Should I or shouldn't I go for MVHR?

[Archie]

We did not consider MVHR when we first commissioned our house.  However with 1st fix nearing completion and plasterboard ready to start we are having second thoughts.  

I paid for a design which works with the existing services. 

House construction is SIP's with a brick outer skin.  Designed air tightness is 7 although I think in reality I will beat this as I have paid particular attention to achieving good air tightness.  

The main reason for considering MVHR now is I am concerned about condensation without it. Obviously there are other benefits with MVHR which are an added bonus.  The problem is it was never budgeted for and there isn't the money spare in the budget for MVHR. But if condensation will be a problem in the future I will try and find the money to install it. 

Can anyone assist in giving some advice on whether it is worth doing based on their experiences. 

 

Many thanks. 

[Jeremy Harris]

Absolutely essential, in my view, as for us it more than halves our overall heat loss.  Mind you, with airtightness that's barely better than the worst allowable under building regs, you probably won't get a massive reduction in heat loss from it, plus you need to make sure there are no nasties, like extractor fans, tumble drier extracts or trickle vents in windows, as they will all defeat the purpose of MVHR.

 

FWIW, our tested airtightness (in the odd UK building regs format) was 1.22 m³/m²/hr @ 50Pa, or 0.43 ACH @50Pa in the more usual measurement system used by everyone else.

[jack]

I've lived in a house with MVHR for over a year now and would never be without it.

 

In your case, I'd be slightly concerned about how late the decision to add it is being made.  Do you have a location in mind for the unit and associated manifolds?  Is there space for you to run ducting to all the required inlets and outlets (as a minimum, usually at least one extract in each bathroom, the kitchen, and utility room, and one supply in each bedroom and reception room)? Where would your main inlet and outlet (through the roof or an external wall) go?  

 

These are non-trivial questions, but need to be given a fair bit of thought given the point you're at now.  

[Alexphd1]

Why not just put the ducting in just now to give you options at a later date. 

[Temp]

When we moved into our house the improvement in air quality was really noticeable. First few nights we woke up feeling like we'd been camping out under canvas. If building again I'd definitely put one in again for that reason alone.   

[jack]

I should add, the sinus problems that have plagued me through much of my adult life have largely gone away since moving into a house with MVHR.

[Declan52]

If you don't go with mhrv how are you going to meet the ventilation regs for your house?? 

In my experience i would put it in just to get  fresh clean air in the house. The fact it recovers heat is a bonus in my book.

 

[Archie]

On 4/23/2017 at 18:18, JSHarris said:

Absolutely essential, in my view, as for us it more than halves our overall heat loss.  Mind you, with airtightness that's barely better than the worst allowable under building regs, you probably won't get a massive reduction in heat loss from it, plus you need to make sure there are no nasties, like extractor fans, tumble drier extracts or trickle vents in windows, as they will all defeat the purpose of MVHR.

 

FWIW, our tested airtightness (in the odd UK building regs format) was 1.22 m³/m²/hr @ 50Pa, or 0.43 ACH @50Pa in the more usual measurement system used by everyone else.

 

I am confident that we can achieve much better air tightness than the design although no where near your value, very impressive!  We do have trickle vents as we were not planning on MVHR at the time of specifying windows.  However these can be sealed shut.  We have no other 'nasties' so confident it would be beneficial.

 

Quotation I have received is for a Rolls Royce system that uses a Zhendair Unit and rigid plastic ducting.  What system do you have installed?  Did you use rigid or semi rigid ducting?  Which supplier did you use?

 

After further thought and advice received on this forum we are going to dig deep and find the cash to install.  So need to get best value.

 

 

[Archie]

On 4/23/2017 at 18:35, jack said:

I've lived in a house with MVHR for over a year now and would never be without it.

 

In your case, I'd be slightly concerned about how late the decision to add it is being made.  Do you have a location in mind for the unit and associated manifolds?  Is there space for you to run ducting to all the required inlets and outlets (as a minimum, usually at least one extract in each bathroom, the kitchen, and utility room, and one supply in each bedroom and reception room)? Where would your main inlet and outlet (through the roof or an external wall) go?  

 

These are non-trivial questions, but need to be given a fair bit of thought given the point you're at now.  

 

I realise its a bit late in the day to make this decision.  However I have had a design done that I have tweaked following review of buildability.  I now have a design that works with the already installed services. 

We have space in the attic for the unit and the design of the house means there is space for the main inlet and outlet to be located on a gable end wall.

Based on research and positive posts on this forum we are going to proceed with installation.  Would be really interested in finding out what type of unit and ducting you used and the supplier you used. 

Many thanks.

[Declan52]

A lot of people here me included have used Gary from bpc ventilation who is even a member on the forum.

https://www.bpcventilation.com

[Archie]

10 hours ago, Declan52 said:

A lot of people here me included have used Gary from bpc ventilation who is even a member on the forum.

https://www.bpcventilation.com

 

Cheers.

