MVHR is Largely Bogus

[saveasteading]

4 minutes ago, ProDave said:

How much are CO2 meters now

Ours is being installed by the electrician, but it seems to be so standard now that it isn't expensive. From memory about £50 plus the fitting.

[Simon R]

6 hours ago, SimonD said:

This is probably one of the biggest advantages of natural ventilation versus whole house MVHR in that you can design in controlled ventilation zones. I have an upside down house that has an 18C downstairs and cooler which is better for sleeping, whereas the living area is 21C.

Our house is upside down and like yours is 18C downstairs and 21C upstairs all running happily with and MVHR. The upstairs living area is largely open plan with vaulted ceilings.  Setting it up this way was a simple matter of setting the flow rates during installation. I've not had to touch it since I did the setup.

Another benefit of having an MVHR beside pollen and spore filtering is the recovery of energy. Here is a shot of our MVHR when the outside temperature is 3.8C the air entering the house is at !6.5C heated from the outgoing air which is at 19.8 as it enters the MVHR and just 6.3C when it leaves the house.  This is something you just don't get with natural ventilation where the incoming air is essentially at the outside temperature making for appreciably cold drafts.

In summer when the A2A is keeping the living area at 21C the outgoing air cools the incoming air, again minimizing energy loss.

 

[SteamyTea]

If you don't have forced ventilation in a reasonably airtight house, how do you guarantee that each and every room is ventilated properly.

 

Regarding schools and ventilation. Many bad teachers will blame anything but their teaching style.

My students never fell asleep in my classes. But everyone did in the materials lectures, and they were first thing on a Tuesday morning. I think the Ofsted inspector fell asleep as 'old John' managed to pass.

[Iceverge]

2 hours ago, saveasteading said:

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.

 

 

Did you take any measurements of air quality post construction? This study suggests at children did less well in under ventilated classrooms in the US in 87 sample cases.  

 

Ventilation Rates and School Performance | Indoor Air Quality (lbl.gov)

 

 

 

 

Separately, using historical examples to justify the natural ventilation of buildings often ignores the influence of a fireplace as an active ventilation device. Open fires used to run for 18hrs+ per day in houses and probably 24/7 in hospitals. They used about 2/3 of their energy simply to drive smoke and air up the chimney. 

 

Even at 7kg of coal per day this would equate to about 35kWh/day or 10% of our annual MVHR usage. 

 

 

 

 

 

[dpmiller]

4 hours ago, Roger440 said:

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

 

Will you be able to achieve the necessary airflow when there's a restrictive filter in place?

[Onoff]

26 minutes ago, dpmiller said:

 

Will you be able to achieve the necessary airflow when there's a restrictive filter in place?

 

I'm thinking directional intake and extract on simple wind vanes. So it intakes on the prevailing side, exits on the lee.

[Onoff]

On 20/03/2023 at 00:05, Gus Potter said:

Yes you may think I'm off the mark here.. but see in ten years time when you can't find replacement parts for you MHVR and home automation other than from China.

 

I reckon they've a 50 year plan. Flood us with craptonics. Control our power distribution. Come the Big Overvoltage & Hack of 2073 EVERYTHING goes bang / catches fire and stops. Of course the electric fire engines won't work...

 

😉

[SimonD]

On 20/03/2023 at 10:32, Iceverge said:

what specialist technique did you employ to ensure natural venting was adequate? Do you have any monitoring of the air quality as this is too often pitiful in non mechanical ventilation systems. 

 

I drew our house so that every bedroom, living room and kitchen has 2 operable windows (T&T) on walls facing different direction to ensure good cross ventilation. Similarly the upstairs hallway.

 

I've just taken CO2 readings and currently in the space I've been doing admin most of the day it's hovering between 650-660ppm and downstairs it's about 430ppm. PM2.5 & 10 are at good levels and HCHO is at healthy level. Having just cooked some dinner and eaten, kitchen RH is 60% and downstairs is 55%. Even though I haven't used a lot of wets in the build, RH was much higher and is still gently on its way down, month by month.

