Featured on Channel 4's Grand Designs, this super-energy-efficient Spanish-style dormer bungalow near Bicester, Oxfordshire, was self-built in 2018 by its current owners. They set out to create a property that would combine a Mediterranean sensibility with modern comfort and efficiency. As they were approaching retirement, it also needed to be low maintenance and inexpensive to live in.
The owners had initially set their hearts on an electric heat source incorporating solar energy to avoid burning fossil fuels, and a fuel cell micro combined heat and power (CHP) system from Viessmann. After reading about the Vitovalor fuel cell system in a magazine, they wanted their new home to be the UK's first to install this incredibly efficient micro CHP appliance. However, in the end, the perfect answer proved to be slightly different.
Planning of the heating, power and hot water system was incorporated into the building process right from the start, to ensure all parts of the structure would work together to reduce energy demand and maximise efficiency. Thanks to its high-tech insulation, the house has an exceptionally high thermal mass.
Working with Viessmann installation partner Matrix Energy Systems, the self-builders were able to specify the perfect, fully-integrated solution through a process of evaluation and elimination. Infra-red and direct electric heating systems were quickly rejected because, although cheaper to install, higher running costs make them more expensive in the longer term.
Although the Vitovalor fuel cell micro CHP was a favoured option, analysis by Viessmann's experts revealed that the house was too well insulated and the demand for heat too low. The Vitovalor is designed to run for 20 hours/day, but the new build's internal temperature only drops about 1OC when unheated for 12 hours, meaning that heating is needed for just four hours in every 24, all year round.
A ground source heat pump was ruled out due to lack of space and unsuitable geological conditions. Matrix therefore advised a system built around the exceptionally efficient Viessmann Vitocal air source heat pump.
The Vitocal heat pump is combined with Viessmann Vitovolt solar photovoltaic (PV) panels and a Vitovent mechanical ventilation and heat recovery (MVHR) system. Underfloor heating has been overlaid by 300 mm of concrete screeding on the ground floor and 50mm on the first floor. Due to its ability to retain heat, the thick concrete layer stays at a constant temperature, stabilising the warmth throughout the home. The building is divided into 11 heating zones, with the bedrooms cooler than the living room and kitchen.
The Viessmann Vitocal 242-S Split heat pump was chosen because it has a nominal heat output of just 7 kW, compared to 30 kW for a standard boiler. It also offers the option to link in a battery later on, to store solar electricity if the energy monitoring shows it to be beneficial.
The ventilation is needed because the building's minimal heat loss also limits internal air movement. The Viessmann Vitovent MVHR system creates an internal flow of fresh, pre-heated or cooled air around the house. It also reuses recovered heat from around the building, reducing demand on the heat pump, allowing it to operate more efficiently when needed and less frequently during winter months.
The Vitovolt solar panels are capable of powering the entire heating system. However, at night and on shorter and more cloudy days, when consumption exceeds generation, electricity is consumed from the grid (or, in future, from stored battery power).
The Vitocal's split refrigerant system means the system does not need antifreeze (which reduces output), but it will never freeze in a harsh winter. It generates hot water for an hour a day at midday, when there is peak sun. Any excess is stored in a buffer tank.
The owners are delighted with the results. "We felt as though the Matrix staff were really understanding of what we wanted. We worked through many options and variations until we settled on what we have today; and I must say that I am overwhelmed with what we have achieved. To intelligently link the components together so that the cheapest form of generation is active is really ground-breaking. Today, we have a highly efficient and versatile heating system that has reduced our running cost and carbon emissions."