Declan52, JSHarris and BitPipe's posts on designing & self-installing an MVHR system (and more, a paste)

[Fallingditch]

Having spent a fair bit of time reading through old posts, I thought it would be really useful to actually capture some of the most useful content, written by people who have actually installed theses systems themselves.

NB there are a number of different suppliers of the components required. These suppliers will often offer a no obligation, free design service. Some custom builders take advantage of this service, and loosely work from that base design rather than a blank sheet of paper. Horses for courses.

Firstly, JSHarris from Sept 2015

"It is, without a doubt, far easier to DIY an MVHR fit using semi-rigid ducting and a radial system ... 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 ...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."

" 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.testolimited.com/testo-405-thermal-anemometer ) purchased from ebay secondhand 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   (not accessible)

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 hol 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."

 

SecondlyDeclan52 from November 2015, working from a component Suppliers plan:

"As far as installation goes, when you have signed the deal and paid for the goods you will get a plan showing you the duct layouts and where the vents need to go. Just make sure you get accurate measurements of the unit and make sure it will fit. There are access holes on the top and side on mine so you use what ever one suits your build. Double check it fits as his duct run layout will be from this spot. 

You will have 4 pipes to and from the unit. Feed and exhaust from outside so you will need vent tiles or wall vents whatever way you are connecting to the outside. The other two are the fresh air back into the house and the extract from the house. You will have a larger diameter pipe maybe 150mm from the unit to a manifold maybe a meter long depending on your layout. From the manifold then it will have a duct going to each extract. They basically twist and clip in very easy to do. I had the full 50m reel downstairs between 2 trestles and a pole through it and just pulled the duct along to where it needed to go. Leave a bit extra just in case and you can trim it to size when it's all in and secured.

Measure all where your duct ends are and record this so when you are cutting the holes in the ceiling you know the exact spot where they are. You can use a hole cutter or a pull saw whatever suits you.

Keep the system off till all the work is done or it will get clogged up with sawdust and all the other airborne materials. Get his flowmeter and adjust the vents to suit. Some adjust at the manifold others on the ceiling. 

It's def not a hard job to do just arkward as you are wrestling with a 20m length of duct that never seems to want to go where you need it to go.

Get all your ducting in first before the plumbers or sparks arrive as it will make your life easier. Both plumbers and sparks will want to be first to make there routes easier but look after number 1 and get in first. If the ducting and manifold are in a cold area like the eaves you will have to cover the ducts and manifolds with the like of rockwool or you will get condensation in them

And finally, Bitpipe, from March 2016:

"Just completed the plenum side of my installation using 65mm internal radial duct, through luck more than design I've pulled it off but there were more than a few head scratching moments. So, where to start 

I used BPC who did a design (which I loosely followed) and supplied all materials. Our frame was already locked at this stage so I had to work around what was there and some issues only became apparent when the frame was up.

If you're still at design stage, give first thought as to where the MVHR unit goes, you want to minimise the runs (typically 160-180mm steel radial duct) from this to the external vents (which should be on the same wall to minimise unbalancing due to wind. 1.5m horizontal separation is the recommended minimum.

Then think very carefully on where the distribution boxes go (one for extract and one for supply, usually sit them next to each other) - note that these vary greatly depending on the manufacturer, some are boxy, some are long and flat. Some have ports on all sides, some just on one. You want to get them reasonably central so that you minimise very long runs but this may not be possible. Some are designed to sit in between pozi joists but this does not always mean you will have room to get the 65 or 75mm duct connections to them as it can get very busy. You may need to create a cupboard (or sacrifice one) to house these and the duct. You'll also need to get the 160-180mm steel duct to these also. Think on how you'll get vertically to different floors, wardrobe or cupboard space is usually the answer.

Then try and eliminate any physical barriers to running the flexi duct from the plenums to the distribution boxes, such as steels (spec penetrations now ) or gluelam beams. They will pass happily through pozi joists and you can get a reasonably tight bend on them if required but it then makes them very tricky to pull through, watch out for pinch points where you may have many ducts going through.

Supply plenums usually sit in the corner of the rooms diagonally opposite the door and extracts above showers, or WCs or about 1m from extractor hoods in kitchens. We have double runs to extracts and single to supply. Good idea to consult with your sparky and plumber to make sure you're not going to make their life too difficult (i.e. foul a line of downlighters with a plenum). It's difficult to leave slack in the ducts for later alterations - always possible to shorten a run but not extend it.

