Dipping a toe into MVHR

[Moonshine]

I am thinking of installing MVHR in my proposed house and have a quote from BPC for the kit (no design provided).

 

However for my own curiosity and to try and determine the PITA factor, i want to see how feasible it would be to integrate into my current house design.

 

Its using I-Joists so i would want to minimise the penetrations through these, so starter questions.

 

• Where can i put the MVHR unit? is an airing cupboard suitable?
• What are the typical duct sizes that need to be accommodated? is it flexi duct?
• Can ducting be rectangular?
• Where in a room do supply vents need to be located? e.g. room centre?
• for toilets, does the MVHR constantly extract at 15 l/s or is there a boost based on the light being on?

 

 

[joe90]

I have i joists and as long as the holes are in the right place (not near ends) it’s quite alright, mine is installed in a warm loft but an airing cupboard is ok, I suppose if it’s central in the house it would be better. I used the 75mm flexi duct, built my own manifold from ply and also made it a silencer I also made my own room vents from cheap bits. I located vents at opposite ends of the room to doors to create cross flow within those rooms. I have not balanced mine yet and have just installed a humidity stat to turn on boost based on RH.

[RandAbuild]

• Easijoists are lot easier to thread the ducts through than I-Joists
• An MVHR unit can go in an airing cupboard if it's big enough. Try to locate it centrally in the house so the duct runs are kept short. Remember you will need a condensate drain
• We used BPC flexi duct. It's typically 75mm in diameter - these can be doubled up where required
• Rectangular ducts are available and can come insulated, but I think they are more expensive
• The general consensus is the supply ducts should go in the opposite end of the room to the entrance door (so a flow of fresh air is provided through the room), and extracts located centrally or close to (not immediately above) the main source of moisture
• The MVHR will be on permanently; there is usually a humidity control in the unit that provides a boost eg during showers. Our en-suite clears within about 5 minutes after a shower

 

[Moonshine]

Thanks both, especially those pictures.

 

I have attached some very rough marked up drawings of the house plans which is where i think the duct runs may have to go, as well as the joist orientation.

 

I am proposing that the airing cupboard is uses as this is on the first floor and has the smallest atmosphere supply and extract runs.  

 

I think that its going will be very tight to accommodate the MVHR and also manifolds.

 

Any comments on the layout and better siting for things (note, there is a mini warm loft area above the master bedroom which could accommodate a manifold, and atmosphere supply and extract runs.

floor plans - 200220 - MVHR.pdf

[Mr Punter]

I once sited a Vent Axia Sentinel Kinetic Plus in a small loft as per the specialist design.  You needed to be a contortionist to clean the filters and I doubt the current owners will have ever done so.

[RandAbuild]

+1 on the filters - you need good access to these and the take out the heat exchanger to clean it.

 

On the plans:

• I would do away with ducts to the basement - getting one down there would be more trouble than it's worth
• All duct runs on the ground floor and first floor have sections that run at right angles to the joists - try to minimise this where you can.
• The airing cupboard is well located but probably too small. Don't forget you'll have 10+ 75mm pipes that have to meet up in there. It will be like Clapham Junction on steroids
• If you locate the unit in a warm loft, you've still got to get all those pipes up to it. This might need the space of a HWC alone

Don't get me wrong, I'm a strong advocate for MVHR as a cost-effective solution to controlled ventilation. But it need planning in now and you need to give up space to have it

[Jeremy Harris]

For best performance try to position the terminals so that there are no "dead corners" in any room.  The idea is to try to get the air to take the longest path from the point of entry to the point of extract through any room. 

 

I made sure that all fresh air terminals were diagonally opposite the extract point from that room, usually the gap under the door.  I did the same with the kitchen and downstairs WC extract terminals.  In the utility room I fitted the extract terminal directly above where we have a clothes drying rack.  In the bathrooms the extracts are over the top of the shower and bath respectively.

[Mr Punter]

Here is the one they designed:  The top floor had skeilings so the vents could not go in the corners.

 

Timber frame designer unhelpfully put a strongback where it got in the way.  This used 125mm rigid duct.  I think the flexi stuff with the manifold boxes would be easier to do.

