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MVHR: More extracts than supplies?


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I'm aware of the basics of an MVHR install, that you want to keep everything balanced, you want to maximise the distance between supply and extract points, and that you want to extract from areas of higher humidity i.e. bathrooms and kitchens.

 

My problem is that with a very small open plan house, I am struggling to find places to put the supply vents. I obviously want an extract in the bathroom, one in the kitchen, and I think it would be good to put one at high level in the loft space as that is where all the humid warm air will tend to go.

These three points would end up as far from one another as it is possible to get. If I then had a single supply vent centrally located, that would maximise the distance between supply and extract points. But it's obviously not great from the point of view of keeping everything balanced.

 

I'll try to attach some pictures of the house and a floorplan later.

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There's no intrinsic reason why the extract terminal number has to match the fresh air terminal number, but it does make getting the system balanced easier.  The main issue is for the high rate extract case, when the ventilation rate is increased from a bathroom or kitchen in order to remove excess heat/steam etc.  That has to be matched by the flow coming in through the fresh air terminals, and with just a single fresh air terminal the flow rate at that terminal could be very high, with the risk that it may be noisy and cause unwanted draughts.

 

If you can make the supply terminal and ducting larger than the extract terminals and ducting then that would help, but if using radial ducting you're limited, because I think that the maximum number of ducts per terminal is two, and that may not be enough to keep the noise down.

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Thanks for those thoughts. The only bedroom (this is a very small house) is open plan to the living room/kitchen.

The only places in the bedroom that I can duct air to are pretty close to the en suite doorway, so I think that any supply placed there will just end up being sucked out through the gap under the door.

 

The loft extraction point would be a trickle really, just to prevent buildup of warm moist air.

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There can be advantages in placing a fresh air feed adjacent to a doorway into a room with an extract.  For example, I have one in our hall that is above the door to the kitchen and the kitchen is connected to the utility room and then the downstairs WC, so effectively there are three extracts pulling air through that single doorway.  By putting a fresh air feed above it, in the hall, I've effectively created an "air curtain" to contain any cooking smells to the kitchen area.  You could do the same with an additional fresh air feed above the door to the ensuite.

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OK as promised, some rough sketches showing the space I have to work with.

First one shows the floorplan. The main room is open to the bedroom via a double doorway- this was originally going to have french doors in it, but I have shelved that plan at least for the time being. There is a cupboard off the main room, and an en-suite off the bedroom.

Second pic shows, in grey, the areas in which it is possible to fit ducts/vents for MVHR. Vaulted ceilings prevent any ducts being taken outside of these zones without recourse to ugly box outs, which would be aesthetically unacceptable.

The third pic shows my proposed vent locations, with one additional extract in the warm loft space, not shown.

Floorplan.jpg

Floorplan with MVHR locations.jpg

Floorplan with vents.jpg

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You have a big "dead" air space to the left, because the air from the centre supply terminal will tend to "short circuit" across the extract duct opposite.

 

Ideally you want a fresh air terminal in the bedroom at the bottom right in that sketch, and another in the living area in the bottom left.

 

As you can't get them there, can you fit one at the far right centre over the bed and would it be possible to run an exposed rigid duct from the shaded area across to the extreme left?  Maybe try to make it into a feature, with a bit of small section galvanised duct, the sort if industrial look?

 

The extracts are OK where they are.

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Thanks for the feedback Jeremy.

One thing that I haven't been able to convey in the sketches is that it would be possible to put vents into the 'internal gable', which is at the left hand edge of the shaded area in the main room. This would allow vents higher up in the room, right up to the apex- but of course that brings them close to the centreline.

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4 minutes ago, PeterW said:

Any chance of just boxing in the apex of the vaulted ceiling ..?

 

A 75mm duct wouldn’t need that much and would put the supply in the right place. 

 

It's really a bit too late for that sort of change. I admit I should maybe have thought this through in a bit more detail at the time, before everything got boarded, plastered, and painted.

