VMC: the pros and cons, advantages and disadvantages

[Christophe]

A debate on the interest of VMC DF was made here in this other topic from here: https://www.econologie.com/forums/post211649.html#211649

The only thermal advantage of the VMC DF is to be able to redistribute the heat losses throughout the entire dwelling.

Thus the calories lost in the kitchen (cooking, fridge ...) can be redistributed in the rooms (provided not to abuse the host ...), ditto for losses in the living room (lighting) ... under conditions to place the insufflation / extraction vents judiciously obviously ...

The VMC DB are too often presented on TV, by mediocre journalists, as a means of "heating" in its own right, it is obviously wrong ...

After that, I am not at all certain that a DB CMV on the sole criterion of the primary heating energy (a CMV runs 24/24 or almost) is really interesting ... For health ditto: the exchangers can become "shit nests" if not maintained regularly ...

A well-designed house made with breathable materials (wood, natural insulation, aerated concrete ...), i.e. the reverse of what is done in 95% of cases in new construction (concrete block + polystyrene + waterproof rain / vapor barrier = air tightness = VMC compulsory) strictly does not need a CMV... but the law requires it (at least nine) ...

Example at my place (ytong house, no VMC): it is currently raining but there is currently only 56% humidity in the living room (for 22 ° C) ... when the weather is nice it can drop to 40% ...

The garage, which is made of uncoated ytong (cellular concrete), is the driest room in the house: unheated, it can drop to 30-35% humidity in summer ... very good for drying wood!

[dedeleco]

This affirmation :

The only thermal advantage of the VMC DF is to be able to redistribute the heat losses throughout the entire dwelling.

is only valid for very rare well-designed houses that breathe through their walls, neither too little nor too much:

A well-designed house made with breathable materials (wood, natural insulation, aerated concrete ...)

If not for almost all poorly designed houses with cinder block and double glazing, or even new to 2012 standards, completely waterproof, like a thermos bottle, the VMC is used to make it breathable correctly and the double flow to save heating heat, a loss which can be significant, if the CMV is either excessive or strong to evacuate the water vapor, if the inhabitants give off a lot of water vapor, by their number (1 to 2 liters of water per day and inhabitant), by baths or showers very numerous (not far from ten liters per bath) or per kitchen.
So the vmc is essential not to be in a humid sauna and see everything go moldy.
If the CMV is poorly designed or adjusted, a money pump for maintenance, it can on the contrary make the air too dry, which is sometimes unbearable as the middle note:

Hello,
An office colleague built his house two years ago.
She is super isolated.
It has double flow ventilation.
Result of the races, he complains of a drought not possible in his house. (Sore throats)
Suddenly, he cuts this thing in summer and suffers in winter.
In addition, he also complains about the compulsory and necessary maintenance of this equipment.

https://www.econologie.com/forums/post211724.html#211724

In my opinion his installer makes fun of him, taking him for a cash cow to charge, by poorly adjusting the VMC which must regulate the humidity at the natural value in France from 50 to 75% and not to 30% ( worse than in the Sahara which has more humidity) if the outside air at 0 ° C (or even below) is heated to 20 ° C and fills the house thoroughly with the box of dehydrated air !!
The VMC must have a flow rate regulated automatically by the actual humidity and not a fixed speed (otherwise it is an absurdity).
Or, more complex, rehydrate the incoming air.
On the contrary, the installer has all the more maintenance work for him to fill his pockets, as he runs the CMV thoroughly by mocking the sore throats of his customers !!!

Typical of some pros, where you never know if it's incompetence or bad faith !!!

[dhaulagiri]

Hello,

I relaunch this subject that I find interesting to share my experience and my difficulties, which seem to me unusual if I believe the reduced number of messages that address it.

The context is that of a "classic" house I think of 30 years ago: breeze blocks, glass wool and hollow brick. When I bought a year ago, no double glazing, very damaged joinery and a VMC installed anyhow with a single extractor in the bathroom and an uninsulated duct to connect it to a box whose unused connections had been sealed with duct tape ...

