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Ant Wilson: How cold can it go?

The recent sub-Zero conditions in the UK got me asking: How cold can it go? The answer is very cold indeed!

Let me give you some facts to support this assertion. According to the Met Office, December 2010 had an average temperature in England of 5.2°C below the norm from 1971-2000.

If you look at heating degree days based on the 15.5°C baseline, for December some locations in the UK were well over 50 per cent higher heating degree days. In the West Pennines they had 530 compared with the 20 year average of 349. In North West Scotland it was 524 compared with 332, a massive 58 per cent increase. Even the Thames Valley region saw 442 compared with 311. (That is cold!)

What is the good news?

We actually had a greater amount of sunshine, with 132 per cent more in Northern Ireland and 59 per cent more in Wales, and it was also considerably dryer with the UK experiencing only 38 per cent of the normal rainfall in December compared to the average between 1971 and 2000. The year as a whole was, on average cooler, sunnier and drier.

As people within the heating and ventilating industry, it is the outside environmental conditions that dictate the type and size of the systems that we need to incorporate in our buildings.

I have been discussing with many people recently why we do calculations on the thermal performance of buildings and the type of input variables that are used. It has often been said about the data input to software packages 'garbage in - garbage out'. Why are we doing the calculations? Are we looking at predicting how hot it gets, how cold it gets, how much energy is used, is it to check for regulation compliant buildings or can it cope with future changes in the climate? All of these will require slightly different input to the thermal models.

As we move forward and experience rapidly changing environmental conditions, it is important that we produce robust designs. It is always hard when from one year to the next mean annual temperatures within the UK can vary by over 2°C and a single moth like November last year contain both the hottest and coldest day on record!!! It is also important not just to look at the external temperatures, as an increase in sunshine hours increases the passive solar gains within our buildings. The clear skies that give the very cold night time temperatures also often mean that we have low wind speeds and therefore less convection from our buildings with increased radiation from the façade surfaces to the night sky.

Based on the complexities on the climate alone it is impossible for anyone to accurately predict how much energy a building will use or how the temperatures will be maintained in internal spaces. But with careful selection of the input parameters we can ensure robust design.

Ant Wilson is a director of building services consultant AECOM
11 February 2011


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