Rigid vs Semi-Rigid

[joshwk]

Hi all. New member to the forum (intro post here). 

Currently looking at MVHR and am trying to determine whether rigid vs flexible ducting is "better" when starting with a new build. So far the school of thought seems to generally be that rigid is more hygienic/longer lasting but comes at the cost of price/slightly harder to install. A salesman was laying on the fact that all Passivhaus certified equipment is tested using a rigid setup (verifying this is the recommend setup), but unclear if this is true and accurate.

 

I don't want to hear it and wondered if rigid was 'quieter' than semi-rigid? 

Any thoughts or feedback much appreciated.

[SteamyTea]

I really don't know, but with the relatively low flow rates and assuming a decent installation, I doubt it makes any real difference.

If flexible ducting was dreadful, it would not be used much.

I think there is also a preference for rigid ducts as that is what is used commercially, and a lot of the MVHR people are old commercial installers.

[JohnMo]

All depends on the quality of the system design and implementation. Both can be good both can be rubbish.

 

It's all about keeping flow velocity low and duct run pressure drops low, so fan speed is low. You need to consider room cross talk with a rigid branch system you don't with a manifold system (rigid or semi flex).

 

Semi flex generally easy to install no joints.

 

17 minutes ago, joshwk said:

So far the school of thought seems to generally be that rigid is more hygienic/longer lasting

Where does that info come from? Not saying it's correct or not, but sounds....

[PNAmble]

We went rigid as by all accounts it’s quieter plus Nuaire certify only with rigid . 
 

designing it in at the start means that in a new build rigid was no more difficult. 

[nod]

4 minutes ago, SteamyTea said:

I really don't know, but with the relatively low flow rates and assuming a decent installation, I doubt it makes any real difference.

If flexible ducting was dreadful, it would not be used much.

I think there is also a preference for rigid ducts as that is what is used commercially, and a lot of the MVHR people are old commercial installers.

Beat me to the punch 

One a salesman will shock us all and recommend the least expensive product When you’ve read the back of the packet You will know as much as the salesmen 

 

[saveasteading]

I know nothing about particular mvhr pipes but do about pioes in general.

Some pipes that are ribbed on the outside have an almost smooth inner surface. 

But in general they are crinkly inside too. This creates a lot of turbulence and  flow resistance: reducing flow and making the fan work harder.

Therefore it's best to use smooth pipes whether rigid or flexible. The odd length of flexi to get round obstructions or smooth a turn would be ok.

[Pocster]

I always go rigid 

[G and J]

I’ve not finalised our design but I suspect I’ll end up using both.  Our MVHR unit is going in our garage and then feeds the house in rigid through a small loft space and through a steel to a central point for downstairs (so probably manifold and flexi thereafter) and also in rigid to another central point in the loft for upstairs, (again thence probably small manifold).

 

I read (and am still reading) lots and I’ve absorbed lots of info from biased and apparently unbiased sources and I’ve concluded I simply can’t trust one company to design me the best as they only use their own stuff.  But each discussion and quote has yielded useful info.  It does mean though that I take responsibility for the design and it’s successful operation.

[SteamyTea]

Worth looking here as a starting point as it can be modelled as a viscosity.

All the workings will be down at the bottom left of the charts.

 

https://www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601.html

 

25 minutes ago, Pocster said:

I always go rigid 

I heard you have a semi.

Edited June 9 by SteamyTea

[Blooda]

We went with Rigid ducting, purely because our first concept/pricing design, [Oct 2019], had to turn into our final design, as most people were sun bathing in their hot-tubs in lock down, and wouldn't talk to us.   Installed [Oct 20].   But if we did it again I think manifold and semi-rigid would be more convenient to install.    

[joe90]

3 minutes ago, Blooda said:

again I think manifold and semi-rigid would be more convenient to install.    

Yes I used semi rigid and far more easy to install pulling it around bends and no joins.