 

Have just uploaded my plans to their website.

[Jeremy Harris]

11 hours ago, Archie said:

I am confident that we can achieve much better air tightness than the design although no where near your value, very impressive!  We do have trickle vents as we were not planning on MVHR at the time of specifying windows.  However these can be sealed shut.  We have no other 'nasties' so confident it would be beneficial.

 

Quotation I have received is for a Rolls Royce system that uses a Zhendair Unit and rigid plastic ducting.  What system do you have installed?  Did you use rigid or semi rigid ducting?  Which supplier did you use?

 

After further thought and advice received on this forum we are going to dig deep and find the cash to install.  So need to get best value.

 

To be honest, 0.43 Air Changes per Hour (ACH), or 1.22 m³/²/hr, is OK, but not a lot better than the 27 year old PassivHaus requirement of 0.6 ACH.  UK building regulations are pretty poor, in terms of energy efficiency standards, and haven't really improved much in recent years, they are still decades behind the requirements of other many other European countries.   Countries like Ireland have massively improved their building regulations, for example, to the point where Dublin are now looking at requiring all new houses built there to meet the PassivHaus standard, I believe.  This is definitely a step in the right direction, but sadly the UK doesn't seem to want to improve new homes, and is still churning out thousands of new houses a year that are pretty woeful in terms of performance.

 

We used a semi-rigid duct system for our MVHR, probably the easiest system to install when there isn't much room.  There are several variations on the same design, as HB+ seem to have licensed it to many other manufacturers.  All the duct of this type has the same dimensions, it's mainly the end fittings and plenum chambers that vary from one supplier to another (plus the cost!).

 

The MVHR we went for is an expensive Genvex unit, as it includes an air-to-air heat pump, so can either warm or cool the fresh air feed to the rooms.  For us it made sense, as our house is in a very sheltered location, cut back into a hill, and faces South, so tends to be in a warm spot, especially on still, sunny, days.

[jack]

10 hours ago, Archie said:

 Based on research and positive posts on this forum we are going to proceed with installation.  Would be really interested in finding out what type of unit and ducting you used and the supplier you used.

 

Great news that this is possible.  

 

Re: putting it in the attic, one thing to consider is access to change/clean filters.  This needs to be easy to do at least a couple of times a year (we do ours quarterly or so).  The (potential) worry with attic installations is that they're out of sight and out of mind.  Maybe set a reminder in your diary?

 

We went with a Brink Excellent 400.  I'm not sure whether I'd do this again, as it was quite expensive relative to, eg, the Vent Axia Sentinal that I know a lot of people use.  Our house is reasonably big (290m²) so that immediately knocked out a lot of smaller units.  We were also considering Passivhaus certification, so needed a Passivhaus certified model, which further limited us.

 

I second the vote for BPC.  We didn't go with them in the end, but I had a long a very helpful chat with Gary at one point.  Given we decided against PH certification in the end, I think we'd have saved ourselves a fair amount of money if we'd gone with them. 

[jack]

51 minutes ago, JSHarris said:

All the duct of this type has the same dimensions, it's mainly the end fittings and plenum chambers that vary from one supplier to another (plus the cost!).

 

There are also a couple of manufacturers that offer ducts with a flattened oval cross section.  Useful if you want to run within a stud wall (as we did in places) or have limited space.  Examples at the bottom of this page and here.  

[SteamyTea]

Why are you concerned about condensation?

A condensation risk analysis should have been done when your house was designed.

Also, what are the details between the SIP panel and the ground/floor, there has been much discussion about this in the past.

[Crofter]

2 hours ago, SteamyTea said:

Why are you concerned about condensation?

A condensation risk analysis should have been done when your house was designed.

Also, what are the details between the SIP panel and the ground/floor, there has been much discussion about this in the past.

 

The OP said that the build was intended to achieve a 7 on the airtightness, but that he thinks he will do much better.

IMHO you can't really 'design' a building to achieve that sort of number, you are taking pot luck with how many holes are left behind by each trade. It's just an assumed number and probably bears little relation to reality.

 

Even very very draughty houses get bad condensation- my own being a good example. I've got bare floorboards in much of the house and the wind whistles through the gaps, yet I still end up with windows streaming with condensation in the morning.

[ProDave]

Just to add to this. A nearby self builder (who's house I am wiring at the moment) is going to great pains to make it airtight but is not fitting mvhr. He has a central extract system for wet rooms and trickle vents in the windows.

 

Today he tells me he has had the air tightness test done today. The figure is 1.7 I don't know what units that is other than it's not ACH. He has been told that is "too good" so me must now introduce positive input ventilation as well now.  I kept telling him to fit an mvhr unit, now he will have mechanical ventilation, but without the benefit of the heat recovery. 

[Jeremy Harris]

The "1.7" is probably the oddball building regs way of describing airtightness (it's unique to building regs, they invented a new system, rather than use ACH like every other country on the planet.........).  As such it relates to floor area, rather than house air volume (yes, it is bizarre).  This means it's probably 1.7m³/m²/hr at 50 Pa.  Without knowing the house volume and floor area it's not possible to directly relate this to the more normal ACH.

