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Mvhr vaulted ceiling


Russell griffiths

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Assuming your build is, like most self builders, a one-off design, you might have to pioneer a solution and I think the ultimate answer will be 'wherever it fits'! Shouldn't make any difference whether its in or out of the airtight envelope; just seal the membrane to the ceiling terminals and/or ducting as appropriate.  I think insulation is important though; where my ducting is outside of the airtight layer (and therefore the 'warm bit') I'm trying to ensure it's got plenty of insulation around it.  Probably best to extract from high level vents, but supply could be low level if it makes it easier to run ducts.

 

 

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I ended up created in 100mm service cavity under the airtight membrane to run the MVHR ducting in.  Created with 2x 50mm battens on top of one another (MBC and PM where going to batten and cross batten until I pointed out that we’re they crossed there was only 50mm of cavity for me to run 70mm ducting!)

 

Have read some people doing it within the insulation layer - but for me that is too many things going through the airtight membrane for my liking.

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We have vaulted ceilings in the bedrooms and just fitted the fresh air feed terminals in the low walls at either side, with directional vanes to blow air upwards:

 

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Others have fitted the terminals in the floor with good results, I believe.  The majority of the effect of MVHR is by diffusion, not active air flow, so it doesn't really matter too much where fresh air feed terminals are placed.

 

It does matter where extract ducts are placed, though, these need to be as high as you can get them.  In our case, this was relatively easy in the vaulted sections, as there were dividing walls where ducts could run to high level terminals, like this extract terminal in the very top corner of our shower room:

 

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Edited by JSHarris
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Our largest bedroom (daughters room) has a full height vaulted ceiling.
 It also has a mezanine floor above the adjacent small bedroom.

 

The duct for both these 2 bedrooms is in a small boxed in area right at the very bottom of the mezanine ceiling slope.  One goes down through the ceiling for the small bedroom under the mezanine and the other comes out of the mezanine overhang for the big bedroom.

 

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22 minutes ago, JSHarris said:

Others have fitted the terminals in the floor with good results, I believe.  The majority of the effect of MVHR is by diffusion, not active air flow, so it doesn't really matter too much where fresh air feed terminals are placed.

 

All our bedrooms have floor-mounted outlets and they're fine.

 

This also reduced the amount of ducting we needed, to the point where we got away with one fewer rolls ducting.

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Can I just confirm the supply outlets can be anywhere, floor, wall or ceiling. I have a vaulted ceiling and was planning to run radial pipe along the inside of the ridge and exit high up, central in room and nearly 4.5 m from the floor. Performance wise does it not make much difference if its in the corner of a room or high up on a wall?

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5 minutes ago, JamesP said:

Can I just confirm the supply outlets can be anywhere, floor, wall or ceiling. I have a vaulted ceiling and was planning to run radial pipe along the inside of the ridge and exit high up, central in room and nearly 4.5 m from the floor. Performance wise does it not make much difference if its in the corner of a room or high up on a wall?

 

 

There is a rule that should be observed when placing terminals, in that you should aim to make the air travel as far as possible from the point where fresh air comes in (usually a fresh air terminal) to the point where air leaves the room (often the gap under a door that then leads to an extract terminal in another room).  I have pretty much all ours arranged on a diagonal, for example, the bedroom doors are close to one corner, the fresh air feeds are diagonally opposite, on the low wall at the eaves.

 

The idea is to allow maximum time for fresh air to diffuse with room air, as it very slowly moves across the room, and to try and avoid any dead spots, where air can bypass across one side of a room, for example, leaving much of the room as "dead" space.

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As we approach door frame fitting this weekend, I was mulling over how much gap we need under the doors for the mvhr system?  Plan is for carpets upstairs and tiled down stairs. Does anyone have any suggestions?  It clearly needs to allow air to flow but not too much of a gap - would 10mm above tiles be too much?  What about above carpet?   

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The recommended gap is either 10mm or 15mm - I can't remember. It's a lot though!

 

There's a formula for the size of the aperture, so you may get away with less if you have a wide door.

 

I did a concealed gap in the top of the door frames, but I'm not sure it's actually worth it, especially given how infrequently doors are actually closed in our house.

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The Domestic Ventilation Compliance Guide states:

Air transfer

a. To ensure good transfer of air throughout the dwelling, there should be an undercut of minimum area 7600 mm2 in all internal doors above the floor finish. This is equivalent to an undercut of 10 mm for a standard 760 mm width door.

b. Ensure that the air transfer provision is unrestricted after floor finishes have been laid (e.g. carpets should not encroach). This should be achieved by making an undercut of 10 mm above the floor finish if the floor finish is fitted, or by a 20 mm undercut above the floorboards, or other surface, if the finish has not been fitted.

 

So it depends on the size of your door!

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7 hours ago, jack said:

 

I did a concealed gap in the top of the door frames, but I'm not sure it's actually worth it, especially given how infrequently doors are actually closed in our house.

