MVHR Design: Feedback & Mythbusting

[joshwk]

Hi all,

My architect has been very debbie-downer on MVHR, driven largely by a lack of understanding of the technology ("they're very loud and not good if you have allergies"). Nevertheless, I'm committed to it so would appreciate any thoughts on the V1 design I've received from our MVHR supplier.

Some context:

• Our house type will be single storey, constructed in SIPs, ~200m2 total size.
• We've opted for a Zehnder Q350 system, which should provide up to 350m3/hour. 
• Rigid metal ducting with metal silencers.
• I understand size of rigid metal ducting in general ranges from 200mm to 125mm and the larger ducts to the atmosphere will be insulated with a 50mm isosleeve (specific details of our sizing are called out on the attached). We have one unavoidable ‘tight’ area transitioning between the utility and kitchen, where the designer has proposed a special rigid metal rectangular duct (220mm x 55mm) to hide a duct in a stud partition.

 

Our architect is pushing the supplier to locate the MVHR system within the attic. The designer has flagged that by doing so, it could cause vibrational noise as the floor is timber made, hence, locating it on the ground floor would be less severe. The majority of systems I've looked at have the unit siting on the floor with silencers vertically mounted and ducting branching off into the attic space, which I I am not adverse to (in fact, it's probably my preference as I understand the filters need to be changed every 6 months), assuming there is sufficient space in the plant room.

As the plant is located next door to a bedroom, we will also sound insulate the plant (e.g. rockwool) for peace of mind.

 

I'd appreciate any feedback on the following:

• The MVHR design in general
• Any thoughts on the size of the ducting? This is the area I'm probably least familiar with.
• Any thoughts on the Zehnder Q350 system and if this is under, oversized or "just right"?
• Should I be concerned about i) the noise of the system and ii) the plant room being located next door to a bedroom? Even though I will sound insulate the plant room?
• Any pros or cons to locating the system on the ground floor or in the attic (outside of space saving)?

Thanks in advance!

26791_V_R1.pdf

Edited September 12 by joshwk

[Mr Blobby]

Why are you running a branched ductwork? 

Far better and easier to install with zhender manifolds with attenuators on top of the zhender unit and then 90mm semi rigid ducting to each of the rooms. 

And probably less noise transmission between rooms (hence all the silencers in the ducts on your plans)

 

Is your loft space warm and inside the airtight envelope?  If not then do not even think about putting the unit in the loft space, too much hassle insulating ductwork and sealing penetrations.

Edited September 12 by Mr Blobby

[Kelvin]

We have the Q350 mounted on an outside wall of the plant room. The room next door is a study. You can’t hear the Q350 running from the study. You can hear the heating however. It’s a very quiet constant hum. I think it’s coming from the UFH manifold which is mounted on the adjoining wall of the study so worth considering what else will be in your plant room and just adding rockwool won’t work. 
 

As Mr Blobby says we have two large attenuators on the supply and extract which then goes into a Lindab manifold. 

[IanR]

2 hours ago, joshwk said:

Nevertheless, I'm committed to it

 

You're building with SIPs, which aren't easy to deliver high air tightness with. What's the Air Infiltration rate you are targetting for your build, and has the SIP supplier and follow on trades signed up to it. MVHR has a negative impact at an infiltration rates greater than 3m³/m².h@50Pa, and only starts to pay for itself, typically, at less than 1.5m³/m².h@50Pa.

Edited September 12 by IanR

[joshwk]

Thanks for the response, @Mr Blobby@Kelvin . I will ask the designer the question RE: branched ductwork but wonder if it is because we are using rigid ducting?

Additional context:

• Bedroom 1, Bedroom and the Kitchen + Dining/Living Room will be vaulted ceilings. There will be a small void here and within the hallway to hide the ducting.
   

I'm not super familiar with a branch vs radial duct system (apart from what I've just read here). My initial thought would have been that a branch layout lends itself better to a house with vaulted ceilings (as there is less ducting to run/hide), however the URL I just pasted mentions that the individual ducts are smaller and can be grouped together, resulting in a smaller ceiling void. Again, I'm not sure if is suitable for us as we will be running a rigid system.

@IanR: Thank you! The MVHR supplier we are using was recommended by our SIP supplier; they have worked together previously.

