Source of "max 2.5m/s" rule of thumb?

[Sparrowhawk]

I've seen it quoted on MVHR supplier sites and here, but where does it come from and why?

 

Is it draughts, or air moving faster than this is audible (in any diameter pipe)?

[SteamyTea]

How timely, I was talking to a man from an MVHR supplier and fitters yesterday at the Swindon show and he said he designed for 3.5 m.s^-1 max.  But if it did go a bit over, then it was generally not a problem.

[JohnMo]

Passivhaus trust say

Within a single dwelling the air speed in the primary ducts should not exceed 2~3 m/s and in the final runs to room valves the speed should be limited to 1~2 m/s.

 

So I assume for noise is one reason. But secondly to keep fan speed low. The higher the velocity the more the pressure drop, for a given flow rate, so the faster the fan has to run to compensate.