Destratification fans are one of the most cost-effective improvements available in industrial heating. They recover warm air that collects at roof level and return it to the occupied zone, cutting heating costs and evening out temperatures from floor to ceiling. But a destratification system only works if it is correctly sized. Get the airflow wrong and the fans will run without ever fully breaking down the temperature gradient they were installed to fix.

This guide explains how to size destratification fans for an industrial building, starting with the rule of thumb Hadar uses for an initial estimate.

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Why Stratification Happens in the First Place

 

Warm air is less dense than cold air, so it rises. In any heated industrial building, the warm air produced by the heating system naturally drifts upward and collects at roof level. The taller the building, the more pronounced the effect. In a high-bay warehouse it is common to find the air at roof level 10 to 15 degrees warmer than the air at floor level.

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This is wasted heat. The thermostat sits in the occupied zone at floor level, reads a low temperature and keeps the heating system running, while a large reservoir of warm air sits uselessly above everyone's heads. Destratification fans solve this by mechanically pushing that warm air back down so the heat your system has already paid to produce actually reaches the people in the building.

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Because the warmest air collects at the very top of the building, destratification fans are mounted as high as practically possible. They need to reach the hottest air at roof level and drive it down through the full height of the space. Mounting them lower would leave the hottest air untouched and defeat the purpose.

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The Rule of Thumb: Two Air Changes Per Hour

 

The starting point for sizing a destratification system is the total volume of air the fans need to move. Hadar uses a rule of thumb of an hourly air turnover equal to twice the building volume.

In other words, the combined airflow of all the destratification fans in the building should be enough to move the entire volume of air in the building twice every hour. This gives a target total airflow that the fan selection then has to meet.

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The calculation is straightforward:

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• Step one: Calculate the building volume. Length multiplied by width multiplied by height, in cubic metres. Use the full building volume up to the roof

• Step two: Multiply the building volume by two. This is your target hourly air turnover in cubic metres per hour

• Step three: Select a number of destratification fans whose combined airflow meets or exceeds that target figure

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As a worked example, take a warehouse 50 metres long, 30 metres wide and 8 metres high. The volume is 50 x 30 x 8, which is 12,000 cubic metres. Multiplied by two, the target hourly air turnover is 24,000 cubic metres per hour. The destratification fans selected for that building should have a combined airflow of at least 24,000 cubic metres per hour.

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This rule of thumb is a reliable starting point and it is deliberately straightforward. Using the full building volume up to the roof is the simple and conservative choice. On a building with a tall, steeply pitched roof it errs slightly towards generous airflow, which is the right way to err. It is better to slightly oversize a destratification system than to undersize it and never fully clear the temperature gradient.

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Factors That Refine the Estimate

 

The two air changes per hour rule gives a sound initial figure. A proper design then refines it against the specific characteristics of the building. The main factors are below.

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Ceiling Height

 

Ceiling height is the single biggest variable. The taller the building, the more severe the stratification and the harder the fans have to work to drive warm air all the way down to floor level. A building with a low roof has a small temperature gradient that is easy to break down. A high-bay building has a large gradient and the air has much further to travel.

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For taller buildings, the two air changes per hour figure may need to move towards the upper end or beyond, and the individual fans must be capable of throwing air down through the full height of the space. A fan that produces plenty of airflow but cannot project that air down 12 or 14 metres to the floor will not destratify a tall building effectively.

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Fan Throw and Coverage

 

Total airflow is necessary but it is not the whole picture. A destratification system can hit the target air turnover figure on paper and still perform poorly if the fans are the wrong type or are badly positioned.

Each fan has an effective throw, the distance it can project air downward, and an effective coverage area. The fans must be selected and spaced so that their coverage overlaps adequately and the whole floor area is served. A building that is correctly sized for total airflow but has fans spaced too far apart will have cold spots between the fans where the air is never properly mixed.

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This is why two buildings of identical volume can need different destratification solutions. A long narrow building and a square building of the same volume have different coverage requirements even though the air turnover target is the same.

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Building Use and Obstructions

 

Racking, mezzanines, machinery and stored goods all affect how air moves through a building. High-bay racking in particular can block the downward flow of air from a destratification fan. The layout of the building affects both how many fans are needed and where they should be positioned.

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The Heating System

 

The type of heating system influences the destratification requirement. A building heated by warm air unit heaters has heat being actively introduced at high level, which increases stratification and the destratification duty. The destratification design should always be considered alongside the heating system rather than in isolation.

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Choosing the Right Type of Fan

 

Once the airflow target and coverage requirements are understood, the right type of destratification fan can be selected.

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The Hadar DSF destratification fan is designed for industrial destratification, recovering warm air from ceiling level and returning it to the occupied zone. A number of DSF units are typically distributed across the building to deliver both the total airflow target and even coverage across the floor area.

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For very large floor areas, a high-volume low-speed (HVLS) fan moves a large volume of air across a wide coverage area from a single unit. The Evel WZ HVLS fan is suited to large distribution facilities where its wide coverage can serve a substantial floor area from one fan. In many buildings a combination of fan types gives the best result, with the selection matched to the airflow target, the ceiling height and the layout.

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Putting It Together

 

Sizing a destratification system follows a clear process:

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• Start with the rule of thumb: Calculate the building volume and multiply by two to get the target hourly air turnover

• Account for ceiling height: Taller buildings need fans capable of throwing air down through the full height, and may need turnover towards the upper end of the range

• Check coverage: Select and space the fans so their coverage overlaps and the whole floor area is served, not just the total airflow figure

• Allow for the building layout: Racking, mezzanines and machinery affect fan numbers and positioning

• Design alongside the heating system: Destratification should be considered as part of the heating design, not as an afterthought

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The two air changes per hour rule of thumb gives a reliable initial estimate that any building operator can calculate. A proper design then refines that figure and translates it into the right number, type and positioning of fans.

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If you want a destratification system sized correctly for your building, contact Hadar Industries today for a free site survey. We will assess the building, calculate the requirement and recommend the right destratification solution.