Ducting shape - does it matter?

[gc100]

 

I have vaulted ceilings thought my single story build and basically very little room between the roof joists and the plasterboard (50mm). I have another 70mm inbetween joists. However I have one room with is problematic (detailed here ) and the suggestion by one member is to use 225mm x 25mm flat ducting.

 

Using the excellent calculators found here  I can see this is equivalent to 85mm round diameter duct which for me kitchen would give about 2.5 velocity for BC boost regs for that room (46m3/h).

 

This on the face it it seems good. However does the shape of the duct have effect on noise but also backpressure which would mean the MVHR has to work harder?

I don't think I can avoid a couple of hard tight 90 degree turns either.

 

I could run 2 of these flat ducts and have 2 extracts, and 2 further ones for supply in that room.

 

Anyone know much about this?

Thanks

 

[Jeremy Harris]

I used one run of the slim rectangular duct in a tight spot, it seems to be fine.  The shape makes a very slight difference to the frictional loss, but not enough to worry about.

[tonyshouse]

Not much but velocity does, how about two ducts and two terminals , I would somehow squeeze in 50mm ducts if pos

[Ferdinand]

Cam you test one duct before you commit to the second?

[tonyshouse]

It could also influence the balance of the system and flow rate in the kitchen 

[Jeremy Harris]

As above, there will be a tiny increase in flow resistance from the increased surface friction from the changed duct cross sectional area to surface area ratio, but the impact of this at flow velocities below 2.5 m/s is going to be pretty tiny, and almost certainly way below the restriction that will be imposed by whatever sort of balancing valve/restrictor that needs to be fitted to every duct run.

 

Assuming this is a radial system, then the total flow resistance is  90% determined by the balancing valves/restrictors, rather than the duct frictional loss, and it's usually pretty easy to just adjust this during commissioning and flow testing  (although this is an iterative process and a bit of a PITA to do).

[gc100]

Thanks all. 
 

The designs drawn up from previous quotes have used a branched layout due to the small routing area of I have (down the ceiling of the hallway).

 

Does this effect overall velocities or calculations or is it just harder to balance?