In an underfloor heating system the primary heating circuit is highly tolerant to varying return water temperatures
Such systems may have response times in excess of
the daily occupancy period and so cannot be operated on a conventional daily on-off cycle. The thermal mass needs to be charged up to stabilise at close to the operating temperature before it will start to respond to control changes. To operate the heating or cooling on a simple occupancy cycle would result in the thermal mass filtering out such short term changes and instead tending to follow longer term swings in the ambient temperature. This may lead to a building becoming uncomfortable a few days into a hot spell, with no means of respite. So, thermal mass is not just significant for night
cooling in naturally ventilated buildings. We must ensure that we understand the natural response of the building fabric when designing any heating or cooling system. But we must also ensure that the controls design and the subsequent operation of the building also take account of the thermal response. These issues will become ever-more important
as we seek increasingly low energy solutions for buildings. We must ensure that we communicate our design intentions throughout the supply chain and inform the end users of our buildings how the natural response will impact on their experience of the buildings, and how to operate them to maintain low energy consumption. If we do not do so, we risk installing systems
which, while nominally of the correct capacity, may not actually be able to control the building to a comfortable temperature, or may result in excessive energy consumption to do so. l © Doug King 2010
Doug King is principal of King Shaw Associates and visiting professor of building physics at Bath University. He was author of the Royal Academy of Engineering report, Engineering a Low Carbon Built Environment.
www.cibsejournal.com August 2010 CIBSE Journal 49
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