Sub-distribution boxes?

[tanneja]

Trying to retrofit MVHR at this stage of our renovation is causing major headaches.  The necessary void spaces to bring down ducting from the loft to the ground floor threaten to really impact our first floor spaces.  Its more complex as well due to solid wood joists, size of steels, and their directions.

 

A way to perhaps make the void requirement less would be to have additional distribution boxes, but I don't know if I can get a balanced system through doing so, I would hope so if it is a valve balanced approach.

 

An example of the ducting would be:

- main feed from MVHR unit to a 6x 75mm exit distribution box (4 exits used to supply the upstairs bedrooms, the other two make their way via voids to the ground floor)

- each 75mm feed that gets to the ground floor has its own 4x75mm exit distribution box...so 12 feeds in total servicing the house, the downstairs feeds going via 2 distribution boxes.

- It would be a similar story for the extract ducting circuit

 

Is this reasonable, or will the downstairs rooms suffer from insufficient flow?  Assuming the balancing is possible, will the "void" runs experience more noise due to faster flow?  Or would noise only materialise at the plenums, by which time the flow speed is reduced (after its volume being divided in the secondary box), so I might be ok?

 

Thanks in advance for any wisdom you can afford to me.

Edited September 26, 2020 by tanneja

[MJNewton]

3 minutes ago, tanneja said:

- each 75mm feed that gets to the ground floor has its own 4x75mm exit distribution box)...so feeds in total servicing the house.

 

I think that could bit could be an issue as you'd be trying to push (or pull) four 75mm pipe's worth of air through one. The increased volume of air through that pipe would likely result in noise not only from air speed but back pressure also. I stand to be corrected but I would imagine it'd only really work with a large diameter pipe to feed the downstairs distribution box, just as would be the case in a conventional setup.

[tanneja]

@MJNewton I thought that might be the case, I am struggling to think of other kinds of distribution boxes to get around this.  I am probably not thinking outside of the 'box' enough yet!

Edited September 26, 2020 by tanneja

[Declan52]

Have you had an actual plan from a mhrv company???

How many rooms are in your house as you don't need to extract from every single room?? 

[tanneja]

2 minutes ago, Declan52 said:

Have you had an actual plan from a mhrv company???

How many rooms are in your house as you don't need to extract from every single room?? 

Yes from BPC, so far just the general location and number of runs for each extract and supply, not the runs themselves.  The only room with combined extract and supply is the open plan living / kitchen.  I'm certain I have the minimum number of valves and runs. 

 

BPC doesn't know the incompatibility of our build with ducting runs, so with it being left to us to find the runs, I brain-stormed things could be more achievable (and less sacrifice to the 1st floor rooms) if we could reduce the ducting runs coming down to the ground floor, and then split from there.

[Declan52]

A floor plan would help to try and figure it out.

 

[tanneja]

@Declan52 I stopped short of that previously as its a bit of a mess to look at, but hopefully it is clear enough.  This was my recent attempt as runs, until I showed my builder.

 

Note that the partially transparent red features are large structural steels, the solid joists run from left to right between the first and ground floor, purple spaces are vaulted.  We have lots of full height cabinetry planned in the kitchen living area, and conceded to have boxed sections under the ground floor ceiling on the east wall to conceal ducts that need to run up and down the house, under the ground floor ceiling.  There are compromises everywhere and yet its still very challenging.

 

Builder has no idea how we would get 10 feeds down through the void in the master bedroom, while the void would be big enough, the wall separating the master and the ensuite is supporting, so its a not viable void candidate unless some major revisions are done.

 

Please note that the loft space is very small, at the apex it is around 1.5m tall after insulation, so the flat roof section will be approx 0.7m max, it is truly a crawl space.  Even placing the unit will be a challenge.  Makes for sub-optimal distribution box placement too, lengthening all the runs.  Had hoped they could be situated above the ensuite but builder thinks unrealistic, especially if i would ever need access.

 

Windows are already ordered without trickle vents.  I would like to avoid a decentralised unit for reasons or cost and in room / outside of house aesthetics, and likely noise.  I could concede to the garage having its own through wall unit as the unit would be hidden around the side of the house exterior.

 

Edited September 26, 2020 by tanneja

[tanneja]

Can I split the main new air feed from the MVHR unit, with the 150/160 rigid pipework, to feed 2 distribution boxes? Not seen it before.

 

If not, perhaps 2 smaller MVHR units would be sensible? Or go the decentralised route with through wall units, with the cost and aesthetic comcessions it brings. 

 

Nuclear option is to have voids in each bedroom to get to the rooms below.  I had hoped to avoid that ti give the rooms layout flexibility, th3y are already boarderline doubles.

Edited September 26, 2020 by tanneja

[tanneja]

Or relocate the unit somewhere on the ground floor, such as the planned airing cupboard position

[dpmiller]

BPC are very good  IME, why not give them a chance to offer a full design by informing them of your "incompatabilities"?

[Declan52]

What height are the ceilings in the ground floor. 

Think your going to have to put a box section along the lefthand side of the kitchen. This will enable you to run the ducts along under the joists to get to where they need to be before they go into the correct joist. You can use the height of the kitchen units to help hide the box. Where it crosses the kitchen towards the Sink you will need another box from here to move to duct into the bottom rooms. The cupboard beside the sink will help hide it. The purple line will be the run and then you just turn the duct into the body and bring it to where it needs to go.

If you really want to use mhrv then your just going to have to look at some kind of boxed in sections in the kitchen and bedrooms to bring the ducts down. 

[MJNewton]

For what's it worth, try and remember you are installing MVHR for its practical benefits not aesthetics and so if some evidence of its existence can't be hidden away then so be it. A bit like radiators. 

 

(Of course I don't practice what I preach. I had many a sleepless night trying to work out how to retrofit our MVHR with minimum visibility. We had a simpler layout to deal with though (although did have 2.5 floors) and I was lucky with some built-in wardrobes, joist directions and the fact they were I-Joists so could have large holes drilled through them if necessary. Still difficult and I look back now with some pride - and surprise - at what I achieved! )

Edited September 27, 2020 by MJNewton

[Levo]

On 26/09/2020 at 23:59, tanneja said:

Can I split the main new air feed from the MVHR unit, with the 150/160 rigid pipework, to feed 2 distribution boxes? Not seen it before.

Isn't that the original "branch" approach used by larger installations?

You can use a rectangular profile pipes which are much easier to fit into gaps, I think.

 

63mm pipes are really good for carrying around 30m3/h.  Here's a rough calculation.

Basically plan for upto 22m3/h for your nominal flows and can go upto 30m3/h at boost with 2.3Pa/m drop. Above that the pressure loss (and noise) increases dramatically. Note that the drop (DeltaP) figures are for 1m only. For a 10m pipe, you need to multiply by 10 and add acouple of Pa per bend on top.