 

The chances are that 1.7m³/m²/hr  is probably a bit better than the threshold where MVHR starts to bring significant benefits, in terms of energy saving.  A general rule of thumb is that an airtightness of around 3 ACH @ 50 Pa is about the point where MVHR starts to work well in terms of energy saving, but it will still give a very useful improvement in air quality at poorer air tightness levels than this, it will just probably never pay for itself in terms of energy saving through life.  Arguably the same could be said about our active MVHR, but I was prepared to pay the extra to get comfort cooling in hot weather.

[ProDave]

I know it's a big house. easily twice the size of mine so of the order of 300 square metres floor area. 2.7 metre ceilings downstairs, and vaulted ceilings to most bedrooms.

[Jeremy Harris]

So, with a lot of guesstimating, this house could be around 1000m³ in volume.  If the airtightness comes out as 1.7m³/m²/hr, then in terms of ACH it's around 2 ACH.  Definitely into the region where MVHR will save a significant amount of energy, so worth doing, I'd have thought.

 

Edited to add:  Disregard these numbers, they are wrong.  Part L1a 2013 now stipulates a limit of 5m³/m²/hr at 50 Pa, based on internal envelope area, as the limiting air leakage for a house with MVHR.

Edited April 29, 2017 by JSHarris
Wrong info

[A_L]

@ProDave  Suitably sized centralised mechanical extract ventilation (MEV) should be sufficient on its own https://beta.gov.scot/publications/building-standards-technical-handbook-2016-domestic/ Section 3.14.11 (bottom of page 196)

 

@JSHarris Is the m² term not the total external surface area of the house? (ceiling + walls (including openings) + floor)

Edited April 28, 2017 by A_L

[IanR]

2 hours ago, A_L said:

@JSHarris Is the m² term not the total external surface area of the house? (ceiling + walls (including openings) + floor)

 

As defined in Part F:

 

Quote

Air permeability is the physical property used to measure the airtightness of the building fabric. It is defined as air leakage rate per hour per square meter of envelope area at a test reference pressure differential across the building envelope of 50 Pascal

 

I've always thought it was external area (rather than internal), but I can't find it actually stipulate that in the approved document.

[Jeremy Harris]

According to my air test chit, it's internal floor area.  Has Part L changed recently?

 

Edited to add:

It seems to.  It now gives a limiting value of not more than 5m³/m²/hr at 50 Pa internal envelope area for a house with MVHR, limit is 7m³/m²/hr at 50 Pa for a naturally ventilated house.

 

I've edited the earlier posts to show the error I made.

Edited April 29, 2017 by JSHarris
Added Part L1a 2013 details

[A_L]

 @IanR -hello, strictly speaking I think this is correct:-

Quote

The UK standard measures Air Permeability, in m3/hr/m2@50Pa (the q₅₀ measurement), or in other words the air leakage per square metre of building envelope. The ATTMA (Air Tightness Testing and Measurement Association) TS1 standard defines the building envelope as everything within the air barrier line ‘along the line of the component to be relied upon for air sealing’. This could be anywhere within the building envelope (even the external render). This is a measure of building envelope airtightness.

 

from http://www.peterwarm.co.uk/a-guide-to-volume-calculations-for-passivhaus-air-tightness-testing-and-the-difference-with-the-uk-method/

[Jeremy Harris]

1 hour ago, A_L said:

 @IanR -hello, strictly speaking I think this is correct:-

 

from http://www.peterwarm.co.uk/a-guide-to-volume-calculations-for-passivhaus-air-tightness-testing-and-the-difference-with-the-uk-method/

 

Yes, that seems to follow the guidance for the current building regs I looked at earlier, although that guidance states "internal envelope", most probably because the majority of houses will have the VCL as the airtightness barrier, and this has to be on the inside (or very close to the inside) of the envelope.

 

The odd thing is that my air test certificate gives two figures, the air leakage in PassivHaus terminology, using the stated house internal volume (0.43 ACH  @ 50 Pa) and also the UK Part L1a figure of 1.22 m³/m²/hr @ 50 Pa, with the only area stated on the certificate being the gross internal floor area.  It makes me wonder if the air test certificate is in error, as the gross internal envelope area is greater than the floor area.  I'd just assumed that the data on the air test certificate defined the measurement, as the version of Part L1a I was working to (2010) didn't specifically mention the area used in the definition. 

 

Using the Part L1a 2013 definition, and taking the internal VCL as the airtightness layer (which it is), then the envelope area comes out at 230m², making the measured air permeability (using the measured leakage rate of 166m³/hr @ 50 Pa from the average of the two tests) 1.38m³/m²/hr @ 50 Pa, a bit worse than stated on the certificate, but not enough to lose any sleep over, thankfully!

Edited April 29, 2017 by JSHarris