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The perfect blend

Doncaster’s new civic building combines natural ventilation  with mechanical cooling to achieve a BREEAM Excellent rating

Branded as a ‘model of modesty’ by its designers, Doncaster New Civic Offices is a possible insight into the future of building cooling, in that it mixes natural ventilation with mechanical air-conditioning to highly efficient effect.

In fact, so efficient is the building that it sports a BREEAM Excellent rating. This is largely thanks to natural ventilation, which is maximised to reduce air-handling, and means the building needs only four weeks of heating and three weeks of cooling per year.

Mark Winter, construction manager at Wates, which was responsible for the build, says the contract specified that the building would achieve the BREAAM Excellent rating.

A key feature in the building’s design was a large atrium, which allows a sufficient level of air circulation. This was matched with an exposed concrete frame, which acts as a “mediator” between hot and cold air, he explains.

“Due to the low heat absorption properties of concrete it creates a balance between heating and cooling,” says Mr Winter. “It provides a thermal mass to keep the building cooler during the daytime. Allied to this is a high level of glass in the structure, which helps absorb sunlight.”

The window system also plays a part in achieving sufficient levels of natural ventilation. It combines a manually opening lower-level system with an upper tier that is activated by the building management system (BMS), which allows the opening of individual banks when required. They work via temperature and CO2 sensors that have been located in the lighting installations for aesthetic reasons.

Mr Walker said: “There’s a slight delay on the activators, as they open up a bit more and close a little less depending on the data it gets back from the BMS. The window actuators are controlled by the BMS, which receives real-time information from a number of sensors within the office space and outside. From this information the BMS determines any changes that are required, and these changes are made gradually to avoid any sudden changes.”

Among the external factors that activate the motors are the wind. However, the actuators operate only periodically to save them from overuse as the wind speed fluctuates.

Between the mechanical AC demands and the natural ventilation, a simple balance is achieved by separating them in the building layout. For instance, heavily occupied rooms – such as conference suites – where there’s a high demand for cooling are located together, allowing a simpler design and layout for the ductwork, and the more open areas at the opposite end of the building use cross-flow ventilation.

Mechanical cooling is provided by a combination of air handling units and variable refrigerant flow technology, with the latter serving internal meeting rooms and general ground floor office accommodation. Chilled water is provided to serve the air handling units with the cooling capacity based upon the fresh air cooling load with an additional future allowance for cooling of 20 W/m.

Refining the system

The building opened in September and is now operational, but is in a period of refinement and adjustment to ensure efficient energy levels are achieved.

“The equipment has all been checked and its ability has already been established. What we don’t know yet, as it isn’t fully occupied, is how it works in reality,” says Mr Winter.

“In the coming months, we’ll find out how well advised the settings of the BMS are in relation to the advice we’ve been given regarding the occupancy.

“There’s always going to be a period of matching the projections with how it works in practice. From experience it tends to take five seasons to refine.”

Mr Winter stresses the importance of the FM staff in being able to interpret information from the BMS to ensure operational efficiency.

“The knowledge and astuteness of the FM staff in being able to fully operate the system to make it do what they want is vital, but it takes time. There’s a lot of information that the staff may not have, but can learn from the functions available. And then you have the feedback from the occupants.

“This all has to be blended to achieve the right balance in efficiency. Once that’s achieved you’ll have a building that, as well as reducing maintenance costs, will save Doncaster Council around £650,000 a year in building running costs.”

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