When installing, locate the plenums first and then pull the duct through from the distribution box location to there. My system just required the end of the duct to be neatly cut and an O ring slipped over the first rib and then pushed home & clipped in. WD silicone spray is essential to lubricate everything. Leave a decent tail at the distribution box end and mark the ducts with a sharpie so you know what's intake vs extract.

I used 50m rolls of duct and put a piece of supported 2x4 through the centre before uncoiling as it can get tangled very easily. If you can draft a few mates to help then this really speeds things up as each tight bend needs manipulated to get the duct through. Watch out for nails through the joists from above and below too.

It can get very congested when all the flexi duct comes together and I spent quite a bit of time rejigging ducts so they would connect to the distribution box neatly. 

I've not cable tied anything just yet, will probably use builder strap to keep them up out of the way and cable tie each connection to the distribution boxes as these don't have clips. The plenums are rock solid.

Just starting on the 180mm duct now, I've already put 600mm sections in the wall for the intake/extract so they could be insulated & rendered around. BPC have given me loads of 180mm duct (i'll likely use 1/3 of what they supplied) plus 45 and 90 bends and some springy flexible duct also to make the final connections to the MVHR. 

And the golden rule is to get in first before the other trades nick the space"

Thanks to all.

Edited June 19, 2016 by Fallingditch
HSHarris hand added; testo URL updated

[Jeremy Harris]

Thanks very much indeed for picking out these threads and copying them over here, it's very much appreciated and I know just what a pain it is to have to cut and paste all this stuff over to here.

[Frogeye]

I'm not sure if I'm supposed to be joining the end of this pasted topic, but....

When have people trimmed the excess of the plenum off? Some where on the old forum I seem to remember someone suggesting they cut it flush with a dremmel before decorating, yet I've got a recollection of BPC telling me to trim it off after the plasterboard goes on but before the skim coat  - which is where I am this evening!

Any input would be handy.

Thanks.

[Jeremy Harris]

I trimmed all mine to length before the plasterboard went up.  I just allowed enough for the end of the terminal tube to be around 3mm short of the plaster skim finish.  This meant sawing the threaded terminal tubes approximately to length, then screwing them into the threaded mounting plates to get them to be about flush with where the ceiling surface of the plasterboard would end up.  Mine came with red plastic caps that fitted inside them and I left these in place when the ceilings were plaster boarded and skimmed.  When the decorating was done I went around pushing the terminals into the tubes.  There's probably 10 to 15mm tolerance on the push-in terminal fittings, so there's no need to be super-accurate with trimming the end of the pipes.

[Guest Alphonsox]

Ours are still full length after plastering - trimming with a multitool looks to be fairly easy, we'll get round to it sometime.

[jack]

40 minutes ago, Alphonsox said:

Ours are still full length after plastering - trimming with a multitool looks to be fairly easy, we'll get round to it sometime.

That's what we did - piece of the proverbial!

[ToughButterCup]

On 6/11/2016 at 19:48, Fallingditch said:

Having spent a fair bit of time reading through old posts, I thought it would be really useful to actually capture some of the most useful content, written by people who have actually installed theses systems themselves.

[...]

Thank you very much indeed. It's exactly this sort of exercise which adds value to users' experience because it makes it easier for them to  drill down to the facts.

Ian

[ProDave]

12 hours ago, jack said:

That's what we did - piece of the proverbial!

For you, yes. I'll bet the plasterter cursed you doing that, and I wonder if the plaster finish around them was compromised either in flatness or quality of finish having to work round an obstruction?

[Nickfromwales]

27 minutes ago, ProDave said:

For you, yes. I'll bet the plasterter cursed you doing that, and I wonder if the plaster finish around them was compromised either in flatness or quality of finish having to work round an obstruction?

+1. Always avoid protrusions and then the plasterer has a flat clear surface and you get a much better plaster job / finish. 

That's the reason I first fix all the downlights, with the cables coiled up loosely, and then drill through the plastered ceiling the day after and fish the cables out. Quite easy if you draw a reference 'map' and note the positions down . 

