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Rigid or semi?


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Most here seem to have taken the semi rigid radial route for their MVHR ducting. If I have it right this is runs of corrugated plastic pipe going directly to/ from a box which has a pipe into the machine (plenum) from/ to input/ extract terminals with no branching. 

As I understand it the alternative is to use rigid galvanised ducts in which case the norm is to have main, larger diameter 'spines', one supply, one extract, with branches off them to the terminals. 

From a practical point of view I can see that the semi-rigid variety is easier to install and a radial system easier to  balance - but presumably these are one-time events. Greenbuildingstore counter this with a claim that semi-rigid pipe has a life expectancy of 15 years, galvanised 40. As replacing ducting would be a major undertaking in a finished house they strongly recommend the rigid product. 

As it stands we will have an I-beam floor which I believe allows a fair degree of flexibility in terms of coring out penetrations, there may well be some steel as well which will require advanced planning whatever we use. 

Does anyone know if the life expectancy above is realistic? If so it seems to be a no-brainer to go for rigid. 

Would a radial layout work in rigid, or is it a better idea to suck up the balancing difficulties of a branched system?

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First I've heard of a 15 year life on semi-rigid duct.  Ours has been in nearly five years now and there's not the slightest indication of any degradation, and I can't see why it shouldn't last as long as any other PVC pipe, like waste pipe or drain pipe.  They don't need replacing every 15 years.

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10 minutes ago, Nick1c said:

 

To me that article looks like it has been written by a marketing person. For example the flexible ducting I have seen being installed is indeed ridged on the outside but smooth on the inside.

Edited by Dreadnaught
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10 minutes ago, Nick1c said:

 

 

Got to ask yourself, which product do they make the most profit on?

 

Not that I'm cynical or anything, but I've not seen any evidence at all to support the view that PVC degrades after 15 years.  In fact I've seen first hand how PVC pipe lasts for decades with no degradation - our last house was built around 1982/3 and still had the original PVC gutters and downpipes when we sold it last year, so they were around 36 years old and still in good condition, plus they had been exposed to the elements all that time.  PVC duct should last longer, if anything, as it's in the dark and only subjected to a small range of temperatures.

 

Our semi-rigid ducting is all smooth on the inside.  I've got an offcut that has been around the back of the house for the last 5 years, exposed to the elements.  I'll dig it out tomorrow and have a look at it.

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1 hour ago, Nick1c said:

Greenbuildingstore counter this with a claim that semi-rigid pipe has a life expectancy of 15 years, galvanised 40.

 

So... uPVC is inert, resistant to all but the most corrosive of alkalis and acids, and does not cause condensation like metal products. Galvanised steel on the other hand is mildly reactive, usually put together with aluminium fixings that

create galvanic corrosion with any moisture in the air, and is also prone to condensation unless properly insulated. 

 

So, I’d say the chance of failure is a lot higher with steel than plastic ..!

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The claim seemed a bit odd to me, but I have no experience of ducting!!!! I may well call GBS tomorrow and get more info. 

Why would plastic ducting be more prone to bacterial growth then steel? I had assumed it was due to a build up of dust in the ridges, but the incoming air is filtered so this seems unlikely, the dusty (with a large proportion of skin) air will be on the exhaust side. 

Has anyone experience of both axial & radial systems? If so when is one preferable over the other?

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