MVHR pro-design needed?

[Dreadnaught]

Do I actually need someone like BPC to design my MVHR ducting system? 

 

For my new-build bungalow, it seems really simple. I have designed the ducting plan myself following the basic rules:

1. equal number of supply and extracts across the house (5x of each)
2. double pipes to every terminal so it is quiet
3. supply to bedrooms and living room (x3 & x2)
4. extract from bathrooms, kitchen, utility room and plant room (x2, x1, x1, x1)
5. room terminals as far from the door as possible

Seems simple! What am I missing? Is a pro-design for the air-change data, which is needed for building control?

 

The reason I am thinking about this now is that I am just about to put the timber-frame in to production. The frame has one single structural steel. And that steel would benefit from some pre-cut holes (x6) to allow the 75mm MVHR ducting to pass through it. I can cut the holes based on my design. Or should I get a professional involved?

[joe90]

I designed my own, no double pipes, can only be just heard on boost which is very rare, made my own manifold and silencer from ply and made my own terminals from cheap stuff (that cannot be seen).

[Gone West]

I designed our system and there were no problems with BC.

[Iceverge]

Another self design here. 

 

We only doubled the pipe to the kitchen extract. Recommended maximum air flow velocity is 2.5m/s. In theory we’re above this in our bedroom but unless it’s on boost you can’t hear it. 

 

If you have plenty space it might be easier to use 90mm ducting than doubling up.

 

[Dreadnaught]

17 minutes ago, Iceverge said:

If you have plenty space it might be easier to use 90mm ducting than doubling up.

 

Thanks @Iceverge. When you say 90mm is that the external dimension? And internal would be 75mm? Is that right?

 

2.5 m/s noted, thanks. I am thinking of using Ubbink Air Excellent System, which 90/75 duct and even higher capacity one that is semi-rectangular in various sizes.

 

Ubbink has good data on their website for each component showing volumes, flow rates and pressures, which are very hopeful.

 

I have calculated my room sizes on-the-back-of-an-envelope, which has been helpful in deciding on duct numbers and sizes. For example, I have quite a large living room and was thinking of putting two supply outlets, each with two 90mm ducts attached. Four ducts in total. Running the numbers showed that a single 90mm duct to each of the two outlets would be fine but, conversely, one single outlet with two ducts attached would have been border-line noisy.

 

 

[Iceverge]

3 hours ago, Dreadnaught said:

Thanks @Iceverge. When you say 90mm is that the external dimension? And internal would be 75mm? Is that right?

 

 

Start your design at your bedrooms.

 

Bedrooms are the most limiting rooms for ventilation for 2 reasons.

 

1.They are the most noise sensitive

2. When occupied the doors are closed and air mixing with other rooms is limited. 

 

CO2 levels need to be controlled to ensure good health. 

 

 

As per the above graph 5l/s per person will keep CO2 levels acceptable. I worked on the principle that maximum bedroom occupancy would be 2 individuals or 10l/s.

 

Every meter of 75/63mm ducting  contains 3.12 litres of air so at 10l/s  in our master bedroom our design velocity is 3.2m/s. It is above the recommended 2.5m/s but I followed Jeremy Harris's notes about noise where he suggested that it was almost exclusively caused by the MVHR unit itself and the room terminals. 

 

From my experience I would agree. Even at boost (4.5m/s) there is little duct noise. As long as you take care to have straight duct runs in the vicinity of bedrooms 2.5m/s is very conservative.

 

More important is to consider your choice of MVHR unit. The passivhaus component database is the only independent comparison of noise I could find. https://database.passivehouse.com/en/components/list/ventilation_small

 

Important also is how the unit is mounted. If it is in the attic it needs to be physically isolated from lightweight structures like ceiling joists. Either by hanging form the rafters or using acoustic mounts. Alternatively mount it on an external wall or on the floor of a plant room. Another thing to consider is the location of  external vents. Don't put them near bedroom windows. 