 

In terms of approach, it was mostly a research based with just a few numbers to crunch. I read a lot of material not just on ventilation but also on building fabric, finding some real world research showing the reduction in RH coming from moisture buffering materials that either simply temporarily store excess moisture and/or pass it through the whole fabric of the building (I'm avoiding the terms breathable and vapour permeable on purpose here due to common misconceptions). This fabric approach means that my ventilation requirement is significantly reduced (I can provide a link to recent research showing this in real world buildings if you want). I do wonder whether this is really important for well functioning natural ventilation.

 

In terms of natural ventilation, I was dissappointed by a lack of recent good quality research on the subject specifically in domestic contexts so my starting point was The Handbook of Domestic Ventilation by Roger Edwards: https://www.taylorfrancis.com/books/mono/10.4324/9780080454580/handbook-domestic-ventilation-rodger-edwards

 

In weighing up the evidence, Edward's book comes out reasonably in favour of natural ventilation.

 

After this I found some books from the middle to end of the 1800s dealing with principles of natural ventilation including some specific figures re flow rates etc. These weresome of the best things I'd read.

 

Putting this all together including current recommended ventilation rates, I found some papers using computational fluid dynamics to model air flow through buildings and rooms (almost entirely non-domestic) and drew upon these together with my local climate to size and locate my vents and stack (stack is just for cooling). I also had to make sure they're sized in a way that keeps BC happy. I spent a few years designing motorcycle exhaust systems so although now rusty, I got a reasonably good feel for fluid dynamics basics which, once you've played with some numbers actually becomes quite intuitive instead of computational.

Edited March 21, 2023 by SimonD

[SteamyTea]

26 minutes ago, Onoff said:

 

I'm thinking directional intake and extract on simple wind vanes. So it intakes on the prevailing side, exits on the lee.

Been done, and failed if I remember correctly.

[Sparrowhawk]

6 hours 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.

 

6 minutes ago, Onoff said:

I'm thinking directional intake and extract on simple wind vanes. So it intakes on the prevailing side, exits on the lee.

 

As readers of my introductory thread will know, I live in a draughty house on a cliff top that gets pretty windy at times.

 

I've yet to see anyone in this thread address the variable nature of passive ventilation and how you ensure you get enough ventilation when it's still outside, and don't overventilate when it's windy.

 

I don't see a way to get away from a mechanical system if you want consistent ventilation.

[MikeSharp01]

It seems to me that we would all do well to recall the old corollary to the addage "if you can't measure it you can't control it" which is "Just because you can measure it it does not mean you can control it."

 

We have a passive stack plan, alongside full MVHR, for our build high level roof lights and low (ground level) windows. Although they are electrically actuated and we have sensors, all sorts everywhere, I have no idea how to control it other than bang bang control until I see the data.

[Iceverge]

51 minutes ago, SimonD said:

 

I've just taken CO2 readings and currently in the space I've been doing admin most of the day it's hovering between 650-660ppm and downstairs it's about 430ppm. PM2.5 & 10 are at good levels and HCHO is at healthy level. Having just cooked some dinner and eaten, kitchen RH is 60% and downstairs is 55%. Even though I haven't used a lot of wets in the build, RH was much higher and is still gently on its way down, month by month.

 

In terms of approach, it was mostly a research based with just a few numbers to crunch. I read a lot of material not just on ventilation but also on building fabric, finding some real world research showing the reduction in RH coming from moisture buffering materials that either simply temporarily store excess moisture and/or pass it through the whole fabric of the building (I'm avoiding the terms breathable and vapour permeable on purpose here due to common misconceptions). This fabric approach means that my ventilation requirement is significantly reduced (I can provide a link to recent research showing this in real world buildings if you want). I do wonder whether this is really important for well functioning natural ventilation.

 

In terms of natural ventilation, I was dissappointed by a lack of recent good quality research on the subject specifically in domestic contexts so my starting point was The Handbook of Domestic Ventilation by Roger Edwards: https://www.taylorfrancis.com/books/mono/10.4324/9780080454580/handbook-domestic-ventilation-rodger-edwards

 

In weighing up the evidence, Edward's book comes out reasonably in favour of natural ventilation.

 

After this I found some books from the middle to end of the 1800s dealing with principles of natural ventilation including some specific figures re flow rates etc. These weresome of the best things I'd read.