 

[Moonshine]

37 minutes ago, RandAbuild said:

But it need planning in now and you need to give up space to have it

 

yep that is the stage i am now. 

 

The floor plans as they stand rely on a standard ventilation strategy, and i want to see how much compromise is needed if MVHR was used. 

[Moonshine]

40 minutes ago, RandAbuild said:

• The airing cupboard is well located but probably too small. Don't forget you'll have 10+ 75mm pipes that have to meet up in there. It will be like Clapham Junction on steroids
• If you locate the unit in a warm loft, you've still got to get all those pipes up to it. This might need the space of a HWC alone

 

I am thinking that maybe the MVHR unit can go in there, but the manifold (first floor) can go up in the mini loft as below, and the ground floor manifold would go in the cupboard below at high level, so that would only be two trunk large ducts going down before the octopus legs spread out.

 

[Jeremy Harris]

This is the way I ran ours:

 

 

I ran almost all the ducts in the floor space between the ground and first floor, threaded through Easijoists.  The bedroom fresh air terminals are wall mounted, as we have vaulted ceilings upstairs.  The bathroom extract terminal ducts run above a false ceiling.  I tried hard to ensure that the air paths were as long as practical, with no terminals in the centre of a room, as that can create "dead spots", where the ventilation air tends to short-circuit from the point of entry to the point of exhaust.

[Bitpipe]

Agree with much of the above, I did not plan MVHR very well into my TF design (had lots of long steels to navigate past, no penetrations) so had to make some adjustments when it came to install.

 

You do need MVHR in the basement (I have one) otherwise it will get very stuffy. Extract not really required unless you have wet services down there (air will get drawn up to the next floor with an extract. Some of my basement runs take a very meandering route (one goes down from ground floor ceiling, through a stud wall, all to avoid a steel) but the airflows are fine.

 

Extracts tend to be double runs of duct to get the required flows, supplies single. Things can get quite congested at the manifold when you bring that many pipes together and you need to avoid very tight bend radii as you'll constrict air flow. 

 

Expect lots of trial and error as you install but it is do-able as DIY.

 

My strategy was to position the ceiling plenums first and be roughly consistent with spacing from wall - more for aesthetics as the downstairs is open plan so you can see more than one vent at a time.

 

I then ran duct from a position near the manifold (but not right next to it) to the plenum and secured it there - silicone spray is very handy to persuade the rubber seals into place. I left a generous tail on each piece. Essential to mark your supply & extracts with different coloured sharpies as you go!

 

Then, when all the duct was back to the manifold location, there was a lot of jiggling and re routing to get the ducts flat and neat as possible, and the last thing I did was trim the tails to get neat connections to the manifold.

 

The much bigger job is getting the external supply and return ducts (mine were 180mm steel) to the appropriate location and getting those insulated.

 

Final connections from MVHR  to manifold and external ducts was made with the flexi insulated hose and jubilee clips.

[RandAbuild]

18 minutes ago, Moonshine said:

 

I am thinking that maybe the MVHR unit can go in there, but the manifold (first floor) can go up in the mini loft as below, and the ground floor manifold would go in the cupboard below at high level, so that would only be two trunk large ducts going down before the octopus legs spread out.

 

 

That change of level would give me a few headaches, as the third dimension becomes more important. But I would have thought that using that cupboard / boxing-in at high level for the unit was possible if you had the headroom in the kitchen

[Russdl]

4 hours ago, Moonshine said:

the manifold (first floor) can go up in the mini loft as below, and the ground floor manifold would go in the cupboard below at high level,

Unless I’ve misunderstood you, that sounds to me as though you expect one manifold for each floor whereas each manifold serves both floors (extract from the kitchen/supply to the lounge. Extract from the bathroom, supply to the bedrooms)

[MikeSharp01]

I am also struggling with this although, think I know where each plenum is going, at the point of providing all the infrastructure, holes in I joists etc, I am held up because I cannot get my total duct air resistance much below the Passive house total of 100 Pa (This doc refers) and balancing the recommended 30M³ per bedroom with the total extract recommendation. Anyone have the air resistance data for the 90mm ducting, have found it for the 75mm ducting (Page 5) but think I may have to use 90mm, cos of overall resistance, but cannot find a reference for the resistance data!

