MVHR is Largely Bogus

[ProDave]

Re mvhr "evening out" temperature.  When we light the WBS, we tend to put the MVHR onto the medium boost speed for a couple of hours in an attempt to spread some of the heat around just a little and it does seem to work, it can easily lift the temperature in the non heated rooms a degree or sometimes more in an evening.

[SimonD]

On 21/03/2023 at 21:06, Iceverge said:

 

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?

 

 

 

This is technically a retrofit so 1st floor is new build comprising a buildup of:

 

Inside
Interior timber cladding (to be fitted)
Service void
11mm OSB - airtightness taped and no additional vcl
140mm stud wall filled with Thermafleece sheepswool insulation

50mm cross battens filled with Thermafleece (this layer is meant to be on the outside of the frame)

Breather membrane
Battens and ventilated cavity

Cladding

Outside

 

Roof buildup similar but thicker insulation and thicker cross batten layer outside the joists. Plywood as ceiling finish

 

Ground floor, mostly existing except for some new extension work:

Inside
Plasterboard/skim (quite a bit still to be fitted)
Service void

Gypsum plaster parge coat for airtightness

Clay brick

Cavity - varies 35-75mm

Clay brick or sandstone (depending on which wall)
140mm Woodfibre EWI
Lime based Baumit thin coat render

Outside

 

Ground floor suspended floor buildup:

Inside

Floating timber floor

Cork underlay
TG4 18mm OSB

Joists filled with 150mm cellulose insulation

Breather membrane

Outside

Interior volume is about 500m3

 

Occupancy is family of 4. Wife works mostly from home. I work a lot from home. Boys are around pre and post school minus evening sports clubs. 1 dog.

 

No trickle vents anywhere. I've used acoustic vents through the walls so that I can have both cross ventilation through the house in two directions and have ventilation through each individual room when a window is opened. I have a centrally located staircase acting as a stack with two large tilt and turn windows giving ventilation out of the top of the house sited on the North face.

 

We have large windows on 1st floor which are North and South Facing but use a designed large roof overhang which almost completely shades the windows from direct sunlight in summer. Solar gain right now is brilliant but need to work on something for late spring and autumn as had some heat issues during those periods last year but wanted to leave it a couple of years to see how it goes over time and work something out.

 

I'm currently running with only one vent completely open in the kitchen area and had a peak of 907ppm CO2 in the TV area after everyone was watching TV for a while during the evening, now settled down to 780ppm as I write this. As the weather gets warmer I'll crack open a couple more vents. I've actually still got some to install on the ground floor.

Edited March 22, 2023 by SimonD

[SteamyTea]

@SimonD

I may have missed it, but do you know your pressure test/ACH figures?

[Sparrowhawk]

10 hours ago, SimonD said:

No trickle vents anywhere. I've used acoustic vents through the walls

Mind sharing which ones you've used as I might need them?

[Mike]

Another reason for choosing MVHR is that it may well increase the value and saleability of your properly.

 

I've just received a set of Energy Performance Certificate simulations for my current French refurb (I wanted to make sure I wasn't just missing a band boundary - I'm not).

 

However - leaving aside the (correct) argument that EPCs don't always match reality - without MVHR I'd have been in the middle of band D; with it I'll be in the middle of band C, with no easy upgrade to B.

 

In France, at least, having a poor EPC (E, F or G) can knock tens of thousands off the property value, due to laws that will make them illegal to rent (G from 2025, F from 2028, E from 2034). F and G are already selling at a discount of between 10 and 20%, while A and B sell for a premium (article, in French).

 

Edited March 23, 2023 by Mike

[SimonD]

18 hours ago, Adsibob said:

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. 

 

I personally don't give two hoots about whether someone chooses to install MVHR in a development and everybody is different in terms of what they desire and feel comfortable with in terms of temperature variation throughout their house. I respect your experience of your home, but what troubles me is the use of limited data to come to conclusions that are then stated as fact about how these systems work more generally, without backup from larger scale research. This often feeds into what I think is becoming a bit of a MVHR evangelism that assumes MVHR is the only benign solution whereas it's actually more of a marketplace with vendors trying to sell us more technical kit which may often be unnecessary and costly over the long term, - this is even the case with the utterly poor paper published by the passivhaus trust which @Kelvinlinked to earlier in the thread.

