Ducting and Noxious Odours (a paste)

[Fallingditch]

Formerly, "What's The Best MVHR Ducting To Use?"

Started by mikesharp01, 27 Aug 2015 03:42 PM

Am working up details of our new build design with our architect and am looking at the range of ducts and duct sizes available. I can do the sums on the air flow needs OK but I am getting mixed advice about the type of duct to choose. What are the advantages / disadvantages of them all? We, the architects have not done MVHR before either, are thinking the semi rigid ducting is the way to go for what I suspect will be a radial system as this seems to offer a number of advantages but I am not sure. What do people think is the right way to go or is it an ask 9 economists and get 10 answers sort of a problem? (TFA is 140Sqm) 

#2joe90

Posted 27 August 2015 - 03:51 PM

Well this is something I have thought about. I like a radial system (Jeremy has one, and he will have done shed loads of homework) it enables throttling to be done at the plenum so noise is less of an issue. I have heard/read that 125/150 mm ducts are best but I will watch this thread with great interest. 

 

#3wmacleod

Posted 27 August 2015 - 04:15 PM

I got a load of rigid 125mm galv bits cheap a few years ago in anticipation of my install and still ended up going with semi rigid radial as it is so much quicker/easier to install. I think most folk are going down the radial route now - it is more pleasant to work with and should have fewer leaks over the lifetime of the system. 

#4jsharris

Posted 27 August 2015 - 05:59 PM

It is, without a doubt, far easier to DIY an MVHR fit using semi-rigid ducting and a radial system. It's the reason we went for it, as it need virtually no design work, other than some really basic stuff like getting the terminals in the right place and making sure any high flow rate terminals that are a long way from a plenum have double ducts. Pretty much everything can be adjusted at the plenum ends to balance the system. The joints are all dead easy too, just push-fit with rubber seals and a locking clip.

So, in terms of ease of design and fitting radial semi-rigid wins hands down. Where it loses out is price, as the cost of the ducting, fittings and plenums is more expensive than a non-radial rigid duct system.

My view is that the saving in not needing to do detailed flow calcs (which you have to do with a stepped size rigid system) plus the much quicker and easier fitting makes radial ducting a no-brainer for DIY. The only other downside I can think of is that adjusting flow rates when commissioning the system is a bit of a pain, as you keep having to move between changing the restrictor rings in the plenum and re-measuring the critical terminal. Against that is that, unlike a conventional system, there is very little interaction between inlets/outlets, which probably compensates for the running back and forth when adjusting things. 

#5vk2003

Posted 27 August 2015 - 06:19 PM

Feeling a bit thick here, but could someone elaborate on what a radial design is?

I know our mvhr was installed using rigid metal ducting (I was very firm in specifying a silent system...) 

#6jsharris

Posted 27 August 2015 - 06:49 PM

A radial system uses smaller ducts from each room terminal (sometimes doubled up for higher flow rates) that lead back to a central plenum chamber that provides air at the same pressure to each duct, and acts as a silencer. In general radial systems use a lot more, smaller diameter ducting, but have the advantage of being inherently quieter (as the noise generating stuff is retained back at the plenum chamber).

Conventional ducting depends very much on getting the detailed design right, and is inherently noisier on boost, as the flow control (and hence restriction) is at the terminal end, and it's the restrictor that makes most noise.

Both can operate equally well, but a radial system is simpler to design and install, and more tolerant of minor design errors. 

#7mikesharp01

Posted 27 August 2015 - 07:04 PM

Dear All.

That clinches it then radial / semi rigid it is, just need to source all the bits. Like the PAUL FOCUS 200 for our project @ 91% (Before adjusting) its got a great efficiency. Will press on designing for 75mm pipes everywhere. Thanks Jeremy et al. 

#8wmacleod

Posted 27 August 2015 - 07:07 PM

vk2003, on 27 August 2015 - 06:19 PM, said:

I know our mvhr was installed using rigid metal ducting (I was very firm in specifying a silent system...)

