MVHR is Largely Bogus

[SimonD]

1 hour ago, Kelvin said:

Is it possible (with examples) to achieve passive levels of airtightness with the above approach of very passive ventilation control.

 

Yes,it is possible, but I'm wondering whether you're missing the point in terms of how this works. You build for airtightness in both scenarios and then in each of those you design for a defined number of air changes per hour to maintain a healthy environment. This means that even if you build a passivhaus that achieves maximal airtightness, you're going to have to change the air in the house with something from outside, even if some of it is preheated using heat recovery.

 

1 hour ago, Kelvin said:

The hospital example isn’t a corollary with houses unless it was comparing it against very modern build methods and designs.

 

Why not? It's actually about occupant health and the principles were certainly backed up by the architect I spoke to who indicated a review of modern build methods (because they don't work that well in the context) and instead using principles developed and then forgotten from over 100 years ago. It's absolutely relevant to our undertanding of indoor air quality as well as the maintenance of human health. The reason the research is going on there is down to budgets, hence why more work is being done in the commercial construction field - do you think our major house developers would have any interest in funding this kind of research for their builds? Of course not, it's easier to just chuck in a mechanical system, just like defaulting to a gas boiler and using the loopholes to continue doing so.

[Kelvin]

Rather than tell me I’m missing the point explain how a house built to passive standards achieves the ventilation needed throughout the whole year without using any mechanical ventilation system which is what seems to be the suggestion. Some examples of where it’s been done would be helpful too. Everything I’ve read says otherwise. 
 

The reason I’m struggling to accept that Victorian standards of hospital building 100 years ago can be used as a comparator is because air pollution in the same period was awful and worse in the areas with the highest coal intensity. They might well have been better ventilated but that air was likely quite polluted. There’s an interesting study done on this that shows men born and living in the most coal intensive areas were over an inch shorter on average (WWI data) Hospitals tend to be in urban areas so it is likely they were in quite polluted areas too. Why that matters is because we still have quite polluted air in urban areas. Clearly one of the benefits of any kind of mechanical ventilation system is it’s likely to be filtering the air. 
 

With regards to COVID then any recirculation of stale air in a building could transfer airborne pathogens around the building. MVHR separates the incoming air from the outgoing air so in theory doesn’t suffer from the same limitations. 
 

 

Edited March 21, 2023 by Kelvin

[Kelvin]

Not had a chance to read this yet but argues MVHR has a place in any nearly every domestic house. 

 

https://www.passivhaustrust.org.uk/UserFiles/File/research papers/MVHR/2020.04.27-The Case for MVHR-v7 new cover.pdf

[JohnMo]

Just had a look at building regs in Scotland and to have passive stack ventilation your house has to leak more than 5m3/m2.  So not really airtight by any stretch of the imagination. Believe you would also need trickle vents in windows or walls as well.

 

On the plus side the room outlets have to be moisture triggered, so it only vents when needed.

[Kelvin]

Plus isn’t there a consultation going through just now about improving air tightness standards to passive levels in Scotland? 
 

If I could achieve the air tightness levels I want and provide the necessary ventilation without putting more equipment into the house I would and I’m sure most of us would. 

[ProDave]

22 minutes ago, Kelvin said:

Plus isn’t there a consultation going through just now about improving air tightness standards to passive levels in Scotland? 
 

If I could achieve the air tightness levels I want and provide the necessary ventilation without putting more equipment into the house I would and I’m sure most of us would. 

BUT in Scotland if your air test is better than 3, you MUST fit mvhr.

 

One self builder near me wanted just to use MEV ventilation but was "disappointed" his air test was so good and BC forced him to fit mvhr.  It was a pig od a job as much of the plasterboarding was done so he ended up with 2 small mvhr units one upstairs, one downstairs.

 

Far better to plan for good air tightness and mvhr.

 

If you really really don't want it, build in some deliberate leak to give a poor air tightness test, a leak that you can then "correct" after the test has been done.

[Mr Punter]

1 minute ago, ProDave said:

his air test was so good and BC forced him to fit mvhr

 

I have heard this can happen but I cannot work out why you cannot create a few leaks and have it re-tested?

[Onoff]

53 minutes ago, Kelvin said:

Not had a chance to read this yet but argues MVHR has a place in any nearly every domestic house. 