 

I used flexible acoustic silencers from BPC ventilation on both the extract and supply sides of the unit. They were cheap and cheerful and I don't know how well they perform. @joe90 diy foam lined distribution boxes may well outperform them. 

 

I only doubled up on the 75/63mm ducting for the kitchen extract. Through experimentation i found that the maximum a 63mm duct could transfer without restricting any other outlets was about 5.1m/s or 16l/s. Again just using this trial case the noise was primarily from the terminal and the MVHR unit. 

 

 

TLDR. 

 

I used 75/63mm everywhere. Only doubling up for the kitchen. We have no issued with duct noise or air volume restrictions but were careful to have straight short duct runs and shallow bends only. 

 

If I was to reduce noise further I would probably install a larger volume acoustic silencer first. Then possible upgrade the MVHR unit to a quieter model. 

 

 

P.S on positioning rooms terminals. Aim to have them:

 

1.Across the room from the door. ( not vital if you can't manage it )  

2. Where high furniture won't obstruct them ( over a window works well) 

3. Not over a bed or other occupied area where drafts might be noticed. 

 

 

 

[Russell griffiths]

@Dreadnaught check prices carefully, the funny rectangular duct is painfully expensive compared to round. 

Also read the adverts carefully some prices look similar but the rectangular comes in 25m rolls and the round in 50m 

i bet a few people have been caught out with that one. 

[PeterW]

On 16/02/2021 at 14:56, Dreadnaught said:

Seems simple! What am I missing? Is a pro-design for the air-change data, which is needed for building control?

Read the Domestic Ventilation Guide especially section 5 which is the sign off section. Work back from that and you have a design. 
 

And you can always bugger around with it when it’s signed off - that’s the easy bit..!!

[Dreadnaught]

37 minutes ago, Iceverge said:

As per the above graph 5l/s per person will keep CO2 levels acceptable. I worked on the principle that maximum bedroom occupancy would be 2 individuals or 10l/s.

 

 

I read your reply with care. Really interesting and highly relevant. Thank you very much.

 

Regarding doubling ducts, what about pressure as well as noise from turbulence from within the pipes. For all longer runs, wouldn't doubling ducts also reduce pressure, which in turn would reduce how hard the fans need to work, which I turn could would reduce noise from the appliance itself. Any merit in such reasoning too? And given that ducting is not a big cost in the overall costs, suggest doubling long runs as a matter of course?

 

2 minutes ago, PeterW said:

Domestic Ventilation Guide

 

Oh yes, I forgot about that document. Thanks for the reminder.

[dpmiller]

31 minutes ago, Dreadnaught said:

 

I read your reply with care. Really interesting and highly relevant. Thank you very much.

 

Regarding doubling ducts, what about pressure as well as noise from turbulence from within the pipes. For all longer runs, wouldn't doubling ducts also reduce pressure, which in turn would reduce how hard the fans need to work, which I turn could would reduce noise from the appliance itself. Any merit in such reasoning too? And given that ducting is not a big cost in the overall costs, suggest doubling long runs as a matter of course?

 

 

Oh yes, I forgot about that document. Thanks for the reminder.

 

yes, less restriction means less fan power to get the required flow

[Iceverge]

37 minutes ago, Dreadnaught said:

For all longer runs, wouldn't doubling ducts also reduce pressure, which in turn would reduce how hard the fans need to work, which I turn could would reduce noise from the appliance itself. Any merit in such reasoning too? And given that ducting is not a big cost in the overall costs, suggest doubling long runs as a matter of course?

 

Good point I didn't think about this. Anyone have a good method of DIY measuring the duct pressures? 

[Dreadnaught]

On the subject of silencers, I notice the Ubbink Air Excellence 800-series manifold box has the option of putting sound attenuation foam within the manifold itself. It is shown in this video. And it seems you can buy extra attenuation foam pieces too to bump it up yet further, although I am not sure about this.