Having moved in at the heart of winter, the first months were difficult: condensation everywhere (windows and corners of the walls) and the appearance of mold in the corners and even under a mattress, while we aired the room daily. Sensation of humidity, therefore of cold, as was mentioned in this subject.

I replaced the windows and installed a VMC very quickly. It was planned anyway from the start, regardless of our bad experience of the first months, which nevertheless allowed me to refine my project. For carpentry, I took the best I could have given my budget. So I must have something fairly common: PVC, low-emission double glazing. For the VMC, I opted for a humidity-sensitive model with low power consumption (by negotiating the price!) With the intention, from the start, of installing a Hygro B system (extractors AND hydroregulable air inlets).

The opening of the extractors is regulated by means of an equipment which expands / retracts according to the humidity (a bit like a bimetallic strip). In the kitchen, a maximum opening is controlled by a push button. In the toilets, same thing but with a presence detector. The electric impulse is given by a 9V battery. Since it only serves this purpose, I think their lifespan is great.
The connection to the box is made with insulated ducts. I still have to install the sheath which evacuates the air extracted from the box towards the outside. For the moment, it is going to the roof.

I did not choose a VDF, because of its price and especially the maintenance. I believe that the cost and the constraint are minimized. In addition, the installers that my father rubs shoulders with on construction sites (he works in construction) told him that many people were disappointed after the fact. what I read above goes in the same direction.

The Hygro suits me well, too bad for the calories lost. I think we have to stop bothering with this, especially when we live in a house designed 30 years ago, at another time. It would never occur to anyone to lock their house in three layers of plastic bags to make it completely waterproof. So, while waiting to build one day perhaps the passive and bioclimatic house of my dreams, I renew the indoor air by heating a little more (even as the air is less humid, we feel less cold at equal temperature)

I now have to finish the installation by installing humidity sensitive air inlets on the windows. I had anticipated the drilling of slots in the necessary places (all parts without extractors). What I had not anticipated is that the window manufacturer and its profile supplier (a company with global reach ...) would make non-standardized slots, on which the CMV manufacturers rely to size their facilities. I noticed this precisely while preparing my purchase. So this is a problem that is rarely addressed on the net: who has already checked the size of the air inlets? This is fundamental for the renewal of the air in the house: if the module is insufficient, the air is not renewed enough and the premises can be depressed. The system is therefore not optimal.

In my case, the slots were significantly too small. So I asked her to go back to the factory to have them enlarged, because drilling on site is prohibited. No luck, the machines are not calibrated to make standard slits! Worse, the profiles are not provided for either, while the company prides itself on having an assent from CSTB (it must relate to another aspect of the profile). The carpenter, who couldn't get over it, told me that the quest for maximum insulation performance had probably led to more insulating chambers in the PVC profiles and to reducing the space available for drilling the slots. I replied that a large company like the one that manufactured my profiles had to employ enough intelligent people so as not to ignore a standard that a Bedouin like me had found in 5 minutes on the net ... So, the attempt to enlargement failed, I asked for the replacement of my doors. My hygro B CMV will wait a little longer ...

[dedeleco]

Interesting, typical problem of most French people !!
what I notice is that from one family to another the release of water vapor is very variable, from one to another in the same accommodation the humidity problem changes completely !!

How many are you in which area and how many baths and showers per day and per week, cooking how ???

Probably in your place I would have solved the problem differently.

My first reaction is to reduce the sources of water and humidity !!!!

One of my sons with his wife had a humidity problem thinking it was raining at home !!
His wife had the hobby to take more than 3 baths per day and therefore condensation by tens of liters on the thermal bridges !!
Short-term solution of a fridge dehumidifier (consumes less than the CMV impossible to change in an apartment) and above all, in the long term, change bathroom habits !!!!