We are currently submitting our Building Regulations (for the second time) and are about to go out to tender (we don't yet have a builder or the other trades on board). I'm not sure what the target air infiltration rate is, where should I source this?

Cheers,

Josh

Edited September 12 by joshwk

[IanR]

7 minutes ago, joshwk said:

I'm not sure what the target air infiltration rate is, where should I source this?

 

Building Regs currently require you to achieve an Air Infiltration/Permeability rate of 8m³/m².h@50Pa on the finished property, proven by a "blower" test. (Where abouts are you based? I think in Scotland it is 7m³/m².h@50Pa)

 

Judging by your answer you've not considered an improved target over Building Regs. If you've not stipulated anything better then any SIPs suppliers or builders you are speaking with will have assumed just a Building regs pass is required, unless you've inadvertently gone for a package that is offering someting better.

 

MVHR will provide no benefits at that level of Air Infiltration, and will infact increase your energy losses and add additional running costs to the finished house.

 

Don't get me wrong, MVHR is a great product installed in the right home, and I'd encourage you to consider your options to improve the building performance over Building Regs requirements, but if you are not doing that, you are wasting your money on MVHR.

[Post and beam]

Use isolation rubber feet to mount the MVHR, this is what i am planning for my loft mounted Brink 400.

[joshwk]

Thanks @IanR.

 

We are targeting an improvement vs building regulations (I actually made a post here to try and understand SIP wall thickness and diminishing returns here). Currently, I am approaching our build on the basis that walls would be 142mm + 25mm PIR to achieve a u-value of 0.15 and roof would be 142mm + 75m PIR to achieve a u-value of 0.11 

I'm less familiar with Air Infiltration/Permeability requirements; is that related to PSI vales, or is that something completely different?

Edited September 12 by joshwk

[IanR]

11 minutes ago, joshwk said:

I'm less familiar with Air Infiltration/Permeability requirements; is that related to PSI vales, or is that something completely different?

 

To use another term, its air leakage, so all the gaps and holes in the building envelope that allow warm air to leak to outside.

 

MVHR itself does not improve a building's energy efficieny, it's improving the air tightness that does. You can improve air tightness down to 3m³/m².h@50Pa without requiring mechanical ventilation as it is deemed there is sufficient natural ventilation.

 

Once you are better than 3m³/m².h@50Pa, buiding regs require you to have whole house mechanical ventilation to ensure there is sufficient ventilation. MVHR then allows you to recover some of the energy in the ventilation that it supplies. Until you are below 1.5m³/m².h@50Pa you are unlikely to ever recover the capital and day-to-day running costs on an MVHR system, so unless something better than that was your target, MVHR is pointless.

[Kelvin]

psi is a measure of heat loss per  metre between two thermal components such as the wall to floor junction. You may have come across the term thermal bridging. 
 

Ian is talking about how air tight the building is so how much air is getting in or out. The lower this is (so more air tight) the less heat is lost. Whereas there’s diminishing returns with the level of insulation you can’t be too air tight but you also need to think about ventilation. The numbers Ian mentioned are the current building regs and easy to hit with no particular effort but are pretty terrible really. 
 

 

[joshwk]

Thanks both, that's helpful. Sounds like the Air Tightness calculation is a question for my architect. Is this something quite standard, which should be calculated as part of the Building Regulations submission?

Outside of questioning the role of MVHR  - any other thoughts / feedback to the initial list of questions would be appreciated!

[Kelvin]

It has to be better than the regs obviously as a minimum. Some timber kit suppliers will provide their target air tightness score but only if they are doing most of the building work.  A few will even guarantee this figure. 
 

That said, you need to decide what level of airtightness you’re aiming for then come up with a strategy to achieve it. The vast majority of trades don’t understand it nor care that much. Given that you’re building a new house you should be aiming for as low an air tightness score as possible. 

[IanR]

36 minutes ago, joshwk said:

 Sounds like the Air Tightness calculation is a question for my architect. Is this something quite standard, which should be calculated as part of the Building Regulations submission?

 

Building Control will want to know your "design" ventilation strategy, but they will not get involved again until you have the results of the physical Air Permeability test, in order to sign the build off.

 

If your Ventilation strategy is to have a better than 3m³/m².h@50Pa air permeability, enabled by whole house mechanical ventilation, they will need more detail on ventilation supplied to each room and a commisioning document for the mechanical ventilation system that shows you have achieved it.