 

So forgive me for a minute if I geek out for a while 😉

 

If you look at temperature variation in the Chartered Institute of Building Services Engineers, for example, the indoor temperature range between rooms can be as much a 10C. As I quoted above it looks like this:

 

• 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

 

Whatever your beliefs about the extent of temperature equalization in homes, the wider research shows that this kind of variation cannot be provided by MVHR systems and this is found to be an issue for occupant comfort (and control). So, in addition to the limited phrase you chose to hang your coat on in the research I quoted above it says:

 

"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]." (bold is my emphasis).

 

But to take some quotes from other research:

 

Title: Ventilation Performance and Hygrothermal Conditions in New-build UK Housing (link: https://core.ac.uk/reader/159923279)

 

"Significantly less variation of temperature and relative humidity levels were observed in homes with balanced mechanical ventilation with heat recovery systems (p=<0.001)"

 

Title: Thermal comfort and IAQ in super-insulated housing with natural and decentralized ventilation systems in the south of the United Kingdom. (Link: https://core.ac.uk/reader/220155974)

 

"Achieving thermal comfort in winter is as dependent on the heating system as the ventilation strategy. Decentralized systems tend to create thermal zones with different temperate within a building, while centralized systems tend to provide uniform temperatures in all rooms. "

 

Title: On the oversupply of heat to bedrooms during winter in highly insulated dwellings with heat recovery ventilation (Link: https://www.sciencedirect.com/science/article/abs/pii/S0360132316302657)

 

"The results clearly illustrates that the supply-air temperature and the temperatures in the living room and bathroom have substantial effects on the thermal conditions in the bedrooms. A one-zone MVHR solution, with approximately the same the supply-air temperature to all rooms, has clear limitations regarding the provision of thermal comfort in bedrooms."

 

With the last quote, just wait for the next iteration of MVHR which would unsurprisingly be multi-zone to squeeze even more money out of us, add yet more complexity to the system, and be much harder to commission, run and maintain properly.

 

If you're still tuned in, I could go on because I have plenty more papers I could dig out, but then it would get a bit silly, if it isn't already. 😉

 

So I'll go back to my original claim with some more specificity in that MVHR (centralised) will provide more uniform temperature through the house than a decentralised natural ventilation system.

[SteamyTea]

5 minutes ago, SimonD said:

Bathrooms 26–27 °C;

Is that when in use, or constantly?

My bathroom is similar in temperature to the rest of the upstairs, mainly because I never shut the door.

[SimonD]

5 minutes ago, SteamyTea said:

Is that when in use, or constantly?

My bathroom is similar in temperature to the rest of the upstairs, mainly because I never shut the door.

 

The CIBSE Domestic Heating Design Guide gives the environmental design temp of bathrooms as 20-22C so my assumption has been that 26-27C is an in use figure.

Edited March 23, 2023 by SimonD

[SimonD]

1 hour ago, Sparrowhawk said:

Mind sharing which ones you've used as I might need them?

 

I got mine from Rytons: https://rts.vents.co.uk/blog/products/acoustic-aircore-and-airliner-sets-background/

[SimonD]

4 hours ago, SteamyTea said:

@SimonD

I may have missed it, but do you know your pressure test/ACH figures?

 

No figures as yet. Still to organise a test as I'm really, desperately hoping to actually finish the house this year. However, given the number of hours spent with airtight foam, tapes, sealants, liquid membrane, parge coating, airtight caulk and all the other myriad of products in this space I'm secretly expecting the result to be okay, but we'll see. It's one of the little projects on my list to build my own fan and test rig, so I might soon be on here asking for some help to do this.

[Kelvin]

Impressive level of research. What’s still unclear to me is how do you achieve passive standards of airtightness and in a free running ventilated house ? 

 

As @ProDave already pointed out in Scotland anything under 3.0 ACH requires some kind of mechanical ventilation albeit as @JohnMomentioned it doesn’t need to be MVHR it’s just that there aren’t many alternatives. 
 