 

https://www.passivhaustrust.org.uk/UserFiles/File/research papers/MVHR/2020.04.27-The Case for MVHR-v7 new cover.pdf

 

Guessing the Passiv Haus Trust aren't going to argue against it! 😂

[Kelvin]

25 minutes ago, ProDave said:

BUT in Scotland if your air test is better than 3, you MUST fit mvhr.

 

One self builder near me wanted just to use MEV ventilation but was "disappointed" his air test was so good and BC forced him to fit mvhr.  It was a pig od a job as much of the plasterboarding was done so he ended up with 2 small mvhr units one upstairs, one downstairs.

 

Far better to plan for good air tightness and mvhr.

 

If you really really don't want it, build in some deliberate leak to give a poor air tightness test, a leak that you can then "correct" after the test has been done.

Indeed which brings us full circle really. You achieve very high levels of air tightness then you need to fit an MVHR system. 

[Roger440]

1 hour ago, Kelvin said:

Rather than tell me I’m missing the point explain how a house built to passive standards achieves the ventilation needed throughout the whole year without using any mechanical ventilation system which is what seems to be the suggestion. Some examples of where it’s been done would be helpful too. Everything I’ve read says otherwise. 
 

The reason I’m struggling to accept that Victorian standards of hospital building 100 years ago can be used as a comparator is because air pollution in the same period was awful and worse in the areas with the highest coal intensity. They might well have been better ventilated but that air was likely quite polluted. There’s an interesting study done on this that shows men born and living in the most coal intensive areas were over an inch shorter on average (WWI data) Hospitals tend to be in urban areas so it is likely they were in quite polluted areas too. Why that matters is because we still have quite polluted air in urban areas. Clearly one of the benefits of any kind of mechanical ventilation system is it’s likely to be filtering the air. 
 

With regards to COVID then any recirculation of stale air in a building could transfer airborne pathogens around the building. MVHR separates the incoming air from the outgoing air so in theory doesn’t suffer from the same limitations. 
 

 

 

With a passive system incoming and outgoing are still seperated. 

 

If you want to filter the incoming air, filter it. 

 

No need for a powered fan to do it.

[Roger440]

2 minutes ago, Kelvin said:

Indeed which brings us full circle really. You achieve very high levels of air tightness then you need to fit an MVHR system. 

 

Could have just fitted intakes. 

 

How is that different from MVHR?

[Roger440]

1 hour ago, JohnMo said:

Just had a look at building regs in Scotland and to have passive stack ventilation your house has to leak more than 5m3/m2.  So not really airtight by any stretch of the imagination. Believe you would also need trickle vents in windows or walls as well.

 

On the plus side the room outlets have to be moisture triggered, so it only vents when needed.

 

Sadly trickle vents is just another regs anomoly/nonsense.

 

As ever, why mandate the solution? Why not mandate the outcome. ie, good quality air indoors and then let smart people figure out different ways of doing it. Mandating MVHR simply means no one will invent something else and we will be stuck with it as a solution forever.

 

 

[JohnMo]

5 minutes ago, Roger440 said:

Mandating MVHR

In Scotland it actually says

"Centralised, balanced supply and extract with / without heat recovery recovery"

 

So MVHR is not mandated, as such, but sure there is much on the market that could provide centralised and balanced other than MVHR.

[Onoff]

I suppose you'd term this "PWV", Prevailing Wind Ventilation:

 

https://sustainabilityworkshop.venturewell.org/node/1028.html

 

Not so great if nextdoor smokes strong weed, likes bonfires or has an open cesspool...

[saveasteading]

We have chosen not to fit mvhr as the cost would have been about £10k. If the building is as efficient to heat as planned, then we think that cost would never be recovered.

We have some areas of high ceiling, which helps air quality a lot. We think that everyday movement will shift enough air for health and comfort. But we will find out!

 

[MikeSharp01]

34 minutes ago, Roger440 said:

As ever, why mandate the solution? Why not mandate the outcome. ie, good quality air indoors and then let smart people figure out different ways of doing it. Mandating MVHR simply means no one will invent something else and we will be stuck with it as a solution forever.

Now there is a thought - but I guess the powers that be are getting more famous for not thinking things through than they used to be. 

[MikeSharp01]

1 hour ago, Mr Punter said:

I have heard this can happen but I cannot work out why you cannot create a few leaks and have it re-tested?

Or just put a few controllable holes in  before the test so you won't get a three and control it down to 3.2 on the day so you can still get a good SAP / whatever passes fro SAP. Hang on, just wondering if this is the reason some of the big house builders just don't bother too much with air tightness.

