Posted By: WillInAberdeenTo use humidity swing to buffer heat, you need to swing the humidity in the room high to condense and low to release humidity and (minus) latent heat from the fabric. Swinging the humidity low requires over-ventilation which might waste more heat than it stores?

On a dry sunny day when you have plenty of energy available from PV, solar thermal, sun coming through the window: over-ventilate to dry out the fabric. The house will be slightly chillier than it otherwise would be as energy is being lost in evaporating the water; that energy gets carried out in the latent heat of the vapour.

Later, on a chilly dull day: under ventilate somewhat (but enough to keep the COâ‚‚ levels reasonable) so the indoor RH rises which causes condensation in the dry fabric releasing heat which helps offset the lack of energy coming from outside.

We hardly bother about washing & bathing in terms of ventilation rates - well except that I turn it down to trickle (50 m³/hr) when I take a shower in the winter to minimise drafts. The exact opposite of the normal advice.

It's not generally a good idea to open a window when the building doesn't need heating. The reason the building doesn't need heating is exactly because the windows are closed and the building is airtight and the heat is being recovered from the ventilation air. It only makes sense to open windows much closer to summer, and currently it makes sense to close them again on sunny summer days.

CIBSE Guide B, which I trust a bit more than Building Regs, suggests that 5 l/s of fresh air per person provides low IAQ. That's 18 m³/hr per person, so they would suggest 90 m³/hr as a base for five people. I tend to use their 'moderate' 8 l/s or 28.8 m³/hr, which I round to 30 m³/hr, as a target for the average ventilation rate unless I have an overriding concern like enabling the post-heater to operate. So for two of us the 50 m³/hr trickle setting is just too small and I alternate it with the 125 m³/hr setting that is the next setting up. 125 m³/hr is just about right for four people in my estimation. Both of those settings are pretty low power and no noise.

I have four settings on my MVHR:

0 = 50 m³/hr - supposedly for when the building is unoccupied, set by the manufacturer
1 = 125 m³/hr - set to conform with the requirements of the Compliance Guide and not changed since commissioning
2 = 165 m³/hr - set to conform with the requirements of the Compliance Guide boost rate and not changed since commissioning - used occasionally
3 = 225 m³/hr - the manufacturer's default boost rate and never changed. Never used and it is definitely audible.

The actual rates of all four settings can be adjusted from 0 to the maximum 300 m³/hr that the unit is capable of but I haven't seen any reason to change any rates after commissioning.

Building Regs set the rates that the MVHR must be capable of meeting AIUI and the Compliance Guide provides intermediate settings. Neither imply what rates you must use. The ventilation rates are designed based on nominal floor areas per person and hence tend to be somewhat higher than many people need.

@Tony, do you adjust your fans' speed down in cold weather and up again in warm weather, or leave them?

@DJH, the CIBSE 8l/s rate you mentioned, IIRC, is applicable for rooms such as cinemas where you might be seated close to lots of strangers. It's for control of 'bodily odours'. If you are using it for a house, you are probably over ventilating, unless you have particularly smelly teenagers at home.... You have mentioned very low RH which suggests that too. Per the example numbers I gave, you need to halve the ventilation rate as you go into winter and then double it again in spring, not leave on the system defaults.

@Mike, the point is that ventilation rates need to be adjusted depending on outside weather/humidity, not just on indoor activities. Push the boost button for all of Spring and Autumn! Not saying that passive systems are any better controlled btw, just bemoaning that most of the MHRV systems on the market lend themselves to users who keep them at the same too-high rate all winter, not adjusting them as you would do with an opening window.

Perhaps I should get round to fitting my Mitsubish Lossnay that's been sitting in my attic for last 5 years!

How important is air tightness in a mid terraced house? All my chimneys are currently vented. I guess they need sealing off for a start

Vic, I seal up chimneys regularly especially if on internal walls

If HVHR is balanced not too much problem with poor air tightness

I appear to have found a bug in the forum software in that I can't directly quote from posts on an earlier page!! Which makes answering individually somewhat challening I'll try to respond to as many of the questions about my approach as possible.