[Jeremy Harris]

24 minutes ago, Nickfromwales said:

+1. Always avoid protrusions and then the plasterer has a flat clear surface and you get a much better plaster job / finish. 

That's the reason I first fix all the downlights, with the cables coiled up loosely, and then drill through the plastered ceiling the day after and fish the cables out. Quite easy if you draw a reference 'map' and note the positions down . 

Our plasterer suggested I both cut all the pipes down to about the level of the plasterboard, so he could skim flat over the holes he'd cut in the boards, and also asked us to just coil up and record the down lighter positions, so the holes could be drilled after he'd skimmed.

In my view anything that makes a blokes job easier is likely to either save you money or result in a better job (or, perhaps, both).

[Declan52]

I left the metal box in the void and cut it flush with the bottom of the joist. Took very accurate measurements and drew them out so when the time came I just cut through and slipped in the ceiling covers.

As nick says a ceiling with no wires  hanging down or holes will end up much flatter.

[Barney12]

On the last house refirb I did i had loads of downlights so I did as other suggested and loosely coiled the feeds and boarded over them and skimmed. I actually mapped all the layouts for the lights on the chipboard floors and then used a plumb-line lazer to place each cable point. Each point was marked on the floor using a small screw. Once the plasterer was gone I then found the points on the ceiling (Again using the lazer) and cut them out.

Worked rather well! The screw was probably overkill but didn't want to lose the marks. I guess a thick sharpie and some lines would have been equally effective.

Edited July 5, 2016 by Barney12

[Inchbyinch]

On these do people have any comments on 65mm v 76mm id pipes? I have been told (by a sales man) 65mm are a lot noisier due to restriction of air but reading here nobody has reported this as an issue?

also HB+ v Lindab? There seems to be quite a discount on the Lindab stuff, or is there something I'm missing? Is the extra worth it for the HB stuff? 

[Jeremy Harris]

We have 75mm OD (63mm ID) semi-rigid ducting.  It's silent, the ducting creates no flow noise at all, any noise (only on full boost) is either from the terminal (pull the terminal off and the noise drops) or transmitted noise from the compressor in the MVHR when it's in cooling mode (as it's doing right now as I type this).

The rule of thumb is to keep the duct flow velocity below 2.5 m/s, so if you have a high capacity requirement (kitchen extract is usually the biggest) then just double up on the duct runs to that terminal.  All the terminal fittings I used will accept two ducts and have a blanking plate if only one is fitted.  I doubled up on the run to the kitchen, but in practice I've found that I didn't really need to, as I've had to throttle that run back a lot to get the flow rate right and balance the system.

On full boost I have some duct runs that are flowing at around 3.5 m/s, yet even these don't seem to generate duct flow noise, so I think the 2.5m/s guidance is a bit conservative.

Ours is all HB+, but the duct seems identical to that sold under several different names, like Polypipe Domus, for example.

[jack]

7 hours ago, ProDave said:

For you, yes. I'll bet the plasterter cursed you doing that, and I wonder if the plaster finish around them was compromised either in flatness or quality of finish having to work round an obstruction?

Actually, it was me cursing the plasterer!  I requested, and was assured, that the outlets would be cut just below flush before the plasterer started, so imagine my surprise when I returned home from work to find that he'd just plastered around them!  But then, I was also assured by the people that did the plasterboarding that they'd fill the large gaps around some of the outlets before the plasterer started, and they didn't bother with that either (boarders and plasterer were contracted via the same company).  Long story why I didn't bother forcing the issue.

Surprisingly, the skim finish is perfect around all outlets.

To make it clear, I certainly wouldn't recommend this approach, particularly if you're planning to skim (rather than just tape and fill).  My point was more that using a multitool to cut off the excess is pretty easy, whenever you do it.

[Frogeye]

I ended up cutting the plenum excess off flush with the plasterboard prior to the skim coat going on (well on the first three done so far at least), I figured this would give me one less tedious job to do down the line. I used a flexible pull saw bought at screwfix (http://www.screwfix.com/p/irwin-pull-saw-14tpi-12-305mm/42618), which made a fairly easy job of it.

I noticed that the plasterers had run a lap of scrim around the opening as well (after I had cut it off), which I guess will help, although on the third one (today) I did run a lap of fortax 6400 (sealant) around the edge of the cutout for good measure.

A couple of before and after photos (but different rooms!)