What ducting material for MHVR

[Happy Valley]

There appears to be a number of options from plastic radial ducting to spiral steel ducting.

Which ones are best to avoid - thinking the minimum I should have is an antibacterial plastic radial. 

 

There are 6 air supplies and 5 extractors in a 160 sq m house (none on landing or in entrance hall). We have 125mm between the joists to run ducting to the ground floor rooms and over 150mm for the exhaust and inlet which will be on a flat garage roof.

[PeterW]

28 minutes ago, Happy Valley said:

thinking the minimum I should have is an antibacterial plastic radial. 

Why..?

[saveasteading]

Drainpipe. Easy to get, smooth flows, solid, proper, sealed joints at junctions. Doesn't rot or scuff or get eaten, has standard brackets for fixing solid, can change to rectangular at tight spaces. 

[Happy Valley]

18 minutes ago, PeterW said:

Why..?

 

I presume it is sold as an upgrade option for a reason by most suppliers.

[markc]

depending on material, some drain/down pipe can create static.

spiral steel does not do this but is more difficult to work with.

[MJNewton]

19 minutes ago, saveasteading said:

Drainpipe. Easy to get, smooth flows, solid, proper, sealed joints at junctions. Doesn't rot or scuff or get eaten, has standard brackets for fixing solid, can change to rectangular at tight spaces. 

 

Is that recommendation from personal experience? I've not heard that approach before.

 

I found 75mm semi-rigid radial ducting perfect for the job and a joy (well, almost!) to use once I'd got the hang of it. No joins (barring the two end points of course!), easy to route around obstacles, swept bends, loose coupling with the house structure to avoid vibrations etc. I didn't find the pricing particularly offensive, especially given how quick it was to install, and it wouldn't surprise me if it was far cheaper the drainpipe given all the fittings you'd need.

 

My case was a retrofit and so solid piping would never have been any option but I think my positive experience would be equally applicable to a new build with much easier access.

 

P.S. Being more specific, I used Airflow Airflex (the red stuff) and whilst Blauberg do a similar (compatible) product in green I had a piece and found it far less flexible and not quite as self-supporting in making a tight bend should that be required.

Edited September 10, 2021 by MJNewton

[markc]

Convoluted/radial/flexible whatever you want to call it is fine for low volume low velocity air but dramatically reduces the efficiency of the system as turbulence is created.

[MJNewton]

Are you saying it's an issue for MVHR (which I assume would be considered 'low volume low velocity')? What sort of issue have you seen?

Edited September 10, 2021 by MJNewton

[markc]

Just now, MJNewton said:

Are you saying it's an issue for MVHR (which I assume would be considered 'low volume low velocity')?

In a house build size MVHR it won’t make much difference. In larger installations the loss of efficiency and increased noise can be an issue.

[MJNewton]

Ah okay. The OP is in a house in this instance.

 

Is drainpipe what's used in large installations?

 

(Hope this isn't sounding passive aggressive - I'm genuinely intrigued!)

Edited September 10, 2021 by MJNewton

[Happy Valley]

Just found this on an installers website:

 

Four kinds of ducting are compatible with a whole house Mechanical Ventilation & Heat Recovery (MVHR) system; flexible ducting, rigid metal ducting, rigid PVC ducting, and semi-rigid (bendy) ducting. Ducting can be installed in either a 'branch' configuration, where one pipe or branch services multiple rooms, or it can be a radial system where every room has it's own dedicated air pipe, all fed from one air distribution box or manifold.

Flexible ducting in a branch configuration is frowned upon by BPEC, the body that certifies our installers, as condensation can pool in the internal spirals and can potentially become a health risk. Its fine if it's insulated and used in short lengths to connect the unit to atmosphere, but is best avoided in the rest of the property.

Rigid metal ‘spiral’ ducting in a branch configuration is difficult to work with and is susceptible to heat loss, so even if the MVHR is 90% efficient, up to 50% of the warm air inside the pipe could be lost on the way to and from the heat exchanger. Large diameter spiral ducting is typically reserved for commercial air handling systems which don't generally recover heat.

Rigid PVC ducting is certainly the most common domestic system, however it’s difficult if not impossible to clean and requires substantial boxing-in making it tricky to retrofit. A ‘branch’ ducting system with flexi, metal or PVC pipe is also prone to acoustic issues, either a whistling from the supply/extract valves nearest to the fan motors or 'cross talk' travelling from one room to another. The last thing you need is TV noise in your bedroom from somewhere else in the house.

By far the best kind of ducting in a domesitic or small commercial property is the new semi-rigid ‘radial’ ducting. The benefits radial ducting include:

• Flexible enough to thread through joists, between stud walls and other tight spaces (it's ideal for retrofit).
• Eliminates the risk of cross talk between rooms as every room has its own dedicated air pipe connected to a sound-proof manifold. The manifold also eliminates any motor noise from the MVHR fans.
• Can be cleaned in the future as it has a smooth internal bore and there are no sharp bends. While advisable at the moment, this feature is likely to become a requirement under building regulations.
• Incredibly low air resistance meaning the MVHR fans have less work to do, making the unit quieter, more energy efficient and (in theory) extending the life of the motors.
• Far easier and quicker to install as it clips together with no gluing & taping. One continuous length runs from the manifold near the unit to each room. The pipe can be cut neatly with a blunt knife rather than awkwardly with a saw. The install time is normally a lot less than that of rigid PVC ducting. It’s ideal for DIYers or builders new to MVHR work. The risk of boxing in a leaky pipe is almost nil.
• Partially insulated to reduce heat loss through the ducting. It can also be lagged with mineral wool without the condensation risks associated with PVC ducting.
• Extremely durable with a higher tensile strength than any other ducting

[saveasteading]

29 minutes ago, MJNewton said:

 

Is that recommendation from personal experience?

Yes. Used it for extract fans, including multiple intakes from an inline fan downline.

 

When you take out old spiral stuff you see how filthy it gets because of all the  shapes and ripples in it. Round pipe you could even rod if you wanted.

15 minutes ago, Happy Valley said:

rigid PVC ducting

that is drain pipe to me.

 

perhaps there is wonderful new stuff I don't know about. this is only my  opinion.

I last used flexi on an extract fan so am not against it...but it is so flimsy.

[MJNewton]

I'm nodding at all those points, although I've never measured its air resistance and it's not clear what the 'less work to do' is in comparison to (other than ducting with higher resistance of course!). Certainly as far as I'm aware it's working perfectly, and is doing so completely silently so I can't really think of any downsides. I suppose there is the issue that you have to buy it in large rolls and so you are at risk of over/under-buying. I sold what little excess I had though as there's often someone looking for just that little extra having run out themselves!

 

If you've never had a piece in your hands more than happy to pop one in the post so you can get a proper idea what it's like. Of particular note is the internal lining which whilst ridged is what I'd call 'semi smooth', which of course is impossible to interpret accurately.

[MJNewton]

16 minutes ago, saveasteading said:

Yes. Used it for extract fans, including multiple intakes from an inline fan downline.

 

When you take out old spiral stuff you see how filthy it gets because of all the  shapes and ripples in it. Round pipe you could even rod if you wanted.

 

It sounds like we might be talking about different things. I can't imagine anyone using semi-rigid radial ducting like the following for extractor fans.

It's got an internal diameter of 68mm so it would indeed be a terrible choice given the flow rate of a typical extractor.

Edited September 10, 2021 by MJNewton

[saveasteading]

9 minutes ago, MJNewton said:

talking about different things

I mean common drain pipe with no ridges.

[dpmiller]

Yep, but the flexible stuff used in the radial systems is smooth bore too...