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For the supply terminal, could you have the terminal not in the ceiling directing air from above and downwards but mounted in the internal gable and facing into the big air space to the left, located as close to the current location that you have shown in your diagram and as far as the internal gable structure would allow? Thus, could it be that the supply air could be directed into the big air space and so minimising the short circuit that Jeremy mentioned, perhaps using a special design of terminal that directs the air?

 

I have no knowledge of MVHR ducting so disregard my idea if you wish.

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4 minutes ago, Dreadnaught said:

For the supply terminal, could you have the terminal not in the ceiling directing air from above and downwards but mounted in the internal gable and facing into the big air space to the left, located as close to the current location that you have shown in your diagram and as far as the internal gable structure would allow? Thus, could it be that the supply air could be directed into the big air space and so minimising the short circuit that Jeremy mentioned, perhaps using a special design of terminal that directs the air?

 

I have no knowledge of MVHR ducting so disregard my idea if you wish.

 

I'm not sure what will work best. I had actually been thinking about putting the extract at the apex of the internal gable, as that would prevent a pocket of warm moist air gathering up there. The problem with placing vents on the internal gable is that you are reducing the space between them the higher up you go.

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2 minutes ago, Crofter said:

 

I had actually been thinking about putting the extract at the apex of the internal gable.

 

I like your idea better than mine. Could you have the extract not at the apex but as high and far towards what I assume is the kitchen and facing towards it, and the supply as you have marked it, facing downwards, so within your constraints maximising their separation.

 

I wonder what an MVHR designer would recommend.

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On 14/02/2018 at 21:14, JSHarris said:

Ideally you want a fresh air terminal in the bedroom at the bottom right in that sketch, and another in the living area in the bottom left.

 

As you can't get them there, can you fit one at the far right centre over the bed [...]

 

I recognise that your suggestion is a compromise out of necessity, but I've been wondering about how careful one must be with siting of supply terminals assuming there is a risk that moving air - even warm air - can have quite a cooling effect on people beneath/near them? I seem to recall one installation manual stating a need to avoid siting them over beds and seating areas, presumably for this reason. Could you (and/or of course any others with MVHR) comment on how 'draughty' supply terminals actually are in practice?

Edited by MJNewton
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On 14/02/2018 at 15:21, JSHarris said:

but if using radial ducting you're limited, because I think that the maximum number of ducts per terminal is two

Could you elaborate on that a bit please J? Do you mean 2 ducts joined into a reducer, say 2x 75mm ( cant remember the common size, maybe 65 / 63mm? ) into 1x 150mm for a single point double capacity terminal?

Im looking at a proposal putting 2x single extracts into a reasonably sized utility ( beefing up to remove a bit more heated air generated from all the white goods that will reside there ), plus the plant room would be in that room too, eg a door into the utility. So the thought is that one may struggle, so put two in, at opposite ends. Outside the utility entrance door there will be a cloakroom WC, so an extract in there too. The utility and cloak doors are linked to a tiny sealed lobby, the seal formed by a third door to the kitchen, so next thought is to increase the single inlet in that lobby to a larger single inlet to match the 3x local extracts, doubling up 2x 75mm ducts into one 150mm. Any issues with a 150mm inlet servicing those 3x 75mm extracts in close proximity?

Also;

Is there any scope to put in some kitchen inlets at low level, even though a typical kitchen 'room' should only have extracts? It'll be an open-plan room, with fresh inlets over the dining / living space just off the kitchen, so a myriad of difficulties to overcome in order to get the right air shifting in the right directions in the right amounts :/

 

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6 hours ago, Nickfromwales said:

Could you elaborate on that a bit please J? Do you mean 2 ducts joined into a reducer, say 2x 75mm ( cant remember the common size, maybe 65 / 63mm? ) into 1x 150mm for a single point double capacity terminal?

I suspect @JSHarris means this where you have a terminal and you can run 2 ducts into it.

MVHR-GF-Rear-Hall-Study2-View.thumb.JPG.6f33d4c5341a3144e331586239aa2717.JPG

 

This is Ubbink 90/75 (90mm external / 75mm internal), I also have some 75/63 doubles.