Just to add, radial systems are less prone to cross-talk problems (sound travelling through the ducts from room to room rather than fan noise) which can be an issue depending on your layout. 

#9jsharris

Posted 27 August 2015 - 07:08 PM

We bought all our stuff from CVC Direct, just for convenience. The same product (HB+) is sold by several vendors and production has been licensed to other companies, for example Polypipe sell exactly the same stuff under their Domus brand name. 

#10mikesharp01

Posted 27 August 2015 - 07:50 PM

Thanks for that Jeremy, interestingly my other half wants a central vacuum system and we had looked at other suppliers so a single source opportunity here. 

#11Alphonsox

Posted 27 August 2015 - 07:58 PM

A central vacuum may not be the best idea for a high spec home. 

http://www.ebuild.co...cuum#entry96869 

#12tonyshouse

Posted 27 August 2015 - 08:09 PM

Rigid spiral ducts for me and central vac is great too. 

#13djh99

Posted 27 August 2015 - 08:31 PM

There are some differences between the various brands. I went for a Brink MVHR unit and associated Ubbink semi-rigid ducting. The system uses preformed insulated ducts from the MVHR unit through an attenuator into the plenums (or distribution boxes) and then through the D-shaped radial ducts. The main difference between this and some other versions is that they first design the system (in a few iterations with you) and run it through some software that calculates the restrictor ring sizes that are needed for each duct and tells you where you need double ducts etc. Then you install it all, which is relatively easy through metal web joists, although getting all the radial ducts to come together nicely at the plenums is 'interesting'. Once it's all finished, you just plug the MVHR unit in and the job's finished*. There's no need to balance anything or do any commissioning, because the MVHR unit is self-balancing and has set flow rates, and the precalculated restrictor rings ensure the correct flow through each duct.

* n.b. after testing that it works, make sure to turn the MVHR off and put the temporary plugs back in all the duct openings until the construction work is finished, to avoid getting carp in the ducts and MVHR filters. 

#14jsharris

Posted 27 August 2015 - 08:45 PM

Ours is actually the Brink/Ubbink/HB+ ducting. The pre-calculation option has very large errors, I found, as if you have a lot of bends in some ducts and relatively few bends in another duct the balance gets thrown out a lot.

For example one of the longest critical duct runs (as in one that had to meet a BR required max flow rate on boost) was the kitchen. The calculator said we needed double ducts, but the reality is that we could have got away with a single duct easily, as there are very few bends in this run. Conversely, some runs with sharp bends are surprisingly less effective than they should be.

Finally, my addition of 38mm waste pipe ducts in a straight run from the extract plenum to the upstairs toilet cisterns flows far more air than they should, according to theory. I can only assume that very smooth bore plastic pipe has a far lower resistance to flow than the standard calcs assume. 

Edited by jsharris, 27 August 2015 - 08:47 PM.

#15mikesharp01

Posted 27 August 2015 - 08:58 PM

Why to the cisterns Jeremy, have not come across that in my reading so far or did I miss it in one of your blogg entries? 

#16Alphonsox

Posted 27 August 2015 - 09:08 PM

mikesharp01, on 27 August 2015 - 08:58 PM, said:

Why to the cisterns Jeremy, have not come across that in my reading so far or did I miss it in one of your blog entries?

#17jsharris

Posted 27 August 2015 - 09:11 PM

mikesharp01, on 27 August 2015 - 08:58 PM, said:

Why to the cisterns Jeremy, have not come across that in my reading so far or did I miss it in one of your blog entries?

I discovered that the air space above a toilet cistern is directly connected to the flush ring round the edge of the bowl, for as long as the toilet isn't being flushed. So, of you connect the MVHR extract to the cistern air space, by opening up the unused second flush inlet hole to accept a 1 1/2" waste pipe, you can have continuous, low level suction all around the edge of the toilet bowl.

