Why is ceiling based air supply used?

The performance of ceiling based air supply equals no more than 30-40 % in terms of ventilation, heating and cooling effect. The supply air will create a mixture of the room air and the contaminants in the room. This is why the system is called mixing ventilation.

Why is the performance of mixing ventilation that poor?

The supply air disturbs the removal of heat and contaminants. Furthermore the entire room is ventilated and cooled, including the unoccupied zones.
If the room is heated in the same way, the air temperature at the ceiling level will be much higher than the temperature at the floor level occupied by people.    Â

"Figure 1.
Down-stream supply air mixes and spreads supply air and used air in whatever way. The outcome is mixing ventilation, excessive energy consumption and poor performance.”

The poor effect of mixing ventilation is easily explained as we think about what happens when a room is being ventilated and cooled by ceiling based air supply: 1. Warm, contaminated air from people and other sources of heat always moves upstream towards the ceiling in order to be exhausted from the room.

1. If colder supply air is being introduced downstream, the warm and contaminated air is partly brought back towards the sources of heat.
2. The supply air at the heat sources is then constantly being heated by the returning exhaust air.
3. With the objective of cooling the unintentional heating, there is a constant need for supplementary cooling energy supply.
4. The air in the occupied zone of the room is constantly being contaminated by the downstream air.
5. In order to make the room air suitable for breathing, more supply air must constantly be introduced into the entire room.
6. The exchange of heat and contaminants between room air and supply air occurs in the entire room instead of being directed toward people and other heat sources.

Furthermore: A human being expires more than half a million contaminants per hour, most of which are harmless, however not all of them. If we try to remove the contaminants using mixing ventilation, they will spread uncontrollably between people and their mouth and nose. This ventilation method always results in poor performance and excessive energy consumption.

Thanks to the Computational Fluid Dynamics – CFD – calculation program, we were, some years ago, able to detect the low effect of mixing ventilation.
This inspired the search of alternatives providing a more efficient heat exchange and removal of contaminants. Today this alternative exists: a ventilation technique based on the natural thermal conditions of the room, providing a stable stratification of the room air as well as efficient ventilation without mixing. A natural and energy saving method, isn’t it?

Displacement ventilation – a mistake in the right direction

The drawbacks of ceiling based air supply have been known for a long time. Some decades ago this inspired a growing interest in a new ‘old’ ventilation technique intended to remove heat and contaminants at floor level without interference of mixing or poor effect. Research studies showed that the room air was temporarily stratified providing more pure air among the people at the floor level and more contaminated warmer air at a higher level.

As that happened, the used upstream air could be removed without being disturbed by the supply air. This enabled the effect of the ventilation to exceed the maximal 50 % effect of mixing ventilation, an interesting fact indeed that generated further research.

Extensive marketing of the method – incorrectly called displacement ventilation – predicted the final discard of the mixing ventilation. However, the development turned out differently as a result of many contributing circumstances:

• Cooling supply air directly into the occupied zone often causes problems in terms of draught and cold feet. Experience shows that there is a risk of poor thermal quality in removing more heat from a room than 35-40 watts per square meter.
• In order to avoid draught the supply air velocity is low generating low-level air supply. And as the supply air isn’t capable of controlling all of the thermal pressure of the room, the air in the room takes control spreading the supply air. This often happens in rooms with many mobile heat sources. The intended stratification of the room air more or less turns into mixing, which was to be avoided.

"Figure 2.
Displacement ventilation with low-level air supply also provides poor performance. Cold air causes draught and cold feet. The supply air velocity is so low that the ventilation becomes mixing when people move in the room.”

• On the other hand attempts to heat i.e. industrial facilities and sport halls using low level air supplies will, for natural reasons, heat the ceiling more than the people. This kind of air supplies is therefore totally unsuitable for heating.
• The installations tend to be voluminous as well as expensive.

Due to combined effect of many factors, ceiling based mixing air supply devices and cooling baffles are dominating the market again – in spite of their poor performance.


Displacement ventilation became the catalyst of  ”new thinking” about ventilation

The principle was right: Cold air moves downstream, warm air upstream.
It worked the wrong way: Low level supply generates mixing.
Shouldn’t the fundamental law of nature apply to ventilation as well?

“Figure 3.
The principle is simple: Cold air moves downstream, warm air upstream. Ventilation should use the natural thermal conditions of the room."

