MVHR Flow Rates to rooms and 2 Ducts or 1

[Ambaz79]

Hello I am designing the MVHR for the house and not sure if to have some rooms with 2 duct pipes running to the plenum or 1.  

 

MVHR Unit will be in the loft and there are some runs that are around 15M. 

 

We are using the 75mm Flexible Semi-Rigid Ducting from BPC.

 

Builder is installing this for me however never done this before.  He understand the basic priciples of the system. 

 

Living room and Kitchen are the ones that are around 15M away from MVHR Unit at Max. Attached the room sizes however this is Block walls so we need to consider the rooms will be dot and dab plastered so will become a bit smaller. Also Ensuit designs is not as per the MVHR drawing not the one with the image with sizes. 

 

I have read somewhere that if runs are more tha 7M then we should run 2 pipes to the plenums. 

 

Is there any rule of thumb we should be working to on this? 

 

Sorry if there is not enough info but can provide whatever is needed. 

 

Thanks in Advance! 

 

 

[Iceverge]

From memory 2.5m/s is the maximum recommended velocity for ducting for noise. 

 

With a 75/63mm internally this translates to 7.8l/min flow at the terminal per duct.  A long duct run will have increased flow resistance but it really doesn't factor on practice once you throttle the flow at the terminals. We are single ducts everywhere except the kitchen is doubled. Longest run is about 15m. 

 

Any noise we had was because of the unit itself. Cured by a DIY silencer. 

 

 

[JohnMo]

You want the flow resistance to be as low as possible, that way the fan can run at its lowest speed.

 

Sorry can't help myself, some comments on your planned layout.

 

You seem to have supply and extracts in the same room.

 

Your study and lounge, should just have a supply only, not extracts.

 

You don't require supply or extract in cupboards, as they are not habitable spaces.

 

You really don't need the supply in the hall either.

 

The kitchen should really only have a extract.  I would have the pantry as extract not supply.

 

Bathroom extracts ideally need to over the source of steam, ie shower or bath.

 

In the bedroom or any room, you want the supply air to sweep across the room, so it needs to as far from the extract as possible, in the centre of the room is not ideal.

[SuperJohnG]

30 minutes ago, JohnMo said:

 

You really don't need the supply in the hall either.

 

Bathroom extracts ideally need to over the source of steam, ie shower or bath.

 

Interestingly I just had design done by BPC, they put a supply in my hall which I wasn't expecting. But they said do not add ducts directly over showers and baths

[JohnMo]

This isn't where I found the advise to install the extracts above the showers, but says the same thing.

 

And also from Scottish building regs, support guidance for ventilation 

Sect 8.8, d, states the extract should be above shower head or bath. 

 

 

 

2030716736_Buildingstandardsdomesticventilationguidance2CNovember2017.pdf

Edited November 19, 2022 by JohnMo
Reattached file

[Iceverge]

One advantage of a supply and extract in the same room is that you don't need a door undercut for crossflow which may help with noise between rooms. 

 

I put a supply in the hall just outside the kitchen door like @Jeremy Harris to keep smells in there and also as I had a spare slot in the manifold. 

 

I read over windows is a good location for terminals as they're unlightly to get blocked by high furniture. 

 

I think there's many ways to skin the cat in reality. The only thing that I've noticed is there's one just above our sofa if it's shoved in the wrong corner of the sitting room. It can be a tiny bit chilly then. 

[SuperJohnG]

On 19/11/2022 at 08:47, JohnMo said:

This isn't where I found the advise to install the extracts above the showers, but says the same thing.

 

And also from Scottish building regs, support guidance for ventilation 

Sect 8.8, d, states the extract should be above shower head or bath. 

 

 

 

2030716736_Buildingstandardsdomesticventilationguidance2CNovember2017.pdf 2.43 MB · 0 downloads

That's strange totally contradicts what BPC say albeit it is a vent axia system they are offering. 

 

Thanks

[JohnMo]

Lots of contradictions everywhere you look.

 

In the end that's why I looked at Scottish Building Regs, the last thing I want is the inspector say no it has to move.

[Ambaz79]

On 18/11/2022 at 22:31, JohnMo said:

You want the flow resistance to be as low as possible, that way the fan can run at its lowest speed.

 

Sorry can't help myself, some comments on your planned layout.

 

You seem to have supply and extracts in the same room.

 

Your study and lounge, should just have a supply only, not extracts.

 

You don't require supply or extract in cupboards, as they are not habitable spaces.

 

You really don't need the supply in the hall either.

 

The kitchen should really only have a extract.  I would have the pantry as extract not supply.

 

Bathroom extracts ideally need to over the source of steam, ie shower or bath.

 

In the bedroom or any room, you want the supply air to sweep across the room, so it needs to as far from the extract as possible, in the centre of the room is not ideal.

 

Hello Thanks for all the pointers.  Here are some reasons I have put the ducts where they are.  

 

Lounge is going to have a Log Burner so I thought I could use the Log Burner to heat the room and the Extract to move the heat around the house.  

Study is actually our temple room where we burn insent sticks and Oil Candle so I just wanted a way to move the smoke out.  I have not really reaseched if its a good idea but just seemed to make sense when I did it.

The Cupboard where we have the extract is where the Manifold for the UFH, Fuse board and a number of transformers for the LED Lights we are running in the ceiling.  So again the idea was to recirculate any heat generated by this. 

I was 50/50 about hallway and could remove this.

Ive been told due to the size of the kitchen it would be better to have a supply and extract there. 

 

Would you suggest I change any of the above? 

 

I will move the extracts in the bathrooms to be directly above the showers.

I will also move the supply to be more away from the extracts. 

 

Thanks allot for all these pointers. 

 

 

[JohnMo]

MVHR flow rates are way to low to move heat from A-B, it will move some but the don't count on it.  The air you supply to a room, will wash though that room, out an open door or under it and make its way to an extract point, so supply air moves the air anyway.  You UFH manifold will be at a low temp so not much warm air to move about.

 

Study I would make that extract only in your case.

 

You can do the kitchen either way.  But watch for grease mist entering the extract terminal, I have mine about 4m away from the hob, so grease is filter by the recirculation extractor first and have a foam pad in the extract terminal to capture anything missed by the recirc extractor.

[Ambaz79]

43 minutes ago, JohnMo said:

MVHR flow rates are way to low to move heat from A-B, it will move some but the don't count on it.  The air you supply to a room, will wash though that room, out an open door or under it and make its way to an extract point, so supply air moves the air anyway.  You UFH manifold will be at a low temp so not much warm air to move about.

 

Study I would make that extract only in your case.

 

You can do the kitchen either way.  But watch for grease mist entering the extract terminal, I have mine about 4m away from the hob, so grease is filter by the recirculation extractor first and have a foam pad in the extract terminal to capture anything missed by the recirc extractor.

Noted