[saveasteading]

I have experimented with ventilation, with client consent.

School classrooms are said to need ventilation at a dustbin quantity per second, or kids sleep.

I think any school built to that soon has it turned down or off.

I've designed and built about 30 classrooms in 5 schools. All have windows, some have high ceilings, none have any forced ventilation. No problems reported or noticed in my return visits. That sounds risky but they were designed to allow retrofit at the same cost, if necessary.

 

So I am bemused at the specified quantity of forced ventilation.

Why do mine work? I can only suggest high ceilings, and multiple door opening allows enough air movement. Then they are empty for 18 hours.

Hence I suspect over-design is general. 

Likewise sports halls. The worst I have experienced had complex and extensive ventilation systems, as designed by m and e consultants, and the hall stank. That musty smell is mostly a lack of oxygen i think.

Our halls had fans at one end and louvres at the other, but mostly self ventilate because of wind suction  and pressure differences at 30m distance. Fans used when necessary but that is seldom.

Done 25 of them so it isn't luck.

(BTW that means 25 schools have saved £50,000 or more each for books and pencils)

 

Again i think that the industry that designs and makes the stuff are overspecifying. 

 

Does this logic apply to new houses? I think so, and will soon find out.

Edited March 21, 2023 by saveasteading

[ProDave]

1 hour ago, saveasteading said:

We have chosen not to fit mvhr as the cost would have been about £10k. If the building is as efficient to heat as planned, then we think that cost would never be recovered.

I find that a staggering cost.  My mvhr unit was about £500 and all the ducting about £1000 all DIY fitted.  Even allowing for doubling due to inflation I don't see where £10K comes from.

[ProDave]

1 hour ago, saveasteading said:

I have experimented with ventilation, with client consent.

School classrooms are said to need ventilation at a dustbin quantity per second, or kids sleep.

I think any school built to that soon has it turned down or off.

I've designed and built about 30 classrooms in 5 schools. All have windows, some have high ceilings, none have any forced ventilation. No problems reported or noticed in my return visits. That sounds risky but they were designed to allow retrofit at the same cost, if necessary.

My recollection of school classrooms is at change of period when you all march to the next classroom previously occupied by the last lot, the air in the room as you walked in was rank, and all the windows were thrown open to make it breathable.  You tend not to notice the stink and lack of oxygen when it is you creating it and it creeps up gradually.

 

Scottish regs now demand a CO2 meter in the master bedroom.  If they were not so expensive I would buy one out of curiosity, as a measure of how well the mvhr is actually doing.

[Adrian Walker]

1 hour ago, saveasteading said:

We have chosen not to fit mvhr as the cost would have been about £10k. If the building is as efficient to heat as planned, then we think that cost would never be recovered.

We have some areas of high ceiling, which helps air quality a lot. We think that everyday movement will shift enough air for health and comfort. But we will find out!

 

It’s not about recovering the costs, it’s about comfort and wellbeing. Suggest that you use a CO2 monitor to gauge your comfort levels. 

[saveasteading]

2 minutes ago, Adrian Walker said:

 use a CO2 monitor to gauge your comfort levels. 

It's compulsory in Scottish regs.

 

3 minutes ago, Adrian Walker said:

not about recovering the costs,

That's not the way I've seen it promoted for 15 years, but I think you are right.

[ProDave]

How much are CO2 meters now?  I remember looking when they first became mandatory (we missed that requirement for ours) and thought HOW MUCH?

[jamieled]

1 minute ago, ProDave said:

How much are CO2 meters now?  I remember looking when they first became mandatory (we missed that requirement for ours) and thought HOW MUCH?

Indeed. Ours was lumped in with our other electrical costs, but I believe there were very few on the market at the time and given they were mandated there might have been a bit of price creep!

 

They seem very sensitive. Ideally I would like one with a logging function, otherwise unless you hold your breath while looking at it, the values spike.

[MJNewton]

25 minutes ago, ProDave said:

I find that a staggering cost.  My mvhr unit was about £500 and all the ducting about £1000 all DIY fitted.

 

Yeah ours was just shy of £1500 all-in. Took a fair bit of shopping around, bit of good fortune/timing for picking up the unit cheap and again all DIYed but I had the time (and inclination - tight Northener here!) to take such an approach. I can see costs being much higher for paid services, single sourcing and of course much bigger houses. 

Edited March 21, 2023 by MJNewton