 

Putting this all together including current recommended ventilation rates, I found some papers using computational fluid dynamics to model air flow through buildings and rooms (almost entirely non-domestic) an drew upon these together with my local climate to size and locate my vents and stack (stack is just for cooling). I also had to make sure they're sized in a way that keeps BC happy. I spent a few years designing motorcycle exhaust systems so although now rusty, I got a reasonably good feel for fluid dynamics basics which, once you've played with some numbers actually becomes quite intuitive instead of computational.

 

Very interesting. A couple of questions if you don't mind. 

 

What is your wall buildup?

What is the interior volume and occupancy rates?

Did you use bog standard trickle vents or something else?

 

 

[Oz07]

13 hours 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?

I got a good one off amazon for 62 quid the other day. Its the tfa dostmann one which logs previous readings for 12 hrs. I think the list is around 80 quid I paid 62. Interesting to take round the house with you and see what the bedrooms say in the morning but you do become a bit of slave to it. Where did you get your mvhr unit for 500 that seems cheap

[JohnMo]

I have two units a big and small one, the big one was £320 and the small £250.  Both from eBay, retail price circa £4k for both from Titon.

[MJNewton]

9 minutes ago, JohnMo said:

I have two units a big and small one, the big one was £320 and the small £250.  Both from eBay, retail price circa £4k for both from Titon.

 

Our is a Titon and was from ebay too (£350). One of many fitted to a development that turned out to not match the spec so they were all replaced with the next model up having only been commissioned. Came in the box of the next model up with the latter's mounting hardware etc still all sealed so the story seemed to check out. 

 

I wouldnt normally factor such a bargain into discussions of value for money were it not fir the fact that it seems quite common to buy new/as-new units off eBay so it's a buying strategy I would recommend if you have the time to wait for one to turn up. 

[ProDave]

46 minutes ago, Oz07 said:

Where did you get your mvhr unit for 500 that seems cheap

It was at the time Kingspan had a little forray into the market and had re badged the Misubishi Lossnay units. They had abandoned that idea and the Kingspan units were coming up on ebay.  I think i paid £400 for the unit and £100 to get it delivered.

[Adsibob]

On 21/03/2023 at 11:28, SimonD said:

I have an upside down house that has an 18C downstairs and cooler which is better for sleeping, whereas the living area is 21C. It's almost impossible to zone in this way with MVHR due to the way the system distributes air throughout the house.

I'm not sure about this logic. MVHR will not equalise temperatures within a house, because flow rates are too low. It might make warmer parts of the house, a little cooler, I estimate 0.5C or so cooler, and colder parts of the house a little warmer, I estimate 0.5C warmer, but it's unlikely to move the needle much beyond that unless you ran the ventilation rates on max all day, and nobody would do that because it would be noisy.
Ultimately the purpose of MVHR is to constantly ventilate a house with fresh filtered air, without losing the heat that is already in the house to the outside. So air of varying temperatures is extracted (say 22C plus from bathrooms and kitchens) and that heat is transferred onto the incoming air which gets distributed to the rest of the house, but not in sufficient volumes to really increase the temp by much more than 0.5C or so.  

[saveasteading]

17 hours ago, Iceverge said:

Did you take any measurements of air quality post construction

No. But I spoke to many teachers  6 or 12 months later, and they all said that it was between good air quality and  the best they had worked in.

I expect that heads, bursers and PMs also visited our previous projects before awarding us their projects. 

 

On the other extreme, our preferred m&e contractor tokd me of new county schools jampacked with computerised ventilation kit. Windows and fans were linked to heating and heat recovery and sensors to a central system. They had been called in because it wasn't working. The janitor explained that the best device was the red button in the plant room, which turned it all to manual.

 

In hindsight a meter would have been interesting, used surrepticiusly. But our noses are fairly accurately programmed.

[SimonD]

2 hours ago, Adsibob said:

I'm not sure about this logic. MVHR will not equalise temperatures within a house, because flow rates are too low. It might make warmer parts of the house, a little cooler, I estimate 0.5C or so cooler, and colder parts of the house a little warmer, I estimate 0.5C warmer, but it's unlikely to move the needle much beyond that unless you ran the ventilation rates on max all day, and nobody would do that because it would be noisy.
Ultimately the purpose of MVHR is to constantly ventilate a house with fresh filtered air, without losing the heat that is already in the house to the outside. So air of varying temperatures is extracted (say 22C plus from bathrooms and kitchens) and that heat is transferred onto the incoming air which gets distributed to the rest of the house, but not in sufficient volumes to really increase the temp by much more than 0.5C or so.  