• The most important function of ventilation is to remove excessive heat and contaminants from rooms and heat sources – not to introduce air into the room in whatever way.
• Is it possible to increase the exchange rate of heat and contaminants in other ways than using ceiling downstream supply air?
• Does the exchange rate of heat at the heat sources increase to the same extent as the contaminant removal rate when ventilation works without interference of mixing ventilation? In that case this phenomenon should be of great importance to our future energy consumption!

Earlier it has been almost impossible to measure how much energy can be saved by using a more efficient heat exchange than that of mixing ventilation. That is why, even at present, exchange of heat is carried out in this ineffective way. Thanks to the rapid development of the calculation program Computational Fluid Dynamics – CFD that has taken place during the last years - we know today that the exchange rate of heat is rather well in line with the air exchange.
This makes it really exciting: Provided that supply air enables a permanent stratification of the room air, the same temperature can be reached in spite of reduced energy consumption!


“Figure 4.
Ventilation works only where it’s needed and concentrates on cooling the heat sources.”

In order to achieve a stable stratification, a notably bigger supply air velocity than that of the current traditional air supply systems is required. For that reason we developed a new series of High-level air supply systems making the supply air inflate a lot of small jets. However, instead of directing the jets into the room, we direct the jets toward various surfaces applying the Impinging Jet Ventilation technique. This turns the jets into a silent draught free supply air flow into the room, providing a stable stratification of the room air.

The tests of the new air supplies indicated such an interesting function that further research for optimizing was transferred to the University of Reading in England in 2003.

After extensive research work the Reading researchers were able to specify the fundamentals of the new ventilation technique, a technique that always stratifies the room air and always enables the intended function in smaller office facilities.

“Figure 5.
Stratifying ventilation provides stable stratification of the room air, controlled ventilation with reduced energy consumption and high performance.”


The research studies of the new ventilation technique – called Wall Confluent Jets Ventilation – provided the following results:

• The heat and contaminant exchange rate constantly exceeded 65 percent compared to mixing ventilation, which indicates that energy supply for cooling can be reduced by 20-30 percent compared to that of mixing ventilation.
• All studies showed upstream convection air from the heat sources. Heat and contaminants were always removed from the room without any interference in terms of mixing and poor performance.
• The thermal quality was constantly superior compared to previous studies of mixing and displacement ventilation.

We call the technique Stratifying High-level air supply.

Because the research results were so positive, the researchers decided also to study the effects in a room where stable stratification had never been obtained – an occupied classroom. Those studies gave the following results:

• The contaminated air in the occupied zone was moving up-stream being replaced by “new” air from the floor level. The stratification of the room air remained stable, even when electric fans were brought into the classroom as an attempt to cause room air mixing!
• The Local air index – the efficiency of air exchange in the occupied zone – was on an average 26 percent higher compared to the level obtainable with mixing ventilation - and 60 percent higher than the average obtainable with mixing ventilation.

The results of the studies were presented by the researchers in September 2004 at the ROOMVENT 2004 conference in Coimbra, Portugal. Complementing information was presented at the Indoor Air 2005 conference in Beijing, China.
Further research studies were presented at the World Renewable Energy Congress IX in August 2006 in Florence, Italy.

Conclusions of research work and experience from stratifying high-level air supply systems until February 2007

In comparison with mixing ventilation the following achievements were obtained:

• 30-50 percent saving in heating energy consumption
• 20-60 percent saving in cooling energy consumption
• 60-80 percent in terms of constant effect
• The air supplies do not cause any jet ranges or near zones in the room. The supplies also have a high silencing effect of their own.
• Removal of heat and contaminants takes place without any disturbances.

All in all stratifying ventilation provides a better indoor climate with reduced energy consumption and less environmental measures than any other ventilation principle known.



“Figure 6.
Traditional heating with supply air makes the ceiling warmer than the floor.”

“Figure 7.

High-level air supplies primarily heat the occupied zone and provide equal temperature in the entire room.”

Current studies of Stratifying high-level air supply ventilation focussing on industrial facilities Studies based on facts concerning heating of industrial facilities show a decrease of ceiling level temperature of more than five centigrades when mixing air supplies are replaced by high-level air supplies. This indicates that the new ventilation technique will be of great importance to our future energy policy. Therefore the method is, at present, subject to extensive research work in an industrial environment, led by professor Bahram Moshfegh.

The above information material has been collected by Bengt Sellö in February 2007. We welcome your comments on the material by e-mail info(at)stravent.fi
You will find more information about Stratifying high-level air supply ventilation at www.stravent.fi


Illustrations: Hans Sandqvist, Bildinfo (08-88 52 30)