A Case Study of an Energy Efficient House

Mark and Natalie
Langley Village Macclesfield

Background

Houses and owners differ. They want or need different solutions. This is our story.

We had lived for over 30 years in a beautiful large Victorian House, big rooms, high ceilings, cellar etc. Great in the summer but cold and expensive in the winter even with some insulation and double glazing etc. We were looking to downsize and were able to purchase a 30-year old agricultural building built with planning permission to convert to a domestic dwelling. We also wanted if possible to move into a net zero house….although we prefer the term energy efficient as the meaning of net zero is somewhat disputed. We also wanted a pleasant environment to live in all year round; frosty, windy, wet, hot, whatever the British weather might throw at us.

It was not a new build but change of use of building from stables to residential use. An existing simple rectangular structure with a duo pitched roof but with tight planning control to maintain aesthetics/appearance of the original structure. Therefore, there was no control over orientation, size, form or window layout with the exception of a few additional Velux rooflights. Despite that, the project was able to achieve a high performing energy efficient property.

We were aware of the Chester Road retrofit that Ariva had done and decided to go with Rick and the team together with Richard Gibson as the architect. We were not in the building when the work was being done.

Performance of the Building

The high number of PV panels meant that the space required for inverters and batteries is considerable. Therefore it was necessary to incorporate a first floor plant room within the roof space. Solar PV panels blend easily into a slate roof and add to the appearance of the house.

We moved in in January so have now been in a year. The final thing in order to be fully functional was an export agreement with Electricity North West in order to export back to the grid and then to have this agreed with Octopus. This came through at the end of March. We now have good data for 9 months covering the period April-December.

How has the building performed? It is very pleasant to live in with a similar environment throughout. This is very comfortable compared to houses that can be hot and stuffy in some rooms but cold and/or damp in others. We keep the house between 19-21oC by heating in the winter. In the summer the cooling was able to keep the internal temperature at least 5oC below the outside temperature. The humidity stays between 40-60% irrespective of the external environment or whatever we are doing inside.

The technical detail. EPC is an A with a score of 115. The house operates on the “seal it tight and ventilate right” philosophy. An air pressure test was part of the build to check that the building did not leak. There is 50mm cavity wall insulation and then additional insulation internally on the walls, under the floor and under the roof. The whole house is then sealed.

Heating is provided by ASHP (Air Source Heat Pump) via heating coil being the overarching heating throughout the house with heat recovery via the MVHR (mechanical ventilation and heat recovery) preventing loss of heat. The stale air as it is expelled passes over a heat exchanger such that fresh cooler air is heated before it is pumped round the house. A heating coil is also part of the MVHR. Heating is supplemented by underfloor heating in the rooms as needed. Each room is separately controlled. In the summer the ASHP can be put into cooling mode and the heat recovery function of the MVHR switched to cool recovery.

The operation of the heat pump and MVHR in heating mode or cooling mode are shown below. The heat pump and MVHR as shown below can also operate in reverse to cool the house.

The house is 100% electric. No gas, open fires, oil or anything else. We have a single 6kWh Air Source Heat Pump. This is our sole energy supply for heating and hot water. There are 48 solar PV cells, 3 inverters and 6x5.3kWh batteries. Whilst we have a very simple roof which is therefore amenable to multiple panels other things are less than ideal. The two aspects are either south east or north west. The south east aspect is good but suffers from a significant amount of shading from multiple nearby trees (the trees are beautiful). The north west aspect is obviously just not as good. Our import tariff is Octopus Go (currently 8.5p/kWh 00.30-05.30 and rest at 31.23p/kWh) and export is Outgoing Octopus (at 15p/kWh). We have an EV with a 64kWh battery. We import almost all our electricity on the 5h night rate. We charge the household batteries overnight to 100%. Car charging is also overnight. All the household needs are then met using these batteries. Only in the most extreme conditions like for example very cold conditions (

The figure below shows the energy use and generation on a month-by-month basis April to December. There are four separate blocks; The load or amount of energy the house uses, generation from the PV cells, the amount imported from the grid and the amount exported to the grid.

The total load on the house during the period April-December was 3.9MWh. The total energy generation was 8.9MWh. The crossover where the load began to exceed the PV generation was in October. We were consistently exporting significant quantities from March all the way through to September. From a financial perspective April-December saw a credit of ~£500 with Octopus (remember there is a standing charge of 50p/day). Some of this will get used during January and February but over a 12-month period the house will generate cash.

Anything we might have done differently? In the summer we do get a lot of solar gain particularly from one very large east facing window. On those heatwave days finding some way to keep that heat out so we don’t have to try and control it via the heat pump and MVHR would be beneficial. Apart from that nothing really.

In summary then the combination of insulation, air tightness, ventilation and then appropriate heating or cooling makes a great house to live in. Of course it does cost more than a standard building but we expect that over roughly a 10-year period it will be cost neutral and of course it is very significantly better than net zero in terms energy utilisation. As a rough rule of thumb the renewables add on approximately an additional 30%.