Is destratification ventilation more efficient at ceiling level?

Question

This question is more of a practical physics question. In buildings with heating systems, heated air tends to rise and create air temperature stratification - hottest air near the ceiling, coldest air near the floor. Traditionally, ceiling mounted ventilation devices are used to counter the effect, sending heated air towards the floor and thus promoting air mixing and temperature homogeneity.

From a physics point of view, and assuming the goal is even heat, is there any reason this process is more (or less) efficient than the reverse process of floor mounted ventilation sending cold air up towards the ceiling? What about a ventilation device mounted in the midpoint of the building, sucking air either from the top or from the bottom?

Since building geometry probably has a pronounced effect on air movement patterns, let's assume the building is a giant square with a heated air source located at the midpoint of one of the square's lateral walls.

Here are some factors that I identified that potentially impact the different processes (there might be others):

• Different densities of hotter and colder air (which is easier to "push" by ventilation?)
• Dust concentration on the floor vs ceilings (contributes to overall density of "floor air" or "ceiling air"?)
• Location of air leaks to the outside (if there are air leaks either on the ceiling or on the floor, maybe the efficiency of the process that also helps "push" hot/cold air out of those leaks is impacted?)

Answer 1

For a typical room stratification, the temperature difference between the warmest and coolest air is only a few Kelvin. This means that the flow may be considered Boussinesq and there is essentially no difference in the energy requirement between trying to push cool air upwards or warm air downwards.

However, that is not the only consideration. For simplicity let us imagine a room stratified into two layers. Within each layer, all the air is at the same temperature. For arguments sake, lets say the fan is on the floor, blowing up. For destratification to occur, we need the air blown upwards to have enough momentum to travel the height of the cool air layer, push through the density interface between the layers, and ideally reach the ceiling, causing a lot of turbulent mixing with the warm air. This will increase the depth of the warm air layer whilst reducing the temperature difference between the layers, i.e. destratification. If the fan is too weak, all that will happen is that air movement will be induced in the layer of cool air in which the fan sits, but no destratification will happen.

For a typical room, the warm air is likely to be confined to a layer near the ceiling, less than half of the height of the room. Therefore, placing the fan in the warm layer means it needs to be less powerful as there is less distance for the warm air to travel downwards before hitting the interface between the layers and interacting with the cool air beneath.

There are definitely more practical considerations as well though. In a room that is tall enough to experience significant stratification, there is plenty of space to mount a fan at ceiling height without anyone worrying about getting hit in the head. By contrast, a fan mounted at floor level would either require guards and use up floor space, or take out people's ankles.

For your other questions about sucking air from the middle (height?) of the room, there are still far too many possibilities. We haven't included heat sources, which will act to stratify the room if they are small area sources like a person or a radiator, but will cause mixing if they are larger area sources like underfloor heating. We also haven't included any ventilation, which will also change the stratification depending on the flow rates and location of vents. Indoor air distribution gets complicated very quickly.