Another experience, in one of my houses in 1974 like yours, concrete blocks, bricks, thermal bridges, without VMC, ventilated by leaks and ventilations of the time adjustable manually, I had no humidity problem at all !! !
Then, a rainy November 15 years ago, persistent musty odors and incomprehensible mold in our room !!
A month to undergo to find the cause: a very large down and washed poorly dried, dry on the surface, and stored in the closet, but full of water that it released enough to make everything rot!
Not easy to find !!!
After removing this source of water, no more problems !!

Also, you can have a weak invisible source of water, much less than a liter per day per room, which provides you with appalling humidity, especially on thermal bridges !!
It may be among the others that I have already experienced:
A leak on a weakened bathtub seal almost invisible because the water evaporates without clearly wetting a paper !!! (lived 35 years ago !!!

Leak of cracked evacuation pipe hidden behind a wall lining, invisible, but very effective to make everything rot elsewhere !!! - (case of my parents, who did not realize and breathed the hidden mold spores for years !!)

A leak of cracked tile on the roof which flows little, invisible, but soaks a large surface of the ceiling by evaporating to keep a very dry appearance !!! personal case 10 years ago)

Crack in the wall allowing a little humidity outside the soil to pass through with rain.

So in my opinion, on my personal experience, before working on that the CMV, (not very useful on these houses with large vents to close more or less and costly in energy), we must seek to find the abnormal causes, often well hidden humidity, which are real traps and cowards !!!

So in the past I lived without VMC, adjustable flow according to need.

This type of house with thermal losses through thermal walls and bridges must be insulated first by the attic and then by the exterior.
Changing the windows first is illusory and a mistake not to make, especially since it will be incompatible with the external insulation with thermal bridges around the windows !! .

[dhaulagiri]

I agree with some points and not with others

The question of lifestyle and quality of construction is obviously important.

As far as I'm concerned, our habits seem to me to be those of people made aware of the need to have reasoned behaviors (well said, right?). So they can only go the right way After, it is obvious that the humidity will be higher if the pressure cooker operates 24/24.

On the quality of the construction, we have to deal with the existing when we buy in the old: the priorities for renovation work that we establish are never motivated by a single objective such as the search for better insulation or 'lower humidity.

When I decided to change my heating mode, I bet that I would lose less money than keeping the existing one, all other things being equal. I also wanted to have a heating based on a renewable and clean fuel.

When I decided to change my joinery, I did not think so much about improving the insulation, paltry if we consider the entire envelope of the house, than better comfort (less current air), aesthetic (the old ones were really ugly) and no longer have condensation on the windows (by the mere fact of double glazing, this problem is almost solved).

Same thing for VMC: by making my house more waterproof after various renovations, I increase the need for ventilation. I could have punched holes in the walls or left two windows open, the efficiency might have been better but ... A hygro system seems interesting to me, because it reacts to the context, unlike a self-adjusting CMV or that an open window without a servant who stays behind constantly. It goes without saying that it is only an imperfect remedy for an imbalance created by the home ecosystem, an imbalance aggravated by the improvement in thermal and acoustic performance and what do I know yet?

This is where your point of view seems contradictory contradictory: you talk about isolating, which tends to make it watertight and therefore aggravate the imbalance, and at the same time you say that you have to limit the sources of humidity. The excess insulation does not create moisture in itself but it confines it inside, which ultimately amounts to the same. You only talk about reducing the sources, as if it were possible to remove them all: for disorders (humidity linked to a broken tile, I experienced a few months ago), I can only be agreement. Afterwards, if you remove the inhabitants from a house, it is certain that there will no longer be any humidity problem: they will no longer sweat, no longer cook, no longer wash ... You can also ask to live outside: ventilation level, we have never done better and we will never do better.

A renovation is therefore a compromise, between lots of different considerations, which aims to produce a balance that suits its inhabitants. In this area, each of us is different ... For you, for example, an ITE would be a priority. In my particular case, it was impossible. Now, if you want to subsidize me up to the amount of the work, my house will become concrete proof that your priority was the right one ...