GreenPaddy,

Thanks for the well wishes, I certainly hope it'll turn out to work!

djh,

I plan to achieve the temperature differential across the house mainly through simple thermostatic control and localised ventilation. I'll have an upside down house thus benefiting from stratification across the whole of the downstairs, the main bathroom probably being the one exception. Given that I'm going to end up with more insulation within the intermediate floor structural than any house I've so far lived in within the UK has had within its external fabric, the stratification should be fairly reliable. There is also a stack effect within the stairwell to the upstairs.

In terms of having a backup plan, I've considered that but won't be going for installing a load of ducting just in case. I've got enough on my plate just building the house and the natural ventilation I've installed so far has been on the conservative side so I have several further options available to me to test.

Mike1,

Re throwing heat away - I think this is only one way of looking at it and doesn't paint the full picture of the pros and cons of the different systems. Taking a wider view there is significant embodied energy within the MVHR and it has a constant energy draw, which, depending on the source of this energy may, again from a wider environmental perspective, reduce its 'environmental COP (ECOP)' as used by Rodger Edwards in his book on domestic ventilation. The study I linked to above indicates that in some instances a passivhaus with natural ventilation may use less overall energy that one using MVHR.

To me what is important is overall energy consumption of a house, not just the efficiency of the heat exchanger to provide heat recovery.

The reality is also that everyone lives and behaves differently in their houses. I have two young boys and I can't count the number of times they just walk out a door, or open a window and leave the bleedin' thing like that. With that goes a massive amount of lovely preheated air. Both my wife and I are guilty of doing similar things sometimes when just popping out with a handful of recycling.

Re windows - I am not having trickle vents cut into my windows, that to me is a sure fire way to wreck the overall window u-value and expose us to unwanted noise.

For me, there are many critical things missing from the research about the various ventilatiojn options (at least the stuff I can get my hands on because I don't currently have access through a university or similar). One of these is the fabric build-up of the house. I went to see a passivhaus being built near me where they'd used sprayfoam insulation across the entire build. This, like a pir insulation, will behave very differently compared to a vapour permeable and hygroscopic fabric. And this will, in itself have a bearing, intuitively I guess a significant one, on the way in which occupants ventilate their home and thus on enrhgy consumption over time.

The other is climate. This has a huge effect on the house, its performance, and occupier comfort, yet in many ways, houses are designed and built as if they're hermitically sealed with little regard if any to the local environment. I think it's a bit mad that we should be trying to build uniformly designed houses across different climatic zones according the u-value and airtightness. It would be far more sensible to take a wider view of this. The Swedish BBR provides (or at least still did a few years ago) for an approach based on energy use that is coupled to the climatic zone and the type of heating system being installed.

There is also the approach of balancing how far we really need to go with all this. A Swedish critic of Passivhaus has suggested that the whole of life energy consumption of a passivhaus insulated to a depth of 490mm compared to a non passivhaus one at 290mm gives less than a 2% benefit.

To get back to ventilation, I think what makes natural ventilation more challenging than MVHR is that the building does need to be designed with all this in mind, but also that the local microclimate and wider regional climate needs to be taken into consideration.

We're somewhat lucky that the position of the old house sits on a way that assists us. We're on the side of a hill and we get a regular, almost constant, breeze coming down the hill. We're sheltered in the terms of prevailing wind. I've therefore decided that given our location it is ideal to use cross-ventilation but with a stack backup which utilises very tall windows that go right up to the top of the highest point in our house and have a tilt mechanism at the top.

I've got room for manouvre in this and hope it works, but overall I don't think this will cause a significant impact on our energy use compared to putting in an MVHR and of course we then also avoid the various downsides to MVHR systems.

We'll see....

Posted By: RobLOur Sentinel Kinetic automatically turns up to control RH, and we added an analogue 0-10v CO2 monitor too. The MVHR just gets on with it, up and down as needed. We used to have a gas hob - using it would make the mvhr turn up. The CO2 thing we added was about £100, ran off 24v, so wired in direct to the mvhr unit. I recommend them, it should be in the mvhr - they’re expensive enough!

Outside air varies lots in RH during the day and the year. CO2 is constant outside at almost 400ppm, and is therefore a much better measure for air fresh/staleness. The correlation between RH and CO2 is practically absent, at least in the data gathered from a bedroom sensor in my house:
shorturl.at/nwzE4

I agree that CO2 control should be integrated with MVHR units. For my unit the CO2 sensor add-on was horrendously expensive, so I hope to DIY the tie in of my own sensor with the MVHR controls later.

