- Richard Weaver
De-Stratification - The Wind From Heaven.
Updated: May 6, 2021
Implementing de-stratification fans is one of the most common and cost-effective recommendations for improvement of energy efficiency in commercial buildings.
We have all seen a hot-air balloon, and know that hot air rises. However, this knowledge often appears to desert us when we consider heating in tall factory and warehouse buildings. For some mysterious reason, the nice new gas-fired fan heaters can’t seem to get it even decently warm for the workforce tending the machines or packing the despatch.
The reason isn’t rocket science; actually it’s balloon science. The heated air simply rises to the roof, and stays there. The antidote is a downward-pointed fan or two to push the warmth back down to the ground: something to stir up the air, and disturb the strata or layers of temperature going from warmest at the roof, to coolest near the ground. A de-stratification fan.
‘Stratification’ can affect any tall space in a building, not just the portal frames and big sheds. Tall reception areas can also suffer, often leaving the poor receptionist having to rely on a little electric heater hidden under the desk. An unusual place for this this recommendation was the chapel of a boarding school: simply another tall building, with the heating lost up into the rafters. A few very quiet fans hidden in the roof timbers would have provided a very real and comforting 'wind from heaven'.
So far we have talked about a useful piece of kit placed close to the roof. But around half of the places which have destratification don't use it or don't know what it's for.The usual misapprehension is that they are cooling fans for the summer. Others have simply forgotten where the switch is. -
To get the benefit, you have to switch them on - at the right time.
Therefore it is recommended that de-strat fans are 'slaved' to the heating burners; meaning that when the heating is on, the de-strat fans come on automatically and stay on until the heating is switched off.
There is a simple 'rule of thumb' in sizing the requirement for de-stratification in a space. The fan(s) must have capacity to move the volume of the zone twice per hour. So for a heated factory with a volume of 5,000 cubic Metres, the fans must have total capacity of 10,000 cuM/hr (more usually expressed as cuM/sec => 2.75 cuM/sec between them ) If three fans are required to give an even distribution in the building, then each should have a capacity of, say, 0.9 cuM/sec. - Easy.
Finally, the benefit of de-stratification. For this we turn to the i-SBEM model used in generating commercial EPCs. This is a model, and may not be perfect in absolute terms, but it allows a very simple with/without comparison for the specific features of the zone in question. Typically the benefit of de-stratification fans is 30 to 40% saving of heating energy consumption. Destratification gives the most benefit in the tallest zones, and where the roof fabric is poorly insulated. As well as an energy-saving measure in ESOS recommendations, this is a good wheeze to try in reducing EPC ratings for MEES.
So the recommendations checklist for de-strat fans is:
If you have a heated zone over about 5M tall, consider the need for de-stratification. If you can, measure the temperature at the top and bottom of the space.
Perhaps get an EPC assessor to model the zone.
If you already have destratification fans - use them correctly.
If you think you need de-strat, use the rule of thumb to get a rough idea of what you need and what it might cost.
As energy-saving measures go, this is one of the best in getting good quick return on your investment. Give it priority..