On the design i received the double runs were on all the extracts but only on the supplies that exceeded a certain length. Similar to yourself its in a refurb that had already been drawn up, i made a few stud walls deeper than originally envisaged and managed to route all but 1 outlet/inlet as designed, this has been reduced to a single run for about half its length ( ceiling plenum has two ducts , but these merge half way back to manifold box, was the best compromise i could manage for that run)
Again like yourself still not received flow rates or any calcs, can’t say i’ve been overly impressed with the suppliers performance , compared to the assurances in the dealings before transferring the cash.

Posted By: modernvictorianI am retrofitting MVHR into an old property so there are obvious limitations with locating duct runs, manifolds etc. The design the supplier has provided includes a lot of 75mm double duct runs. I am wondering if this is an absolute requirement or if I can reduce a couple of the double duct runs to single runs (back bedroom and loft?). There were no figures for air changes, flow rate etc. provided which probably doesn't help with the decision.

Insist on being told the flow rates and the linear air speed in the ducts and at what settings on the MVHR unit. The flow rates need to meet building regs at least and the linear speed is what determines noise levels. PH has more stringent requirements but as I remember we only have one double duct, from the kitchen. We have the D-shaped ducts though (AE35), which will make some difference I suppose. If you can fit larger ducts in place of the 75 mm ones you may be able to avoid the double runs. But the first thing is to check that the design flow rates are not too high. Definitely worth minimising bends where possible as well.

It's the speed of the air through the ducts that creates the turbulence and noise. The fan noise is reduced by the attenuator and sound absorption built in to the distributor boxes.

RobL said: "I think from memory 4m/s was the critical flow speed to stay under"

That sounds about right. PH standards are a bit tighter. There's a very good guide to MVHR at http://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/2018%20MVHR%20Good%20Practice%20Guide%20rev%201.1.pdf

It says "Another limiting factor of air velocity in ducts is noise. High velocity air generates noise, and therefore air speed limits are recognised for ducts in different locations to minimise noise problems. 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 for my radial semi-rigid system I believe the design limit was 2 m/s.

They look like the standard calculations, which is good.

1 Litre per second (L/s) = 3.6 cubic metres per hour (m3/h)

You can check the flow speed through the ducts if you know the internal cross-sectional area of the ducts. It might be worth seeing what the speed in the duct to the lounge is with just a single duct. I suspect the other thing they were considering is the pressure loss in the longer runs, which will mean the fans have to work a little harder.