 

If you choose to set an air permeability target beter than buiding regs then your Architect needs to know to ensure they have designed for that target, and your SIPs supplier, erectors, and window fitters need to have signed up to delivering that target.

In all honesty, if you wish to target a better than building regs result, then it's a fundamental decision to be made before you decide on construction methods, as it will/may require non-standard approaches to some construction aspects that will have a greater or lesser cost impact depending on the construction method you choose to go with. 

Edited September 12 by IanR

[Kelvin]

One of the  things that has surprised me is the number of self-builders that don’t care that much about air tightness. This seems to correlate with how involved they are with the build with the less involved being less interested. I’ve met half a dozen people who live in houses built for them that couldn’t tell you what their air tightness was nor particularly care. I’ve met two who actively made their air tightness worse to save putting MVHR in. As Ian says, it’s a fundamental decision you need to make right at the start. 

[Simon R]

6 hours ago, joshwk said:

My architect has been very debbie-downer on MVHR, driven largely by a lack of understanding of the technology ("they're very loud and not good if you have allergies").

I'd strongly urge you to take an in depth look at airtightness, it's fundamental if you want a draft free low energy house and needs to be designed in from the start. MVHR noise even with a loft installation should not be a problem, just a very low background hiss that you only hear when there is no other noise.

Re-allergies, our kids and visitors lover our house during pollen season as they don't suffer with their hay fever.

[JohnMo]

6 hours ago, joshwk said:

Our architect is pushing the supplier to locate the MVHR system within the attic.

Not best location, keep inside the heated envelope. You also need good access to service.

 

6 hours ago, joshwk said:

also sound insulate the plantroom

It's pretty much mandatory to sound insulate all internal walls. Mine is mounted on a stud wall in a cupboard between two bedrooms. Did two layers of 18mm marine ply to mount it on. You cannot hear it at all.

6 hours ago, joshwk said:

Should I be concerned about i) the noise of the system and ii) the plant room

It's not an ideal location next to bedrooms, MVHR is only one noise source.

 

4 hours ago, joshwk said:

I'm not sure what the target air infiltration rate is, where should I source this?

Your architect should be discussing with you and it should be clearly stated on the house specification and drawing.

 

If you are not going airtight better than 3. Don't waste a load of money on MVHR, do MEV or dMEV with demand activation controls. MVHR requires airtightness or it's just another draft, that costs quite a bit to run, for a negative impact on heating costs - not a positive one.

 

As mentioned all this should be sorted at and defined as part of the design stage. Adding and changing stuff, without a sound strategy, is a good way to waste lots of money later.

Edited September 12 by JohnMo

[joshwk]

Thanks for the replies.Without going into too much detail, our self-build project has evolved dramatically over the past 4 years from "full conversion" to "full demolition and rebuild" due to structural issues.

We were strongly advised by our architect to pursue SIPs, due to their strong overall performance, which we committed to back in 2020. At the time, the building was ~50% conversion & ~50% rebuild so the "full demolition" opportunity allowed us to build in 100% SIPs. My understanding is that SIPs were good for air tightness but the "3m³/m².h@50Pa air permeability" figure is truthfully the first I've heard this referenced.

I'll enquire with our architect now RE: air infiltration/air tightness but if we're using SIPs coupled with a high performing window (e.g. thermally broken aluminium windows), should they be able to provide a simple calculation i.e. "better than 3" or do they outsource this to a specialist company to calculate the numbers?

 

Edited September 12 by joshwk

[JohnMo]

Air tightness is all about details. Builders will do what is asked if them in general. If airtight details are not specified, you will not get them. This includes how window and doors are installed and then sealed in place. Are joints between sip of panels taped etc...

 

Be careful with aluminium windows make sure you get the U_W value from the supplier. Some are absolutely rubbish.

 

Scottish building reg mandate MVHR is airtightness is 3 or better, MEV or dMEV for above 3 to 5, what ever you want after that because it's drafty. There have been studies that looked at it also they concluded similar. I binned all my references when I changed my phone 

[Post and beam]

3 hours ago, IanR said:

Until you are below 1.5m³/m².h@50Pa you are unlikely to ever recover the capital and day-to-day running costs on an MVHR system, so unless something better than that was your target, MVHR is pointless.