Posted By: djh

Posted By: bhommelsOutside air varies lots in RH during the day and the year. CO2 is constant outside at almost 400ppm, and is therefore a much better measure for air fresh/staleness.

That's because RH does not measure a physical quantity. If instead you refer to absolute humidity, you'll find there is a much better correlation, since a change in absolute humidity actually requires molecules to move to another place whereas a change in relative humidity does not.

The main factor that causes the regular diurnal changes in external RH is the temperature. Hopefully the temperature in your house is more stable and consequently the RH inside will be more stable too. Control of inside RH is actually a much better measure than either control versus external RH or control of CO2 levels, neither of which matter much in a real sense in domestic circumstances. Internal RH however is quite important in determining the growth of mould etc at one extreme and creating an uncomfortably dry atmosphere at the other end of the scale.

I agree that maintaining RH in a certain range is essential for a healthy indoor climate. As you indicate, humidity can vary quite a bit beyond diurnal timescales. I never meant to say that external RH could be used to control a MVHR btw.

I am not sure what you mean by "...of CO2 levels, neither of which matter much in a real sense in domestic circumstances". CO2 ppm needs to be kept below certain levels to maintain a healthy indoor climate, no matter what the RH is, so CO2 sensors are excellent input to MVHR controls, and deal with occupancy much better than just RH as there is hardly any correlation between the two.

No, CO2 levels are about the least important criterion for fresh air. There is a very wide tolerance in levels and when they do get too high they produce a noticeable stuffiness that people respond to.

Right, so you rely on people feeling stuffy to turn up the MVHR?

Posted By: Ed Davies

Posted By: bhommelsRight, so you rely on people feeling stuffy to turn up the MVHR?

No. AIUI, if you keep the indoor RH well within the comfort/safe range then the COâ‚‚ won't be much of a problem - i.e., almost always humidity is the pacing factor which determines the amount of ventilation required. So, you're not relying on people feeling stuffy, you're relying on the humidistat to turn up the MVHR.

This is what I tried to illustrate with a RH/CO2 correlation plot (which I might not have shared correctly)
https://drive.google.com/file/d/1cA0zMAgMxSpQm2cYuV7XT0j-RMSUtPl7/view?usp=sharing
High CO2 coupled with low humidity does occur, so an humidistat alone will not cut it.

Since the plot is compiled using data from March to October this year, the low RH area is not very densely populated, which will probably change once I have a winter worth of data.

BTW I have the feeling our experiences differ from the number of house m3/occupant available.

Posted By: SimonDI went to see a passivhaus being built near me where they'd used sprayfoam insulation across the entire build. This, like a pir insulation, will behave very differently compared to a vapour permeable and hygroscopic fabric.

There are different types of spray foam, some vapour permeable, some not. Our house is insulated with 350mm Icynene which is vapour permeable and not like PIR insulation at all.

Posted By: PeterStarckOur house is insulated with 350mm Icynene which is vapour permeable and not like PIR insulation at all.

and is largely vegetable based, not petroleum. Good stuff.

Posted By: djh

I'm not sure it would make much difference. Yes, sure, the humidity in a hygroscopic house will behave differently to the humidity in a 'plastic' house but hygroscopicity doesn't affect other pollutants.

I understand that, my point was more to do with how human perception of and relationship with the environment will affect behaviour, consciously or unconsciously, and thus impact on overall energy consumption of the house.

Due to my background in psychology I have a particular interest in ecological psychology which is essentially about how people relate to their environment. I've only in recent years really gotten into the construction industry and it has driven me potty to read about building products and systems only to find that context (social and environmental) is so commonly overlooked and/or ignored. (yes, they do this a lot in psychology too)

For example, in a study I read about MVHR the conclusions noted that in a majority of test houses they found RH levels below that recommended in bedrooms at night, finding that occupants were opening their windows. To me this is an indication that there is something wrong with the MVHR system and the environment it helps to create rather than deviant occupant behaviour that needs to be trained and modified! (Please excuse the gross generalisations ) I'd also note here that given the RH levels were really low, this did not appear, in itself, to effect occupier comfort, probably the opposite by the looks of it.

