smallest I have seen is semi flexible 75mm outside diameter, about 60mm internal. sometimes you use two 75mm pipes to keep the noise down - I would guess that 50mm would be very noisy (higher velocity air movement)

I could probably stretch to 75mm, will take out most of the studwork but do-able. I've had several lengths of 50mm wastpipe I've been trying to find a home for ages :-)

Posted By: XT600I could probably stretch to 75mm

That will give you a windspeed of 5 MPH to move 1 litre a second.

How much air are you looking to shift?

<blockquote><cite>Posted By: SteamyTea</cite><blockquote><cite>Posted By: XT600</cite>I could probably stretch to 75mm</blockquote>That will give you a windspeed of 5 MPH to move 1 litre a second.

How much air are you looking to shift?</blockquote>

I've no idea. When the wind blows through my open keyhole I can feel the draught from some distance, so a 75mm hole seems massive in that respect! :-)

For MVHR in a kitchen would require 10 l/s - a 50mm pipe requires a flow speed of 5m/s for that capacity (Steamy is a factor of 10 out in his calcs I think). The flow speed needs to be down to less than 2m/s ideally.

For laminar flow - which you need for quiet operation - the flow velocity profile is parabolic not uniform. So you need to double the peak speed. So a 50 mm pipe is way too small for 10 l/s.

See e.g. https://www.pipeflowcalculations.com/pipe-valve-fitting-flow/flow-in-pipes.php

PIV should be banned, like you say it pushes cold air into the house,

It does work in terms of no mould or condensation, probably would make wood burner glow red.

The bad thing about it is that it continuously pushed warm air out through all gaps and cracks!

And the bedrooms would become very cold and more noisy

<blockquote><cite>Posted By: gravelld</cite><blockquote><cite>Posted By: XT600</cite>The other thing about PIV is they normally get put in the landing ceiling, so how does that help circulate air around the bedrooms?
</blockquote>The bedrooms have trickle vents which is where the positive pressure is pushed out.</blockquote>

My bedrooms are now highly insulated and airtight apart from gaps left under the doors. The windows are triple glazed without air vents. This is to reduce traffic noise from the road, and to avoid pollution from the road side. My plan is to push fresh air into the rooms from the loftspace, which will circulate through the room, under the doors, and leave through the 'open' chimneys downstairs. The issue now is to decide how to control the amount of air pushed into the bedrooms, without introducing extra noise :-)

Make a box out of cardboard or hardboard etc and tape it in place? Filter material from http://www.fairair.eu/en/

Thanks for the links guys, will take a look at them.
Do you think that by using in-line filters in each room duct this will help avoid noise transference between rooms?