This neatly extracts noxious odours at source, and stops them escaping into the room, they just get sucked out via the extract. For best effectiveness it's best to close the seat immediately after use and flush, as that way the vast majority of noxious odours go straight out through the MVHR extract.

In our case, adding this to the upstairs bathrooms was very easy, as both had toilets with cisterns built in to the bathroom furniture, and both backed on to the eaves dwarf wall behind which all our services run, so putting in a discreetly hidden system was easy. 

•  

#18mikesharp01

Posted 27 August 2015 - 09:36 PM

brilliant, should be possible in our design as provided the 38mm pipe can be run through the service void as all the bathrooms are above or adjacent to the plant room. Balancing by changing rings seems like a problem looking for a solution, surely someone has developed a simple adjustable iris ring approach that would be simpler. 

•  

#19vk2003

Posted 27 August 2015 - 09:44 PM

Thank for the explanation Jeremy. I don't believe mine is a radial system. Funnily enough our MVHR was done by Total Home, who I have to say were very professional, and have done a good (looking) quality installation. Shall have to see how it is when commissioned, which should be happening in September. Our build is finally drawing to a close, and we will be in by the end of October  

•  

#20Nickfromwales

Posted 27 August 2015 - 09:44 PM

mikesharp01, on 27 August 2015 - 07:50 PM, said:

Thanks for that Jeremy, interestingly my other half wants a central vacuum system and we had looked at other suppliers so a single source opportunity here.

Central vac is a great idea and a terrible reality. I wouldn't have one if it was fitted free. 

#21mikesharp01

Posted 27 August 2015 - 10:20 PM

Work to do on central vac idea by the sounds of it, what is so wrong about it Nick 

#22Nickfromwales

Posted 27 August 2015 - 10:31 PM

The hoses for a start. Long, heavy horrible things which drag against corners and paintwork. 
Big noisy cumbersome central unit. 
Tools and plate / union where hose meets wall don't last. 
One hose really needed per floor. 
Going to change the hose from outlet to outlet to vac a whole floor. 
Loads of outlets needed to make the hose less like a firehose. 
That means more ducting, cabling, labour. 
Oh, and you'll end up just buying a dyson after all the above which will make you want to pay someone from a rugby background to kick you when you've finished kicking yourself and have become tired from all the kicking.
Then your wife will kick you some more when she realises that the money could have gone towards a nicer kitchen. 
And breathe.................. 
Shall I go on 
Regards, nick. 

#23notnickclegg

Posted 27 August 2015 - 10:33 PM

jsharris, on 27 August 2015 - 08:45 PM, said:

Ours is actually the Brink/Ubbink/HB+ ducting.

Ubbink is, at least in the UK, pushing their newer semi-circular cross section version of this. It has a completely different connection system to what I understand is used in the HB+ system (they definitely don't turn and click, for example).

The narrower design is great for squeezing through narrow spaces but I found the drawbacks overall were such that I'd never use it again. The main issue was that the non-circular cross section means ducts want to only bend in horizontal or vertical plane, not both. This means that you can't easily change vertical level by a small amount while sweeping around a curve, for example, because the end of the duct twists, leaving you unable to plug it into the next connector. I found this characteristic unexpected and utterly infuriating.

It's also quite a bit more expensive than the HB+ stuff, I believe.

Jack 

#24ConstructionChannel

Posted 27 August 2015 - 10:49 PM

Nickfromwales, on 27 August 2015 - 10:31 PM, said:

The hoses for a start. Long, heavy horrible things which drag against corners and paintwork.
Big noisy cumbersome central unit.
Tools and plate / union where hose meets wall don't last.
One hose really needed per floor.
Going to change the hose from outlet to outlet to vac a whole floor.
Loads of outlets needed to make the hose less like a firehose.
That means more ducting, cabling, labour.
Oh, and you'll end up just buying a dyson after all the above which will make you want to pay someone from a rugby background to kick you when you've finished kicking yourself and have become tired from all the kicking.
Then your wife will kick you some more when she realises that the money could have gone towards a nicer kitchen.
And breathe..................
Shall I go on 
Regards, nick.

you missed emptying the now massive drum of carp it has sucked up! 

