equipment in line with current TSB BPE programme standards. This included installing electricity meters to the ground source heat pump and Mechanical Ventilation with Heat Recovery (MVHR) unit. Temperature, humidity and CO2 sensors were installed in the lounge, study and a bedroom to monitor environmental conditions. In addition temperature sensors were added to the ducts of the MVHR, ie both supply and extract to/from the dwelling and atmosphere. Finally the rainwater harvesting system was monitored through a meter on the mains top-up pipework, and the electricity generated from the photovoltaic panels was taken from the export meter.

Unfortunately it was not possible to install our own meters on the incoming electricity supply, so the analysis was left to manual meter readings and electricity bills.

Electricity

The annual electricity consumption is around 13,500 kWh excluding the amount generated by the PV. This gives an energy intensity of 63 kWh/m², which compares well against the benchmarks. Data from the English Housing Survey 2010 show that post 1990 built dwellings are typically 250 kWh/m² per annum.

The PV generated 8023 kWh over the year, some 10 per cent more than predicted by SAP, which is good on two fronts; 1 – that SAP appears to be reasonably accurate and 2 – the installation generated more than predicted in a year that had the dullest summer since 1987 and the wettest in 100 years.

Taking the PV out of the total consumption brings down the energy intensity to 25 kWh/m² per annum. While not zero carbon, this means that the total annual CO2 emissions are below 3 tonnes a year or 13.4 kgCO2/m² per annum. The Zero Carbon Hub has best practice benchmarks of 10 kgCO2/m² per annum for regulated loads in detached houses, while data from DECC’s housing energy fact file produced benchmarks of 12.7 kgCO2/m² per annum for appliances and 2.2 kgCO2/m² per annum for cooking. This would give a combined total benchmark for regulated and unregulated loads of 24.9 kgCO2/m² per annum. This means that the vicarage is performing at just over half the benchmark for emissions.

The areas performing above the SAP predictions are the energy consumption of both the ground source heat pump and the MVHR unit. This is not the first site to experience difficulties with these technologies. Looking through the back catalogue of a number of journals, magazines or news websites, it isn’t difficult to find a story about nonperforming installations. But what is the problem? Well for the vicarage this isn’t known without further investigation, but typically it is related to incorrect operation or commissioning issues.

One issue that can be confirmed is the additional energy consumed by the electric heater within the heat pump. The monitoring has shown that this is coming on over significant portions of the year, mainly in the colder months. Why is this? Well there are a number of possible reasons, such as controls, sizing of the heat pump, storage size. As can be seen in the graph of instantaneous power, the heater mainly comes on during the heating season.

Comfort

A building is a sum of its parts, hard materials and services. But a home is a place where people feel comfortable. So, naturally occupant comfort is an important factor in producing a sustainable home. A vicarage is not typical in terms of home occupancy patterns as it is a work-live set-up, leading to higher than average occupancy hours. This may have led to part of the difference between the SAP calculation energy usage and the actual consumption.

Temperature measurements show that the internal temperatures remained relatively stable at around 24°C. During the warmer periods of the summer the internal temperatures never reached the highs of the external temperatures, but still increased. Conversely during the cold spell at the beginning of 2013 internal temperatures remained comfortable, and only dropped below 20°C in the study and bedroom during periods when the building was unoccupied. This was confirmed by the occupant interviews. When asked about the temperature in winter the response was: “excellent, we never feel cold in the house.” They also felt that the air quality was “definitely better than average.” They do however like to open the windows both in summer and winter. “In summer we needed to open the windows as we feel we need more fresh air in the house,” the vicar commented. But added, “with the windows open we felt the air quality was sufficient.” Talking about colder times of year he said, “I think overall in the winter it is a very comfortable place to live.” The amount of natural light was also a positive point for the occupants, as was the image. “Most people who come to the ground floor think it is the best house they have ever been to. It has a very good image to visitors. Sometimes people even take pictures of the house.”

The biggest complaint from the occupants is the lack of storage space and shapes of some of the bedrooms. The master bedroom is too long and narrow and doesn’t work very well.

But the real positive aspect has been how moving into the new vicarage has affected the way the vicar and his family think: “if you live in this house you become very sensitive about how much water and electricity you are using. It really makes you think about what you are using”.

The overall view

So while the building has not matched the predictions of the assessment software, this is not an uncommon occurrence. SAP makes many assumptions, which do not necessarily match the operational nature of the building. Other teething problems, such as issues with the heat pump, can compound the increase in energy consumption over the predicted values.

But is the project a success? Well from the occupant point of view it is a building they like. It has made them more aware of their energy consumption, and is a pleasant environment to live and work. These positives can be built on to help drive the energy consumption down, and learn lessons for future developments.