 

But it isn't logic, it's based upon research and models used to design MVHR, especially in low energy houses like Passivhaus - see for example PHPP. Here is some research with plenty of references regarding variations in temperature if you want to read. As always the picture is more complex than much of the argument on this thread:

 

Quote

The Passivhaus standard suggests assuming an internal temperature of 21 °C and to a certain degree the use of MVHR evens out the temperatures of different rooms providing uniform temperatures throughout the building. Variations in the temperature may be expected if the auxiliary heating system is localized such as with a wood stove. Only minimal variations can be expected where a heating coil is included in the supply air of the MVHR. Two aspects are to be questioned. The first is the set temperature of 21 °C and the second the uniformity of the temperature throughout the building.

A temperature of 21 °C does not always provide the most comfortable environment. Research into thermal comfort supports the view that occupants of free-running buildings experience a comfort band, which relates to external temperatures and is wider than that experienced in mechanically ventilated buildings [15,16,17,18,19,20]. Field studies by Humphreys and Nicol [19] showed a linear relation between indoor temperatures and comfort temperature, whereby the latter varied from around 17 °C to 32 °C and changed through the year as external temperatures changed. Furthermore, Humphrey (discussed in [19]) showed a linear relation in free-running buildings between the external temperature and the internal comfort temperature while the relation in conditioned buildings was more complex and non linear. In particular, in free-running buildings the comfort temperature rose in a linear fashion from approximately 17 °C where the external temperature was approximately 13 °C to a comfort temperature of approximately 29 °C where the external temperature was approximately 32 °C. The impact of the external temperatures on perceived comfort has to some degree also been incorporated in design standards such as the ASHRAE Standard 55 [21], which differentiates between winter and summer settings.

Furthermore, highly insulated buildings typically have a building fabric with a warmer surface temperature than poorly insulated buildings. This reduces the radiant cooling effect experienced in less insulted buildings. Taking into account that radiant heat losses can make up 45% of the body heat exchanged with the environment, lower air temperature can feel comfortable in such environments [22,23].

It is also understood that comfort depends on the perceived and actual ability to adapt oneself and the environment to become more comfortable [16]. Baker and Standeven [15] identify how the ability to open windows, draw a blind, use a fan as well as change their clothing increases the level of comfort. Baker [24] further identified that even if the alterations to make a space more comfortable are not implemented, the fact that they could be implemented increases the tolerance to the environment. Humphreys and Nicol [19] suggest where the ability to adapt does not exist, such as in conditioned buildings, the comfort band may be as narrow as ±2 °C, and as the ability to adapt increases so does the comfort zone. Brager and de Dear [17] go further to suggest that a variety of temperatures is in fact preferred by the occupants.

This research suggests that highly insulated and reasonably airtight but naturally ventilated buildings where occupants are in control of their environment and are able to make adjustments to maintain comfort would not need to achieve a fixed temperature of 21 °C to feel comfortable and occupants could tolerate lower and higher temperatures.

A uniform temperature in all rooms is also arguably not desirable [18,25]. Heschong’s book Thermal Delight in Architecture [25] describes the variations in temperature within buildings as enriching the building experience. The CIBSE Guide B1 Heating [26] recommends higher temperatures in living areas and lower in sleeping areas. In particular the recommended temperatures are as follows:

• Bathrooms 26–27 °C;
• Bedrooms 17–19 °C;
• Hall stairs landing 19–24 °C;
• Kitchen 17–19 °C;
• Living rooms 20–23 °C;
• Toilets 19–21 °C.

The CIBSE range of temperatures suggests the potential for up to six degrees difference between the main rooms. Temperature variations between rooms can therefore not be considered detrimental to comfort.

