Ventilation system with round or flat-oval ducts?

Hello everyone,

for our construction project, the Zehnder Comfoair 350 is to be installed. The catalog offers two different duct systems to choose from – on the one hand the InFloor system, where round flexible ducts are used, which are integrated into the concrete ceiling, and on the other hand the OnFloor system, where flat-oval flexible ducts are used, which are integrated between the concrete ceiling and the insulation layer.

I once read somewhere that the round ducts are the better alternative because they achieve optimal flow behavior and the air resistance is lowest here.

The general contractor also initially promoted the round ducts, but now suddenly the flat-oval ducts are to be installed... a bit strange.

Does anyone have experience with this OnFloor system? Are there increased flow noises during the operation of the ventilation system due to higher air resistance here, which perhaps already fall into a range that is perceived as disturbing? Or maybe someone has another ventilation system with flat ducts and can say something about it?

Hi! I have no experience with the system you mentioned.

We have a Pluggit Avent P310 and the flat ("oval") pipes are installed in the screed.

In the normal operating range of the system, I cannot detect any disturbing flow noises. Only at the highest setting can you hear a slight rustling, but directly at the outlets and inlets.

Every manufacturer has their own systems and pipes here. Everything is tested and I would almost say it works perfectly with every manufacturer, as long as they have the well-deserved good reputation (which is the case with you). Additionally, the required and actually occurring air velocities in the pipes for regular air exchange are not high at all.

I still think that round pipes are preferred when the ventilation outlets are not on the floor, but in the ceiling or wall. And the oval pipes – also because of the floor heating height – for installation under screed.

Best regards Thorsten

Hi,

the choice between oval or round pipes is less a question of flow behavior than of the installation method. Yes, theoretically a round pipe has the best ratio between wall surface and cross-section and thus the lowest wall friction. But if my memories of my fluid dynamics lectures don’t completely fail me, you can safely forget about that at the low speeds and mass flows. Just look in the engine compartment of your car, where round, oval, square, and flat cross-sections are mixed freely in the intake tract.

You take the round (mostly 75 mm pipes) if the pipes are to be laid in the thin ceiling.

Advantage: you often get away with less additional floor construction. And you don’t have a breakthrough through the ceiling, which can be an advantage in the upper floor ceiling if there is an unfinished/uninsulated attic above, as the vapor barrier is then not interrupted.

Disadvantage: The laying of the pipes must be perfectly coordinated with the concreting of the ceilings. And that for every ceiling, i.e., your “pipe layer” may have to come three times.

And many structural engineers get a crisis, because these pipes in the ceiling have to be taken into account in the statics. This is actually nonsense, because the pipes lie in the middle of the ceiling and thus in the shear/tensile stress-free area. The outer sides of a beam carry the load, not the middle, but on construction sites, they are not really calculated precisely, but (from my point of view as a mechanical engineer, so please don’t get me wrong) only “roughly estimated and worked with tables.” Therefore, they don’t get such issues under control in the calculations. It would probably be too costly anyway!

You put the oval pipes, in contrast, on the finished ceiling, under the screed. They then disappear in the screed insulation. And so that the insulation does not become unnecessarily thick, the pipes are simply “flattened a bit.”

Advantage: You can first calmly build the shell, and then the “pipe layer” only has to come once and install all channels. This makes site coordination much easier.

Disadvantage: The floor construction becomes thicker because you need at least 60-70 mm of screed insulation.

For us, it will definitely be the oval pipes in the basement ceiling and ground floor ceiling in the screed. It may be that we will lay some round pipes in the upper floor ceiling. The system planner is currently figuring that out.

According to two suppliers who offer both systems, there is no difference in noise. At a normal low level, you shouldn’t hear anything.

Best regards,

Andreas

Another disadvantage of the bare concrete floor: You have to coordinate plumbing, heating & electrical so that nothing intersects. And that can become incredibly difficult...

I would always put them in the concrete ceilings, then you don’t have that stress...

It also works with flat ducts made of zinc on the raw floor, as in commercial buildings... we installed this solution...