How to fit rigid ducting: a series of silly questions

[dnb]

Looking good!

 

5 hours ago, Omnibuswoman said:

Steico provide detailed tables for this...

 

Finishing this job off myself and can say that Steico do indeed make a very detailed document saying what you can and can't do.  I spent ages reworking the design to minimise joist penetrations and keep the holes away from the edges and the other (electrical) constraints - it was like doing circuit board design all over again.

 

 

 

 

 

[MVHRDesigner]

Hello. I am an MVHR Designer and I thought I would weigh in on some of the posts made, as I think they are a bit misleading. 

 

On 25/03/2024 at 15:41, Iceverge said:

I don't think what your aiming for is actually possibly without having one million joints in the rigid ductwork. It'll have to go back. 

 

Semi rigid radial ducting is lightly your only option.

 

Some companies are a little precious (read snobby)  about this but so long as you put adequate silencers and size the ducts appropriately it's idiot proof. I would say 95% of DIYers use it. 

This is incorrect. Where i-joists are used, rigid ducting is the norm. The major advantage being the ease of cutting holes in i-joists, and the amount of holes that can be made (following guidelines on the data sheets). People often think of semi-rigid (flexi) ducting as space saving, but clearly with this type of joist you would require far more holes because there are far more ducts.
A good designer will ensure the guidelines for holes are not breached.

 

 

On 25/03/2024 at 16:14, BotusBuild said:

I've been cutting that rigid ducting recently - about 8-10 times so far with an angle grinder and cutting disc, with a couple more to do. Believe me, you do not want to go there.

 

The people doing the design need to know the orientation of your joists, and what they are made of so they understand what is possible. Insist on NO RIGID DUCTING.

It is straightforward to cut duct with a hand-held grinder, even with a 1mm cutting blade. See my post above, rigid ducting is the norm where i-joists are used for the floor structure.

 

On 25/03/2024 at 16:46, Iceverge said:

You can calculate and design it pretty easily. 75mm rigid ducting is ok for 2.5m/s flow speed so 28m3/HR.  Assuming normal ceiling heights and a desired ACH of 0.3 then each pair of 75mm ducts will supply 37m2 floor area. 

28m3/hr down a 75mm duct is fairly standard for a lot of companies, however, an optimal design will use two ducts for this flow rate (or higher). Bearing in mind that lower air velocity = lower pressure = lower sound.
Most people would rather spend a bit more, have more ducting etc to have an inaudible system, over saving a few quid but having a system they can constantly hear.

 

On 25/03/2024 at 17:23, Iceverge said:

Me too, without personal comparison I'm led to believe the rigid ones are better. I think it's worth the investment if you have good hearing. 

This is generally true. Rigid steel ducting tends to be lower pressure/sound.

Semi-rigid does get unnecessarily harshly judged in my opinion. I have designed perfectly good systems for Passivhauses.
In my experience it is due to many early systems being in semi-rigid, and were poorly designed and/or poorly installed.
Even though it is straightforward to make connections airtight, I see more often semi-rigid systems with leaks due to improperly connected ducts to manifolds, plenums, etc. With rigid steel duct you just push it together once and it is airtight.
And lastly the high volume of air many companies design for each duct. When I design I aim to put less than 26m3/hr down a 75mm (ID) duct, and less than 18m3/hr down a 63mm (ID) duct.

To the OP, it looks like a nice system.

 

[SteamyTea]

17 hours ago, MVHRDesigner said:

lower air velocity = lower pressure

Not according to Bernoulli.

 

https://sciencing.com/fluid-dynamics-overview-basics-terminology-equations-13723386.html

[Omnibuswoman]

@MVHRDesigner thank you for sharing this information.
 

One of the buildhub conventions is that people who work for companies that we might purchase goods or services from indicate their interest/affiliations, so that buildhubbers can take account of that when considering any position put forward. So I would just like to invite you to do that, if you will. 
Thanks ☺️ 

[LnP]

6 hours ago, SteamyTea said:

Not according to Bernoulli.

 

https://sciencing.com/fluid-dynamics-overview-basics-terminology-equations-13723386.html

Maybe he meant lower pressure drop. Even though the pressure in a Venturi drops as the velocity increases, once the flow leaves the Venturi, it won't recover to the full upstream pressure, i.e. there's a pressure drop across the Venturi which increases as the flow increases. But I'm curious though, is Bernoulli and Venturies something we need to consider when designing MVHR systems? I've read this thread with interest as I will be installing an MVHR.

 

[SteamyTea]

5 minutes ago, LnP said:

Maybe he meant lower pressure drop

Could have.

 

5 minutes ago, LnP said:

Venturi

Strange things and quite counter intuitive at first glance.

But very useful.

I wonder why there was a not a venturi driven MVHR system.

Probably because electronic motor controls are cheap and reliable.

[Iceverge]

@MVHRDesigner

 

Welcome to the forum. Always good to have more experience as most of us are self taught. Like mentioned above maybe you could do an introduction. 

 

I self installed our MVHR. It was making noise until I made a DIY silencer out of foam and MDF. I have a thread on it somewhere. (It's since fallen apart as I only used airtight tape to make and it only lasted 2 years or so, I must redo it soon, my laziness and corner cutting knows no bounds!)

 

The semi rigid duct lengths didn't seem to make any difference to noise in my experience, if anything the shorter runs nearer to the unit were noiser. 

 

I appreciate that there is much work designing ventilation systems especially in large and complex buildings.

 

However most sensitive sized houses can be designed with a few rules of thumb. I'm yet to be convinced that domestic MVHR is any different.