#25Nickfromwales

Posted 27 August 2015 - 11:28 PM

I knew there'd be more downsides. Just thought that the first 8 should suffice. 
 

#26mikesharp01

Posted 28 August 2015 - 08:07 AM

What is so bad about it Nick - I think we get it and has anybody got any experience with polypipe DOMUS radial? 

Edited by mikesharp01, 28 August 2015 - 09:10 AM.

#27djh99

Posted 28 August 2015 - 09:05 AM

notnickclegg, on 27 August 2015 - 10:33 PM, said:

Ubbink is, at least in the UK, pushing their newer semi-circular cross section version of this. It has a completely different connection system to what I understand is used in the HB+ system (they definitely don't turn and click, for example).

Yes, that's what I meant when I said D-shaped. It's definitely not the same as the HB+. The software explicitly takes account of the resistance of all the bends that are included in each run, so I don't know why there would be a problem. I haven't seen any such problems. In our case the main concern with the design was achieving the low flow speeds required by the Passivhaus standard and that was what dictated our double run to the kitchen, for example.

Quote

The narrower design is great for squeezing through narrow spaces but I found the drawbacks overall were such that I'd never use it again. The main issue was that the non-circular cross section means ducts want to only bend in horizontal or vertical plane, not both. This means that you can't easily change vertical level by a small amount while sweeping around a curve, for example, because the end of the duct twists, leaving you unable to plug it into the next connector. I found this characteristic unexpected and utterly infuriating.

I agree the duct sometimes has a mind of its own. The best technique I found was to use plenty of the restraining clips where necessary, using extra blocks and noggins on the joists to provide suitable spots to place them. It only really matters at the ends of the run where it plugs in to a connector; it can be left to happily do its own thing over most of the run.
Oh, and I found it was much easier to install when the ducting was warmer. It made it more flexible and willing to follow instructions!

Quote

It's also quite a bit more expensive than the HB+ stuff, I believe.

I regard it as one of my more successful choices. 

#28Trw144

Posted 15 September 2015 - 09:27 PM

jsharris, on 27 August 2015 - 05:59 PM, said:

It is, without a doubt, far easier to DIY an MVHR fit using semi-rigid ducting and a radial system. It's the reason we went for it, as it need virtually no design work, other than some really basic stuff like getting the terminals in the right place and making sure any high flow rate terminals that are a long way from a plenum have double ducts. Pretty much everything can be adjusted at the plenum ends to balance the system. The joints are all dead easy too, just push-fit with rubber seals and a locking clip.

So, in terms of ease of design and fitting radial semi-rigid wins hands down. Where it loses out is price, as the cost of the ducting, fittings and plenums is more expensive than a non-radial rigid duct system.

My view is that the saving in not needing to do detailed flow calcs (which you have to do with a stepped size rigid system) plus the much quicker and easier fitting makes radial ducting a no-brainer for DIY. The only other downside I can think of is that adjusting flow rates when commissioning the system is a bit of a pain, as you keep having to move between changing the restrictor rings in the plenum and re-measuring the critical terminal. Against that is that, unlike a conventional system, there is very little interaction between inlets/outlets, which probably compensates for the running back and forth when adjusting things.

Jeremy, can I ask how you specced and setup your system - I have trade account with Lindab and ship items in every month from Denmark so am looking to go your route. However, I obviously need to size, plan and commission the system somehow? 