For both health and comfort, I believe an environment needs to be variable, not uniform and consistent which it appears the many MVHR designs are attempting to achieve.

Posted By: djh

houses are designed and built as if they're hermitically sealed with little regard if any to the local environment. I think it's a bit mad that we should be trying to build uniformly designed houses across different climatic zones according the u-value and airtightness.

I'm not sure I understand this. Are you referring to Building Regs? I don't think there are many on here that think they're adequate or especially well-designed. And most people here have built or are planning to build to better standards.

No, I'm referring to a tendency to design & build houses in the same way regardless of the environment within which they are going to be placed and how they're likely to be used. Accepting of course that there are outliers and those that do design houses according to their intended location and likely use, probably many of whom are on this forum

For example, from a study I read, airtightness in a timber frame building is likely to vary across the seasons, with airtightness reducing during the winter months. Now, in a cold climate, snow load will reduce this loss of airtightness of the building due to its load on the building, but in countries like ours we don't have that environmental benefit . This comes down to understanding how our buildings behave dynamically through the seasons and lifetime and thus designing and building them accordingly - or just ignoring it as not relevant and too much hard work (I'm aware of products, like some airtight membranes that do indeed adapt according to the environment so clearly there is a commercial movement in this direction, but maybe not a sufficent wider understanding, including mine at this moment in time although I am learning).

Apart from how much I love to learn new things, these are some of the reasons I took over the detailed design of our house from our architect. He had a great eye for design, but this knowledge about the function of the house was just terrible.

Rant over

300mm Warmcell walls and ceilings. MVHR installed in order to get a C rating, but not actually wanted.


Have lived for 9 years in our new house. MVHR off. Bedroom and bathroom windows on a one click ajar setting. 150mm hole in suspended floor to allow MORE outside air in (front room). Can get a bit damp in bedroom (lower ground floor) in winter, (2 peeps, 3 dogs) but window opened more overnight.

Sometimes a bit too dry in winter with high pressure dry air from Russia.

No mould, no feeling unwell.

5kWh wood burner heats whole small house, expect this draughty system costs an extra log a day on average.

Posted By: SimonDFor example, in a study I read

It'd be nice if you provided links to these studies you mention so other people can refer to them.

Posted By: djh

I'm not sure it would make much difference. Yes, sure, the humidity in a hygroscopic house will behave differently to the humidity in a 'plastic' house but hygroscopicity doesn't affect other pollutants.

I understand that, my point was more to do with how human perception of and relationship with the environment will affect behaviour, consciously or unconsciously, and thus impact on overall energy consumption of the house.

I'm not sure I understand that, so again a link to some explanatory material would help.

No, I'm referring to a tendency to design & build houses in the same way regardless of the environment within which they are going to be placed and how they're likely to be used

It depends what you're talking about. Obviously designers tend to use techniques and materials that builders and regulators are familiar with, and reuse parts of designs where they can to a greater extent than I and many others would prefer (as a matter of cost-reduction). But all the planning applications I've seen recently of any scale have talked about orientation with regard to solar input and street design and so forth. Some are better designed than others but that is inevitable.

airtightness in a timber frame building is likely to vary across the seasons, with airtightness reducing during the winter months

I don't see why that would be the case, given that the timber frame doesn't contribute to the airtightness? In general, timber does move over time with changes in temperature and humidity contributing to it, so it is important to detail the house to allow for that continuing movement. Much like there are movement joints in large masonry structures, but necessary also on smaller scale buildings. I'm sure there are some timber buildings that have not been designed correctly, but you can't blame the material for the designer's ignorance.

This comes down to understanding how our buildings behave dynamically through the seasons and lifetime and thus designing and building them accordingly - or just ignoring it as not relevant and too much hard work

This all depends on the building techniques that are used and again its clear there are preferences to use techniques and materials that require less effort and thus cost, because cost is important and so is reliability. I live in a straw bale house so I'm prepared to take on the necessity of learning all its foibles and I took care to find an architect who was also willing to learn about the requirements as well as being a certified passivhaus designer and having good client references.