 

Quote

While the Passivhaus model is a robust model for ultra-low energy building in cold continental climates, the post-occupancy evaluation of a building in the mild southern UK climate designed to Passivhaus standards was shown to perform more efficiently and without compromising comfort without MVHR. This study also identified that research in field of thermal comfort supports the premise that thermal comfort can be achieved with non-uniform temperatures of 16–21 °C in a building as well as the uniform temperature of 20–21 °C as required by the Passivhaus model and that indeed variable temperatures are preferred by certain individuals. Furthermore, naturally ventilated buildings allow for user control which is also understood to enhance user comfort. Therefore, a naturally ventilated building in a mild climate is as appropriate a building solution, if not more appropriate, than a building solution with MVHR.

 

Link: https://www.mdpi.com/2075-5309/3/1/61

[Adsibob]

7 minutes ago, SimonD said:

 

But it isn't logic, it's based upon research and models used to design MVHR, especially in low energy houses like Passivhaus - see for example PHPP. Here is some research with plenty of references regarding variations in temperature if you want to read. As always the picture is more complex than much of the argument on this thread:

 

 

 

Link: https://www.mdpi.com/2075-5309/3/1/61

I don't think what I said is contradicted by what you've quoted. I said that MVHR won't equalise the temperature across a building. It will increase the temperature of the colder rooms marginally and decrease the temperature of the warmer warms marginally. I estimated the margin to be about 0.5C.

The material you've quoted says:

 

"The Passivhaus standard suggests assuming an internal temperature of 21 °C and to a certain degree the use of MVHR evens out the temperatures of different rooms providing uniform temperatures throughout the building." (my bold emphasis)

 

Ultimately, it's horses for courses. My house is not close to being passiv standard. We have insulated a 1930s semi as well as we could, upgraded the glazing to 2G and installed 3G rooflights. We have a very efficient MVHR system and there is a 3 degree variance between the coldest room in the house and the warmest (at the moment). I imagine a better insulated house with 3G windows might have less variance. 

[ToughButterCup]

4 hours ago, saveasteading said:

No. But I spoke to many teachers  6 or 12 months later, and they all said that it was between good air quality and  the best they had worked in.

....

 

I have supervised undergrad teaching students in classrooms of thirty or more children (+1 teacher and 2 teaching assistants - ie. nearly 40 people in one room) where - in winter - the classroom teacher refused to open even one window : because she was cold.  

 

You could chew the atmospheric fugg in the classroom.

 

On to the next classroom: male teacher just as many children, most of the small windows open, on the same day.

 

"If they're cold, they can wear a sweater if they want. By the way, he said, one child this age is the equivalent of a one bar fire"

[SteamyTea]

9 minutes ago, ToughButterCup said:

 

I have supervised undergrad teaching students in classrooms of thirty or more children (+1 teacher and 2 teaching assistants - ie. nearly 40 people in one room

I can top that.

I went to boarding school.

16 14 year old boys in one dormitory.

An interesting smell.

 

Also.

Try 30 16 year old school failures who wanted to become .com millionaires in a room, with 30 200W PCs going.

Not one of my students ever felt drowsy.

 

12 plumbing apprentices learning ITC, no problem, till the girls doing sport started playing soccer right outside.

Only one student took any notice then, and I can say I did let my eyes wonder.

 

[Furnace]

2 minutes ago, SteamyTea said:

and I can say I did let my eyes wonder.

I reckon your eyes wandered, and your mind wondered....

[SteamyTea]

5 minutes ago, Furnace said:

I reckon your eyes wandered, and your mind wondered....

Not as much as my imagination.

 

[jack]

43 minutes ago, ToughButterCup said:

I have supervised undergrad teaching students in classrooms of thirty or more children (+1 teacher and 2 teaching assistants - ie. nearly 40 people in one room) where - in winter - the classroom teacher refused to open even one window : because she was cold. 

The school my son went to had a strict "no coats or jackets inside" policy. Teachers would stand at the door from the outside making children take their coats off before they entered, even though the halls weren't that much warmer than outside.

 

Even during Covid when it was cold and they had the windows open, a couple of miserable fvckers still refused to allow coats or even an additional jumper inside. Of course, the teachers doing this were were standing at the front of the class in a puffy coat over a thick jumper.

 

In one class when the weather was particularly cold there was nearly a riot - some of the kids were shivering and a couple had blue lips.

 

It took a surprising amount of parental backlash to get the school to relent even temporarily.