#29jsharris

Posted 16 September 2015 - 08:05 AM

The spec was pretty easy, as all I needed to check was that I could get the flow rates needed for the critical rooms (those subject to building regs max extract rate requirements) without exceeding the max velocity limit in the ducting of 2.5 m/s. In practice you can go over 2.5 m/s when on boost I've found, as the duct noise doesn't start to get intrusive until around 3 m/s. These flow rates were just from the building regs requirements in Part V.

As a double check I looked at the whole house trickle ventilation rate from building regs, but that turned out to be unimportant in terms of system design if you have terminals in every room, as the flow velocities in the ducts are very low for trickle ventilation.

I designed the layout of the terminals so that the air had to travel the longest path from the terminal to the extract point in each fresh air feed room. In those rooms the extract point was the door (specifically the gap under the door) so the fresh air terminals were fitted diagonally opposite the door in most cases, and usually in a corner that might otherwise be a stagnant air pocket. I didn't bother with detailed duct loss calculations, as I started doing them for the high flow ducts (kitchen, bathrooms) and quickly realised that for a house of our size the ducting was more than capable of delivering the flow rate needed without incurring either high losses or excessive duct air velocity.

I ended up just running the ducts by the easiest route and doubling up on the kitchen extract duct (two parallel ducts to the same terminal) to ensure I could reach the high extract rate on boost. In practice I'm pretty sure that a single duct run would have been OK.

I fitted the manifolds as close to the MVHR as possible, to minimise the length of 150mm duct needed. I doubt this is really critical, as a 150mm duct will flow a lot of air, but it was pretty easy for me to put the manifolds close to the MVHR. They need to be somewhere where you can get easy access, as when setting the system up you'll be fitting throttle rings to the ducts in them. Some of my shorter ducts are throttled back to the smallest hole in the ring, which is something like 20 to 25mm diameter, so that may give a feel for how little air is moving through these ducts most of the time.

I set the system up with a hot wire air flow meter set into a 100mm diameter bit of duct attached to a 150mm duct adapter to make a home-made cone to fit over the terminals. I put some felt around the bit that would be pressed to the ceiling to get a better seal and to prevent scratching the paintwork. The flow meter I used was a Testo (the previous version of this model: http://www.testolimi...rmal-anemometer ) purchased from ebay second-hand for around £30, but still in cal. This made it useful for leak testing around the house as well, as the sensing probe is small and very sensitive to low air flow rates. Here's a photo of my test setup:

  Ventilation test method.JPG   10.35K   17 downloads

I adjusted the flow restrictors to get the right rates on boost to start with, to comply with building regs extract requirements from kitchen, bathrooms, utility and WC, then balanced it on trickle ventilation rate by adjusting the flow rates on the fresh air feed ducts. To fine trim it and get the feed and extract matched I adjusted the fan speeds slightly in the MVHR, so in trickle mode one fan is at 25% the other is at 28%, just to overcome a small imbalance overall.

Setting the MVHR up was a bit tedious, with a lot of running back and forth and changing throttle rings, but at least a radial system isn't as interdependent as a continuous duct system, where adjusting one terminal upsets the flow rate of other terminals that have already been adjusted on that duct run. There is a tiny bit of interaction on the radial system, but very little I found, and it only affected the boost rate extracts.

The whole house ventilation rate on trickle was measured both by summing the individual duct rates and also by measuring the flow in the fresh air feed and exhaust air ducts. These are 150mm diameter, so I drilled a small hole in the side of them, big enough to take the flow meter probe (about 12mm I think) and measured the velocity in those ducts directly. I then calculated flow rate from that and compared it to the sum of the individual measurements to make sure they were about the same.

I found that the flow rates fluctuate a fair bit with very small outside air changes, so you have to average a few readings to get something meaningful. Even a light breeze will cause the ventilation rates to change by 50% or more up and down over a period of a few seconds, as the pressures in the ducting are pretty low and easily affected by small changes. Ideally you'd do the testing on a dead still day, but they are few and far between, even in our sheltered location.