 

Could such a manifold also act as a silencer too I wonder? Two birds with one stone.

 

 

[Iceverge]

Yup can do. 

 

They're expensive mind you. 

[Nickfromwales]

I fit inline silencers ( attenuators ) between the unit and the manifold boxes. Excellent results, and about £150-£200 for the 1000mm long ones, with the 500mm ones annoyingly close to that price. Last install I put 3x 500mm ones in, and reports are of it being graveyard quiet. 3rd one went on the exhaust duct to atmosphere, to ensure that there was no nuisance noise to the adjoining neighbours property.

The flexi duct attenuator option is the cheap seats, but does start to alleviate the issue at least somewhat, however I do not bother with those now as there are better options for not much money. I would  not be in the cheap seats for the one device that is running 24/7/365 and which could annoy me whilst I slept.

31 minutes ago, Dreadnaught said:

On the subject of silencers, I notice the Ubbink Air Excellence 800-series manifold box has the option of putting sound attenuation foam within the manifold itself. It is shown in this video. And it seems you can buy extra attenuation foam pieces too to bump it up yet further, although I am not sure about this.

 

Could such a manifold also act as a silencer too I wonder? Two birds with one stone.

I often get those delivered by mistake, but never fit them. More filters, more points of maintenance, and additional resistance where I would not welcome it. Just fit 1x decent inline attenuator on the supply air prior to the manifold if you wish to save money, but one on each is my entry level offering. Too little money to split hairs over IMO.

[Nickfromwales]

For completeness, the inline attenuators have a full bore uninterrupted airflow through them. 180mm hole from end to end, ergo zero influence on flow, maximum reduction of nuisance sound. 

[Dreadnaught]

Could someone explain how to make an airtight seal for the supply and extract ducts to atmosphere please?

 

The ductwork work will be 160mm Aerfoam insulated mass flow ductwork. My external wall will be a timber I-beam wall at 600mm centres. Cladding will be brick slips.

 

Is it simply a matter of cutting the holes through, inserting the pipes and then using a pair of airtightness grommets on the inside airtightness layer?

 

 

[PeterW]

Just now, Dreadnaught said:

Could someone explain how to make an airtight seal for the supply and extract ducts to atmosphere please?

 

The ductwork work will be 160mm Aerfoam insulated mass flow ductwork. My external wall will be a timber I-beam wall at 600mm centres. Cladding will be brick slips.

 

Is it simply a matter of cutting the holes through, inserting the pipes and then using a pair of airtightness grommets on the inside airtightness layer?

 

 

 

Yep got it in one. 

[Dreadnaught]

2 minutes ago, PeterW said:

Yep got it in one.

 

How would I seal the outer face? Foam around and then just tape the duct to the breather membrane?

[Dreadnaught]

2 minutes ago, Dreadnaught said:

How would I seal the outer face? Foam around and then just tape the duct to the breather membrane?

 

When is an Intumescent Pipe Collar needed?

[Nickfromwales]

1 hour ago, Dreadnaught said:

 

When is an Intumescent Pipe Collar needed?

BCO needs to answer that. 

[Nickfromwales]

1 hour ago, Dreadnaught said:

 

How would I seal the outer face? Foam around and then just tape the duct to the breather membrane?

For a 160mm duct ( penetration ) you mark a 300mm square, with a pilot hole, drilled from

indide to out, as the centre point. Cut down the two sides and along the bottom to create a ‘cat flap’ of membrane. 
Cut the 165mm hole out with a jigsaw, or recip saw if you’re brave, and then foam the duct into the hole. 
The remaining flap can then be cut into 30mm fingers and shortened, and then air tightness taped to the duct and to the membrane.  
Use A/T tape to seal the open end of the duct ready for the blower test ( if not yet done ) otherwise just use a bag and some gaffs tape to stave off wind-driven rain.