As @SteamyTeaasked what was your airtightness result if measured? 
 

 

[SimonD]

1 hour ago, Kelvin said:

Impressive level of research. What’s still unclear to me is how do you achieve passive standards of airtightness and in a free running ventilated house ? 

 

As @ProDave already pointed out in Scotland anything under 3.0 ACH requires some kind of mechanical ventilation albeit as @JohnMomentioned it doesn’t need to be MVHR it’s just that there aren’t many alternatives. 
 

As @SteamyTeaasked what was your airtightness result if measured? 
 

 

 

I'm certainly no designer of passivhaus, but in terms of airtightness and ventilation, you need to look at them as seperate processes (while taking into consideration the ventilation in the house design of course). With airtightness you are looking to minimise as far as possible, the uncontrolled infiltration of air into and out of the house caused by a non-airtight fabric. Once you've done this, you have a more known volume of air that needs to be exchanged within the house to keep it healthy and comfortable. There are a number of calculations that can be used to predict average volumes exchanged through natural ventilation based upon localised climate and temperature differentials between exterior and interior that can then be used to understand the size and quantity of vents required. For example, a Passive Stack Vent would have a calculated diameter based on room volume and predicted pressure differences in order to make sure it neither over or under ventilates the house.

 

Bear in mind here that because a naturally ventilated house does not have constant exchange, you need to consider the average ventilation rates, so there may be times when RH peaks above recommended levels. But this doesn't matter over short periods of time. Mitigation can then be achieved through strategic use moisture buffering materials. Another strategy is of course to supplement the natural ventilation with lets say a mechanical extract vent switched for particular peaks of RH or CO2, for example.

 

In terms of building regulations, I'm in England and with highly airtight buidings I believe that the current regulations permit the use of natural ventilation providing there is a specialist design.

 

In terms of Scotland, they seem to have been more prescriptive and shot themselves in the foot slightly I wonder. However, reading the document, it simply says that for low infiltration dwellings, mechanical systems should be used to "augment, complement, and/or improve natural ventilation." It then gives some examples of systems that can be used. In this instance, if a natural ventilation system is designed by a company specialising in these systems, then first have a discussion with building control and then secondly look to add a something mechanical.

 

In any case the Sottish regs don't appear to mandate MVHR.

[SimonD]

@Kelvin and anyone else in Scotland - a paper that might be of interest highlighting ventilation aspects that affect both MVHR and natural ventilation performance:

 

An assessment of environmental conditions in bedrooms of contemporary low energy houses in Scotland

[Onoff]

I wonder if houses in Chicago would fare any better on the natural ventilation front?

[saveasteading]

Summary of the summary. People are preferring warm bedrooms recently. They have high co2 in winter. Mvhr is sometimes appropriate but only if well designed and very easy to control.

Lots of new builds don't comply with bldg regs.

[saveasteading]

10 minutes ago, Onoff said:

Chicago

I've just come down from the Windy City reference?

How about Edinburgh?

[SteamyTea]

[Kelvin]

9 minutes ago, saveasteading said:

Summary of the summary. People are preferring warm bedrooms recently. They have high co2 in winter. Mvhr is sometimes appropriate but only if well designed and very easy to control.

Lots of new builds don't comply with bldg regs.

Summary of the summary of the summary. Poorly designed houses perform poorly. It’s all too complicated for most folk. 
 

None of the houses were particularly airtight either. 

[saveasteading]

10 minutes ago, Kelvin said:

None of the houses were particularly airtight either

I don't know about Scotland, but in England they test 1 in 10 (?) on an estate. The developer knows which they will be and they scrape a pass. The rest are porous.

These houses were all on large developments.

[Adsibob]

10 hours ago, SimonD said:

The CIBSE Domestic Heating Design Guide gives the environmental design temp of bathrooms as 20-22C so my assumption has been that 26-27C is an in use figure.

I agree with your assumption. But does this account for the following:

whilst it is obviously perfectly comfortable to shower in a 27C bathroom, it would not be comfortable to sit on the toilet in one that hot for very long. The reality of most houses with MVHR is that it is the hot water from the shower/bath that raises the temperature of the room whilst it is being used for showering/bathing and that heat is cleared by the boost on the MVHR being active for 20 or so minutes after showering/bathing or longer if boost is not used. 