 

This is exactly my view. Doors are almost never shut in my house. Thus I’m going for standard gaps as I think the aesthetic of large gaps is just odd.

If someone one day buys the house who likes doors shut well they can go to the trouble of having the doors trimmed :) 

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22 minutes ago, Barney12 said:

 

This is exactly my view. Doors are almost never shut in my house. Thus I’m going for standard gaps as I think the aesthetic of large gaps is just odd.

If someone one day buys the house who likes doors shut well they can go to the trouble of having the doors trimmed :) 

 

 

What about the toilets and bathrooms?  They are rooms that need a fair bit of unrestricted fresh air access in order for the extraction to work - no gap means the pressure drops with the door closed and the extraction rate significantly decreases.  One of the benefits of MVHR is that toilets and bathrooms stay much fresher and drier, preventing that by sealing them up doesn't seem to make a lot of sense.

 

If you don't want gaps under doors, then you can always use transfer vents instead.  These can be quite discreet and if they are arranged so that they take an indirect path through an adjoining wall then they can reduce sound transmission.

 

Also, remember that the fresh air feed effectiveness is only as good as the extract effectiveness.  Block off the major extract rooms and you also restrict the fresh air feed, as the fresh air feed room pressure will increase, because of the much reduced extract rate and that will then reduce the fresh air feed rate (flow rates in MVHR are extremely pressure sensitive, hence the need for, and the hassle of, balancing the system).

 

Finally, remember that the heat exchanger effectiveness is critically dependent on the air flow being balanced on both sides.  Significantly reduce the extract flow rate by closing off most of the extract rooms and the system will be imbalanced and only recover a fraction of the heat it could otherwise recover, plus the fan power will probably increase, because of the pressure conditions being imbalanced between the two side (fan power efficiency curves are very far from being flat).

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I think gaps under doors have to be larger than 10mm or the air flow will be faster creating draughts. I like @JSHarris's idea of transfer vents, wish I'd thought of that before fitting all the pocket doors. Like others have said I doubt many of our doors will be closed anyway.

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I presume you should balance the system with the doors either open or closed depending on how you expect them to be used? It must make quite a big difference to the balance, unless you have a supply and extract in each room.

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Just now, Crofter said:

I presume you should balance the system with the doors either open or closed depending on how you expect them to be used? It must make quite a big difference to the balance, unless you have a supply and extract in each room.

 

I think you should balance the system with the doors how you usually have them. For example, our bathroom door is closed for less than half an hour a day. Bedroom doors are almost never closed. The only doors that're regularly closed are those on the utility room and TV room.

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We have 826mm doors mostly and went for an 9mm gap, can't say you really notice it at all. We have hard flooring (Karndean, resin, wood) throughout.

 

Our pocket doors still allow a lot of airflow around the sides when closed (we don't have nylon brushes as they are 40mm doors in a 45mm gap) so we didn't trim those.

 

Given you balance the system based on the airflow immediately entering or leaving the terminal, I don't see how open/closed doors will affect readings.

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9 minutes ago, Bitpipe said:

Given you balance the system based on the airflow immediately entering or leaving the terminal, I don't see how open/closed doors will affect readings.

 

If you room is fairly airtight and you close the door, it will increase the resistance on the system (like closing down the vent) (as it has to pull/push past the closed door) so if you balance with a closed door on an extract room, when the door is open, the resistance drops and it will extract faster and the extraction elsewhere in the house will consequently drop.

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7 minutes ago, le-cerveau said:

 

If you room is fairly airtight and you close the door, it will increase the resistance on the system (like closing down the vent) (as it has to pull/push past the closed door) so if you balance with a closed door on an extract room, when the door is open, the resistance drops and it will extract faster and the extraction elsewhere in the house will consequently drop.

 

 

For us, this is noticeable.  The door from the hall to our kitchen/dining room is the only source of air for the kitchen, utility room and downstairs WC.  Even with a gap underneath, and a flush stone threshold, it has a slight resistance to being closed, just from the suction from three fairly high rate extract terminals.

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1 hour ago, le-cerveau said:

 

If you room is fairly airtight and you close the door, it will increase the resistance on the system (like closing down the vent) (as it has to pull/push past the closed door) so if you balance with a closed door on an extract room, when the door is open, the resistance drops and it will extract faster and the extraction elsewhere in the house will consequently drop.

 

Interesting - I had all the doors open while balancing the system as there was lots of running from room to room.

 

Would be interesting to see if whoever is currently balancing their system can measure this effect to see if it's noticeable or negligible in the readings.

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5 hours ago, JSHarris said:

If you don't want gaps under doors, then you can always use transfer vents instead.  These can be quite discreet and if they are arranged so that they take an indirect path through an adjoining wall then they can reduce sound transmission.

 

Do you have any example of domestic/discreet versions? A Google just seems to turn up very industrial looking commercial vents.

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