Phase II of the National Energy Centre (2004)

An exemplary low energy office

The second phase of the National Energy Centre in Milton Keynes is a low energy office building, with a gross floor area of 430m² able to accommodate up to 36 people, including storage and limited meeting space. This has increased the total occupancy capacity of the National Energy Centre to almost 100 people.

The new building is highly insulated and uses a 13kW Viessmann heat pump to provide warm water to an underfloor low temperature central heating system, avoiding the need for radiators on the limited wall space. Initial plans to marry the heat pump to a highly efficient condensing boiler were changed when it was realised that cost savings through omitting the gas supply, boilers and sequencing controls would permit the entire building to be heated from a heat pump unit. On bright days, the heat pump will be powered by a photovoltaic (PV) array and hot water for the kitchen and toilets will be pre-heated by a Thermomax evacuated tube solar collector. A small pellet-fired wood burning stove has been fitted as an auxiliary heating system, although it should not be necessary in normal operation. The National Energy Foundation has worked closely with Avebury International in creating this design for a low energy building; Avebury also provided the construction management services for the new building.

The west elevation of the building has a mixture of horizontal and vertical glazed areas including high level natural ventilation and vertical/horizontal shade panels. This has been constructed using a timber frame, with brick walls below the windows and on the North and South aspects, and low maintenance 'Thermowood' cladding produced from sustainable (Scandinavian) timber elsewhere. The photo above, taken on 19 February 2004, is after initial occupation, but before the solar overshading has been added. The east elevation is partly buried to achieve thermal stability and reduce the visual impact from the office area of the Phase I building. A four-ridged mono-pitch North light style roof sloping at 7.5° towards the East and 15° to the south has been used to improve the angle for the PV installation.

Energy & Environmental Elements

Construction Details   | Heating System |   Other Features

Construction Details

Foundation/Substructure: The floor level of the building has been set at a similar level to Phase I to minimise the disposal of subsoil from the site. The East elevation was partially buried to help shield the Phase II building from solar gain and to use the stabilising effect of the temperature within the mass of the adjacent sub-soil. The ground is sloped towards the building to provide shielding up to a height of approximately 1 metre.

Roof Construction: The mono-pitch roof has an outer skin of Rheinzink, supplied through Boss Metals, to ensure a long design life and ease of workability in fitting the PV and Sunpipes. The ridged design assists with the generation of natural cross-flow ventilation of the building through the eaves level vents in the East and West elevations. All construction elements (roof, floor, walls and glazing) were built to higher standards than mandated under Part L2 of the UK Building Regulations.

Wall Construction: The internal skin of the building is a load bearing timber frame construction, with a high level of yellow glass wool insulation. This was internally faced with plasterboard, and painted in light colours (to reflect natural daylight) with a water-based low-emission paint. The South elevation brickwork is extended beyond the sides of the building to provide shielding from the high summer sun.
Windows and Ventilation: The windows are gas-filled low-emissivity doubled glazed units with frames from sustainably sourced softwood. The roof projects over the high level opening windows to shade them in summer. The high level vents on the West elevation can be seen here above the windows. This natural ventilation should reduce perceived temperatures by between 2-3°C in summer months.
Floor Construction: The floor consists of several layers to maximise energy performance. From the ground up, it starts with 250mm suspended concrete beams, which offers limited summer cooling from its thermal mass. The beams are insulated with two layers of insulation: 100mm of expanded polystyrene (shown stacked on the photo) beneath 25mm of polyurethane foam, with a heat reflecting coating on each side. Underfloor heating pipes were laid above this in a screed of heat transmitting "Gyvlon".
South Façade: The South wall of the building is of straightforward brick construction with no glazed areas to minimise the chance of overheating in summer. As an additional shading feature, a trellis has been installed on which deciduous plants will be grown. The flue for the pellet stove can also be seen in this photograph, inside the red O.

Heating System

The heating system is built around a Ground Source Heat Pump feeding a low-temperature underfloor heating system.

Soil temperature remains almost constant year round deeper than around 1m below the ground surface, as the heat of the sun percolates through the layer of topsoil. Ground Source Heat Pumps capture solar energy by exploiting this effect, using a long water-filled coil to extract heat from the ground.

The Phase II building uses a Viessmann heat pump installed by Geoscience Ltd on behalf of the Foundation, and part funded through a Clear-Skies grant.

The Foundation needed to calculate the heat demand from the building very carefully, as on both cost and performance grounds it was important to avoid over-sizing the system. Calculations suggested a total fabric heat loss from the building of around 7kW (as a design temperature difference of 27K), with a similar maximum loss from ventilation. It was decided to install a 13kW Viessmann Vitocal 300 heat pump unit (above left), which although theoretically very slightly under-sized should operate at a high load for much of the time. The heat pump collects heat from ground loops containing a water/antifreeze mixture laid into three 50m trenches. Each trench contains approximately 250m of polyethylene pipe coiled into a "slinky" of around 1 metre in diameter, laid horizontally at a depth of 1.4m. The external loops are pumped by a Wilo Salmson TOP S30/7 high efficiency pump (above centre).

The heat pump feeds a 200 litre buffer tank, from which a secondary circuit supplies the underfloor heating. To deal with exceptionally cold weather, a manually operated 3kW immersion heater is installed in the top of the buffer tank, but in the first winter of operation it did not prove necessary to use it. The building also has a biomass pellet stove as back-up heating.

Internally the heat pump drives an underfloor central heating system consisting of Rehau Universal pipe laid in a Gyvlon floor screed on the ground floor (above left). The self-levelling screed was a nominal 35mm thick, allowing greater responsiveness from the system and enhanced insulation - this arrangement is shown schematically below. On the mezzanine, the heating pipes needed to be set above timber joists, so were set into Rehau's high conductivity aluminium plates (above right). In all, there are three underfloor heating circuits, individually controlled through zone thermostats, fed from the top of the buffer tank.

The National Energy Centre also includes a demonstration pellet stove installed as part of the Logpile project and to provide top-up heating on very cold days. It is located quite close to the main entrance to the building, where the air is likely to be coolest, and well away from the thermostats controlling the main heating system. The stove is an Enviro Evolution, capable of delivering 7.2kW heat at 82% efficiency. Sustainably produced wood pellets are fed from an integral hopper into the stove, and the output can be adjusted by a variable speed auger changing the volume of pellets delivered. A convection fan helps distribute heat into the building.

Other Features

For more information, please contact Ian Byrne at the National Energy Foundation on 01908-665555, or you can download a more detailed 20 page brochure about the centre (PDF file, 1.1Mb). There is also a page on this website about the demonstration plots of energy crops in the grounds of the National Energy Centre.