 

[ProDave]

7 hours ago, SimonD said:

@Kelvin and anyone else in Scotland - a paper that might be of interest highlighting ventilation aspects that affect both MVHR and natural ventilation performance:

 

An assessment of environmental conditions in bedrooms of contemporary low energy houses in Scotland

Interesting read, I have not yet fully digested it.  but a few initial thoughts.

 

Most of the houses tested are >10 years old, hardly cutting edge, and only a few have MVHR.

There is a clear conclusion that passive or trickle ventilation is not adequate, and the houses with mvhr perform better.

There is the strange observation that most bedrooms are overheated and the notion of wanting a cool bedroom is unachievable in a well insulated air tight house, which is strange because we have been managing this since our house was built.  There really is merit in not heating the upstairs at all if a cool bedroom is your requirement.

 

[ProDave]

7 hours ago, Onoff said:

I wonder if houses in Chicago would fare any better on the natural ventilation front?

It might be the Windy city, but they would certainly have to work over a very much wider seasonal temperature range than UK housing.

[LionessHeart]

On 18/12/2019 at 20:59, DavidHughes said:

 

 

If I were to do it again?

 

No MVHR.

High quality adjustable wall vents low down in the corners of all bedrooms (the rooms which you want to be coolest).

Similar ceiling vents in living areas to let the air out.

Bathrooms with good, possibly motorised, flap valves on the extractors.

A recirculating cooker hood fan or total extract with similar good flap valve.

Chase and fix drafts religiously including floors with insulating and low air permeability underlay.

Hire a cone flow meter to see what's really happening. https://www.bsria.com/uk provide this service  for £80-140 depending on whether you collect or have it delivered.

Set the system at about 0.05-0.1 ACH

On a two storey house a duct and low power fan to circulate the air top to bottom (my last house had this and it was excellent)

If the layout of the house were amenable ( I have solid floors so no chance) I would probably attempt to draw air in over the foundations of the house to either pick up heat in winter or let go of heat in the summer.

Follow the building regs guidelines section F which do have some good rules but don't bother telling them unless you absolutely feel the need to - it's just some wall vents in a house...

 

 

Hello David!

 

We're embarking on a barn conversion project - it'll be our home in the country, so my husband and I will likely be there over weekends with the children and partners joining us possibly 3 times a year. We're seriously looking at NOT installing an MVHR - not only because of the cost, but to [a] minimise energy wastage (there'll be largely 2 occupants using the 4-bed house intermittently), and [b] keep things low-tech as much as possible. We will, however, have an ASHP, UFH, and solar PV.

 

I've read your post with keen interest. 

- We'll install wall vents and good extractor fans in bathrooms and in the kitchen, as you suggest

- But, are reluctant to bore holes into triple glazed windows for trickle vents

 

My question is, how can we achieve the 1 ach if we're not using mechanically driven ventilation? Should we consider a Positive Input ventilation (PIV) and fans (for the hot days of the summer)?

 

Thanks in advance for your input!

 

[PeterW]

1 minute ago, LionessHeart said:

 

My question is, how can we achieve the 1 ach if we're not using mechanically driven ventilation? Should we consider a Positive Input ventilation (PIV) and fans (for the hot days of the summer)?

you can’t… 

 

you have PV - so stick in MVHR, when you’re not there you’ll be using the significant generation capacity to offload the running costs (we are talking 80w/hr) and then have the comfort of ventilation and energy saving.

 

3 minutes ago, LionessHeart said:

keep things low-tech as much as possible. We will, however, have an ASHP, UFH, and solar PV

so the ASHP is probably the most technically complex thing you can install as the control systems required are more complex than a standard boiler… 

 

What is your plan for hot water ..? And what uValues are you aiming for on the overall build ..?

[LionessHeart]

@PeterW Hoping the ASHP will serve UHF and hot water (there's no Gas on site, just electricity).

 

Architect has been asked to take a fabric-first, low-energy, high-insulation approach. Hoping to have the uValues tomorrow.... will share when I have those numbers.