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Heating efficiently with destratification

18 August 2016

Combining destratification with warm air heating can result in significant energy savings for buildings with high roofs, says Stuart Parris, general sales manager of Powrmatic. Here, he discusses the design considerations

The Carbon Trust has estimated that destratification in high buildings such as factories and warehouses that use warm air heating can reduce heating energy consumption by 20%. This is because the laws of physics dictate that warm air will rise to the highest point – the ceiling or roof space – and in doing so will push cooler air downwards into the occupied space.

This can lead to a temperature difference between the occupied zone and the roof space of as much as 5°C (typical pitched roof building with an average height of between 5m and 7m). Clearly, such a temperature differential is indicative that a significant amount of the energy consumed by the heating plant is being wasted.

This situation also accelerates the rate of loss through the building fabric, particularly the roof, because there is a high temperature difference between the air in the roof space and the outside air. 

For example, if the set point temperature for the occupied space is 19°C and there is a 5°C temperature differential between it and the roof space, the air at roof level will have a temperature of around 24°C. On a cold winter’s day, with an outdoor temperature of -1°C, there will therefore be a temperature difference of 25°C.

In contrast, if the warm air is distributed evenly throughout the space at 19°C, the temperature differential between indoor and outside air is only 20°C. Thus the temperature gradient is shallower and the rate of heat loss is slower, compared to a building with no destratification.

Temperature gradients also have an impact on the natural rate of ventilation through leakage points in the building fabric – again most pronounced in older, ‘leaky’ buildings. Some of the rising warm air will leave the building through these leakage points, creating a small negative pressure that draws cold air in through lower leakage points. 

Destratification

The obvious solution is to return the warm air back to the occupied space using destratification fans. A well-designed destratification system will ensure there is only a very small temperature gradient between the occupied zone and roof space. In many cases this will make it possible to maintain the required temperature with fewer warm air heaters, reducing capital and running costs.

In an industrial building where manufacturing or processing machinery generates heat, this heat will also be evenly distributed by the destratification fans, further reducing the load on the heating plant. Destratification fans can also be used without the heating to provide cooling flows of air in warm weather.

Optimum design 

There are essentially two types of commonly used destratification fan. Low velocity blade or propeller fans work by churning the air and these are most appropriate for ceiling heights between 5m and 10m. For spaces 10 to 20m high, a high velocity axial fan will be required to send a significant amount of air at high speed towards the floor.

Fan controls should be configured to suit the requirements of space. Typical options include only operating fans when the heating is on, controlling the fans independently of the heating, linking fan operation to air temperature at ceiling height and controlling the speed of the fan to prevent draughts. There may also be a summer option for cooling.

In all cases, whether retrofit or new build, destratification systems should always be designed by experienced professionals, taking account of the key design criteria that have been discussed here.

 
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