Pointless ? mandated though 

[IanR]

38 minutes ago, joshwk said:

We were strongly advised by our architect to pursue SIPs, due to their strong overall performance, 

 

SIPs is a reasonable product when combined with a masonry skin, for volume builders, delivering building regs houses. There are less positives to SIPs for a self builder, building a bespoke design and wanting better than building regs performance.

 

38 minutes ago, joshwk said:

should they be able to provide a simple calculation i.e. "better than 3" or do they outsource this to a specialist company to calculate the numbers?

 

It's not something that can be calculated from a design. It's set as a target as part of an overall set of targets (U Value, psi value and air permeability) in order to achieve an overall energy loss target for the building and then designed, engineered and built to achieve those targets.

If you are happy with a buildings regs level of performance and comfort then there's not much for you to do as everyone supporting the design, engineering and construction will be delivering their normal service. If you aspire to achieve something better for lower bills, improved comfort and/or reduced ecological impact then you need to look to professionals and construction methods that can deliver those aspirations as cost effectively as possible and help you to make the right decisions based on your requirements.

PassivHaus performance levels are about as far as it's worth going while delivering value for money, but you can also choose somewhere between PH and Building Regs.

With regards to MVHR, unless you are looking for PH or something close, then I wouldn't bother. I'd target 3m³/m².h@50Pa, have trickle vents in your windows and MEV or dMEV in wet rooms as suggested by @JohnMo

 

16 minutes ago, Post and beam said:

Pointless ? mandated though 

 

Yes, I wasn't clear. Unless you're targeting better than 1.5m³/m².h@50Pa, I'd back that off to >3m³/m².h@50Pa, but as close as possible, no MVHR, trickle vents in your windows and MEV or dMEV

 

2 hours ago, Kelvin said:

 I’ve met two who actively made their air tightness worse to save putting MVHR in.

 

I can't blame them for that strategy, it's saved a lot of cost.

 

But having said that, my own preference would be <1m³/m².h@50Pa with MVHR and enjoy the comfort and lower bills.

Edited September 12 by IanR

[Mike]

To add a more rounded picture, MVHR isn't all about cost recovery / return on investment - the same as many kitchens and bathroom suites will never recover their investment.

 

Apart from any cost savings (which can be real), if MVHR is properly specified an installed it can filter the outside air, maintain a healthy internal atmosphere, avoid noise transmission through trickle vents, reduce condensation & mould risk, and avoid blowing warm air out through rattling extractor fans. And probably some other things I've not thought of.

 

As for air-tightness, the Building Regs standards are very unambitious. Passivhaus requires no more than 0.6 air changes per hour under 50pa of pressure, and values as low as 0.2 are possible. However that is only achievable if the architect designs-in adequate airtightness measures, if the various contractors and installers know what they're doing, and if someone is checking the quality of the work and materials used.

[IanR]

3 minutes ago, Mike said:

if MVHR is properly specified an installed it can filter the outside air, maintain a healthy internal atmosphere, avoid noise transmission through trickle vents, reduce condensation & mould risk, and avoid blowing warm air out through rattling extractor fans.

 

If the air permeability rate of a building is already above that deemed sufficient for ventilation then the MVHR just adds additional air exchanges on top. ie. through natural ventilation there is sufficient air changes to maintain a healthy environment and reduce mould risk. Adding an MVHR in this case just increases energy losses. The air it is filtering, is additional, unrequired air.

 

The 3m³/m².h@50Pa air permeability rate, deemed as sufficient for ventilation, is measured with trickle vents closed. The trickle vents are for adding additional ventilation above this level so should not need to be left open, if you live in a particularly noisy area.

 

The energy loses from extract fans will be lower than the day-to-day running costs (incl. maintenance) of the MVHR. Fit a single room MVHR if you must. Maintain it so it doesn't rattle.

 

To avoid condensation and mould risk, in the first instance design out thermal bridges that bring the internal building fabric temperatures close to, or below the dew point.

[JohnMo]

1 minute ago, IanR said:

The trickle vents are for adding additional ventilation above this level so should not need to be left open, if you live in a particularly noisy area.

Just to provide a bit more detail -MEV and dMEV require trickle vents to be open or self modulating to provide sufficient cross flow ventilation. Testing was completed by BRE and many councils and shows without the trickle vents open unhealthy CO2 concentrations will existing bedrooms etc.

 

This may not be the case with intermittent fans.