Rigid vs semi rigid ducting?

[SBMS]

This might have been asked before but speaking to a number of companies it seems they recommend a branch system with rigid ducting versus a radial with semi rigid? For those who have installed MVHR which route did you go down?

[MJNewton]

Semi rigid, not least given that kine was a retrofit and rigid would've been impossible. I'm not sure of the benefits of rigid though - semi rigid seems to tick so many boxes in terms of ease of use, absence of joins etc. 

[Russdl]

Semi rigid if you’re doing it yourself, its easy - depending on the house build of course.  

[Conor]

I did a mix of both as we have a three story building. I used a mix of 160 and 125mm galvanised ducts to each floor. Then branched off using tees to semi rigid or to a manifold then semi rigid. Advantage is lower pressure losses an quieter system. Disadvantages, much more time consuming to install.

[JohnMo]

Semi rigid 90mm via plenum box.  Ease of DIY install, but also very quiet in use and no cross talk.  Most of mine is also going through Posi rafters and there was not enough room to install rigid.

 

If doing rigid, use metal pipes only, most plastic bends the radius this too tight and you get a large pressure drop.  Blumberg may be the exception, as their bends have a larger radius.  You will also need an attenuator at each outlet to stop cross talk.

[Gone West]

10 hours ago, SBMS said:

For those who have installed MVHR which route did you go down?

I designed a branch system using rigid plastic ducting.

 

 

[epsilonGreedy]

Is pressure drop a real concern given that most MVHR owners run their system at the lowest trickle rate once past BC inspection?

[gavztheouch]

7 hours ago, Conor said:

I did a mix of both as we have a three story building. I used a mix of 160 and 125mm galvanised ducts to each floor. Then branched off using tees to semi rigid or to a manifold then semi rigid. Advantage is lower pressure losses an quieter system. Disadvantages, much more time consuming to install.

Hi Conor, what size is your flexible duct 75mm or 90mm?

 

 

[crispy_wafer]

6 hours ago, Gone West said:

I designed a branch system using rigid plastic ducting.

 

 

 

How does the plastic piping interface with the metal silencers, any issues?  I'm planning on doing similar but without getting my hands on stuff it's hard to know about how interchange the plastic is with metal fittings.  Could I ask what sealant you have used on the pipe joins please?

 

[Gone West]

1 minute ago, crispy_wafer said:

How does the plastic piping interface with the metal silencers, any issues?  I'm planning on doing similar but without getting my hands on stuff it's hard to know about how interchange the plastic is with metal fittings.  Could I ask what sealant you have used on the pipe joins please?

It was a while ago now but IIRC the metal silencer had a rubber 'O' ring on the outside which sealed against the inside of the ducting and I don't remember any problems.  I bought all the ducting etc from i-sells. I used Titebond PU gel adhesive to to glue the ducting together.

https://www.axminstertools.com/titebond-greenchoicetm-polyurethane-construction-adhesive-311ml-952217

 

[Conor]

@gavztheouch 90mm (76mm internal diam)

[gavztheouch]

3 hours ago, Conor said:

@gavztheouch 90mm (76mm internal diam)

? thanks

[Rishard]

@Gone West- not an mvhr question but, What are the bowl looking things you have your lighting cables running into in your voids? Thanks

[Adsibob]

On 05/02/2022 at 10:44, epsilonGreedy said:

Is pressure drop a real concern given that most MVHR owners run their system at the lowest trickle rate once past BC inspection?

I didn’t know that. Is the BCO’s flow rate / Air Change requirements excessive then?

[Nickfromwales]

26 minutes ago, Rishard said:

@Gone West- not an mvhr question but, What are the bowl looking things you have your lighting cables running into in your voids? Thanks

Intumescent fire hoods for lighting. They offer some acoustic benefits also, as they are fully encapsulating the fitting, plus the acoustic insulation can go over / all around them.

Edited March 13, 2022 by Nickfromwales

[Rishard]

@Nickfromwales- cheers, I thought they may be something for fire safety. Is there a discussion somewhere stating pros/cons of first floor insulation for a near passive house? I’m sure I read somewhere about people leaving it out? Seems daft as like you say, acoustic insulation will be beneficial. Apologies for straying off piste of the mvhr discussion. 

[Nickfromwales]

23 minutes ago, Adsibob said:

I didn’t know that. Is the BCO’s flow rate / Air Change requirements excessive then?

Your commissioning agent should set the trickle rate accordingly, but you can then set the flow rate per setting. So say factory set position 1 may be 35%, but you can then alter that to 25% for eg after sign-off. Just buy cheap humidity and CO2 monitors and check you haven't dropped to low.

[Nickfromwales]

3 minutes ago, Rishard said:

@Nickfromwales- cheers, I thought they may be something for fire safety. Is there a discussion somewhere stating pros/cons of first floor insulation for a near passive house? I’m sure I read somewhere about people leaving it out? Seems daft as like you say, acoustic insulation will be beneficial. Apologies for straying off piste of the mvhr discussion. 

Digression is a popular pass-time here lol. BCO may well request acoustic attenuation between rooms and floors, so, as you say, its daft to leave it out as walking upstairs would otherwise convey a lot of unwanted noise to the lower floors.

Edited March 13, 2022 by Nickfromwales

[Gone West]

15 hours ago, Rishard said:

@Gone West- not an mvhr question but, What are the bowl looking things you have your lighting cables running into in your voids? Thanks

They are plain compressed glass fibre hoods, not intumescent fire hoods, that I used for all my downlighters. I used them to keep the insulation away from the lights. I bought them from TLC many years ago but they don't seem to sell them any longer. They were about 250 to 300mm diameter IIRC.