[Jeremy Harris]

I used the 75mm ducting and found that we ended up with much higher airflow rates than expected, to the extent that the double run of ducting I used for the high flow rate extracts probably wasn't needed, as I ended up fitting restrictor rings that throttled the flow right back.   Meeting the BR requirements for extract flow rates is pretty much always the dominant factor as far as duct size goes, just because the duct flow velocity needs to be kept below 2.5m/s.   The need to keep the duct velocity down below 2.5m/s is the reason I opted to double up the ducts on the high extract rate rooms.  All the fresh air supply ducts run at lower velocities, because the flow rate through each is a fair bit lower than something like the kitchen extract (there should be more fresh air ducts than extract ducts anyway, to make balancing easier).

 

Some of our rooms have a fairly high volume, due to the vaulted ceilings, but ultimately it's the people in the house that pretty much dictate the actual ventilation rate needed, I think.  I reduced the flow rates on our system to below the guide figures as we were clearly over-ventilating our 130m² floor area, 338m³ volume, house.  I tend to use the measured CO₂ level as a guide to how effective the ventilation really is, and it's rare to see that go over about 700ppm; most of the time it tends to be around 550ppm.

[Moonshine]

On 24/02/2020 at 20:51, Russdl said:

Unless I’ve misunderstood you, that sounds to me as though you expect one manifold for each floor whereas each manifold serves both floors (extract from the kitchen/supply to the lounge. Extract from the bathroom, supply to the bedrooms)

 

I think that i have mis understood that part then, so its one manifold for return air, and one for supply.

 

In that case i think that it could work by having the MVHR in the Airing cupboard, with ducting to the loft above to the side (see section C-C above), and both the manifolds in that loft space (may be tight! its 750mm x 800mm section), and then i put a boxed in riser in the master suite which will take a number (no. 5-6) 75mm flexi ducts down to the joists of the floor below for supply and return air

 

 

 

 

 

[Russdl]

@Moonshine It does sound like it'll be a bit tight. Where do you plan to route the inlet and exhaust for the MVHR? Straight up through the roof? It'll looks like it will definitely get a bit crowded up there.

[Moonshine]

9 hours ago, Russdl said:

@Moonshine It does sound like it'll be a bit tight. Where do you plan to route the inlet and exhaust for the MVHR? Straight up through the roof? It'll looks like it will definitely get a bit crowded up there.

 

tbh i want to minimise the amount of roof penetrations as its a flat roof and paranoid about leaks, my plan would be to take them out of the exterior walls of the loft area on separate facades as below. The Loft is looking as though it is going to be a MVHR duct area.

 

[Bitpipe]

You want to ensure your external vents are on the same wall face to minimise unbalancing when the wind is blowing.

 

You also need to keep them about 1m apart to prevent the intake consuming the exhaust.

[Russdl]

@Moonshine, as @Bitpipe says, you want them on the same elevation. Could they both go out of the west elevation above your master suite? 

[Moonshine]

16 minutes ago, Bitpipe said:

You want to ensure your external vents are on the same wall face to minimise unbalancing when the wind is blowing.

 

That is a new bit of information, thanks.

 

I assumed best to be on separate facades to stop mixing, looks like they can be both on the west elevation with a decent (> 1m) elevation.

[Moonshine]

8 minutes ago, Russdl said:

Could they both go out of the west elevation above your master suite? 

 

absolutely

[Bitpipe]

Even when MVHR is on full boost, the combined external airflow is quite modest so risk of mixing is low if that minimal separation is maintained. 

[Moonshine]

I have been giving this a bit more thought and have sketchup'd the following arrangement in 3D, with the MVHR in the airing cupboard, and ducts running down a riser

It seems that it works (Just!), with the return manifold on the floor, and the supply manifold on the the ceiling, as below

 

Section through loft (short section)

 

Long section through loft