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2 hours ago, le-cerveau said:

I suspect @JSHarris means this where you have a terminal and you can run 2 ducts into it.

MVHR-GF-Rear-Hall-Study2-View.thumb.JPG.6f33d4c5341a3144e331586239aa2717.JPG

 

This is Ubbink 90/75 (90mm external / 75mm internal), I also have some 75/63 doubles.

 

Yes, that's what I meant, that's the same as our terminal fittings (except ours are blue, and HB+ ones)

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2 hours ago, Plumbersmateuk said:

Slightly off topic but no mention of an extract in the kitchen pull out bin cupboard, any reason or overlooked?

ooops I now realise this is no use to the original question my apologies

Cracking idea. 

Maybe just a 32mm or 40mm PVC waste pipe would suffice. @JSHarris, what size pipe did you run for 'operation stench' ?

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11 minutes ago, Nickfromwales said:

Cracking idea. 

Maybe just a 32mm or 40mm PVC waste pipe would suffice. @JSHarris, what size pipe did you run for 'operation stench' ?

 

I used 40mm waste pipe, Nick, which works very well.  The internal overflow in the cistern to the pan, which is how the suction gets to the pan is only about 20 to 25mm, so that's the main restriction.  In practice, because the person sitting on the seat practically blocks the thing off, air gets drawn (gently!) from the small gap between the seat and the pan and sucked out via the rim flush slot.  For best effectiveness you need to close the lid and flush promptly when finished, as the lid works much the same as sitting on the seat to block the pan off.

 

For a waste bin, then I'd have thought either 32 or 40mm would be OK.  Seems a good idea, especially if you have a set of separate bins for recycling, composting etc in a unit (we have one with three bins, one for plastics and cardboard, one for composting and one for general waste, a German unit, a country that's been separating out waste for a lot longer than we have).

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23 minutes ago, Nickfromwales said:

Cracking idea. 

Ah gold star at last?

I'm guessing it has been overlooked then?

How I came to think about this was one of the daughters had an expensive (money no object) kitchen installed and you open the cupboard with the bins in and it STINKS

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1 minute ago, Plumbersmateuk said:

Ah gold star at last?

I'm guessing it has been overlooked then?

How I came to think about this was one of the daughters had an expensive (money no object) kitchen installed and you open the cupboard with the bins in and it STINKS

 Definitely a gold star (well, the closest we can get here!).

 

I can understand the bin smell, the problem is the green bin, the one for composting stuff.  That tends to get ripe pretty quickly, especially if not emptied every day.  The unit we have is identical to this one, except the bins themselves are coloured and ours is on a left hand hinged door.  The lid system works pretty well; as you open the door the lid lifts first then the bins swing out from under it.  This does mean that the bins are covered pretty well, but the green bin still tends to be problem:

 

bins-top.jpg

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  • 1 month later...

So, today has largely been spent clearing all the junk out of my loft space in preparation for MVHR install. I'm amazed at the amount of stuff that I had squirreled away up there, in a house that isn't even inhabited yet. Mostly packaging and various offcuts kept just in case.

 

Anyway, it's decision time for my MVHR install. As noted in my posts above, I will have an extract near the kitchen and one above the shower. Another will go in the loft space.

One supply will go in the living room.

 

I can relatively easily add a second supply in the bedroom, but it would be directly above the head end of the bed. As MJNewton says:

On 2/17/2018 at 22:07, MJNewton said:

 I've been wondering about how careful one must be with siting of supply terminals assuming there is a risk that moving air - even warm air - can have quite a cooling effect on people beneath/near them? I seem to recall one installation manual stating a need to avoid siting them over beds and seating areas, presumably for this reason. Could you (and/or of course any others with MVHR) comment on how 'draughty' supply terminals actually are in practice?

 

Is this a legitimate concern? The bedroom ceiling is vaulted so the supply terminal would be nearly 12ft above floor level.

 

Finally, is there merit in having a trickle supply in the loft space, to match the extract?

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