[dedeleco]

Agree on the principles, but no concrete answer on the points that I raise in particular: the way of life on the surface and the in-depth research of the invisible and unacceptable sources which make the body thoroughly rot as I lived by removing them quickly but which for others remain hidden all their life (case of my parents) !!
The well-conducted scientific method is effective at least for me !!

You have to get out of general theoretical considerations to go to the concrete and find the cause of the problems, which for humidity is difficult to be certain of the real cause and therefore of the right solution !!
An architect, poorly supervising the work, gave me a boost of humidity even without living there !!
Also it is a work of scientific detective !!
It is necessary to list all the clues without preconceived idea, easily inaccurate, or to be offended because one is mistaken during the investigation.

I solved the problem with my hands instead of the architect, who lost his lawsuit with his bad liver !!

I ask the questions how much to live on how many m2 ???
How many baths and showers per day !!

What effects of this humidity?
clue given:

condensation everywhere (windows and corners of the walls) and appearance of mold in the corners and even under a mattress, as we air the room daily. Sensation of humidity, therefore of cold, as was mentioned in this subject.

This indicates given the ineffective ventilation, a possible source of invisible and abnormal humidity to be searched relentlessly ???

daily the piece

gives the impression that only one room in the house has this problem ????
If the basic index case to work !!!

[dhaulagiri]

4 people, including 2 1/2 servings on 140m². Nothing special.

4 showers of 5 minutes each, with water flowing intermittently, or 20 minutes. In total, 2 hours a day at the bathroom, morning and evening included (2x15mn / person). From the properly installed CMV, there is no longer any trace of condensation on the glass of the bathroom. Before, there were still a few, even with the new double glazing.

The traces of humidity had been roughly hidden by the former owner under a coat of paint (wall / ceiling angles for example). So it existed before us and I don't know how these people lived.

I told you to agree with you to identify the humidity linked to the disorders (who would not do it?) And not to the simple activity of the inhabitants. This is what I did: in addition to the mold in one part of the ceiling caused by a broken tile, I identified the cause of other molds on the ceiling: a roll of glass wool placed in the attic, on its plastic packaging! = condensation.

For the mold on the mattress, I also identified the source: my son sweats a lot while sleeping. Should we consider it a mess and deny it where to consider that there is an element of imbalance with which one must obligatorily compose. I will tell you at the end of this winter if VMC was the right solution but I am practically certain that it is ...

[Did67]

Without repeating everything:

a) yes, we are not equal before sweating (as in front of many other things: the ability to store fat, cholesterol level, the risk of heart attack ...). Me, I sweat a lot and drink liters. My wife never! However, we are often together ...

b) ditto for the houses: the capacity of the materials to let the vapor pass is very variable (especially in the old - according to the composition of the mortar, more or less "fatty", lime or no lime ...), etc. ...

c) at home too, a recent house (built in 95) bought in 99, walls in "bisotherm" monoblocks (blocks of pozzolana) + traditional plaster + wax coating: I had mold in one of the rooms; I added a VMC SF hygro with also humidity sensitive outlets, others on sensors (WC); moreover, I put my VMC on programming - in winter, the air is drier; so I run just after the showers, the kitchen, and once in a while 1/4 h to renew the air and extract the residual moisture; roughly speaking, it only runs half the time; I am therefore well below the recommended speed but it is sufficient - so it is completely heretical, but it works! Before you make the holes bigger, don't you want to try ??? Because the standards, they are what they are: standards ... therefore "stupid" (often "to take no risk").

d) the hardest part: teaching the members of the family to close the doors so that the air sweep is done properly; beware of the Velux "bars" that remain open, for example: the air enters from the top of the Velux and goes directly to the mouth! Result: the calories discarded but little efficiency question "extraction" of odors / humidity ...

[dhaulagiri]

I want to do all the possible experiences but ...

I prefer to make sure that the installation is compliant (from a normative point of view, I mean) before, even if that should not ultimately have any impact. I think it is easier to ask for corrections right after the site, rather than asking for them 2 years later, when we have realized that the experiments attempted are insufficient. I may be overly procedural, but I want to make sure I'm out of trouble.

In addition, I forgot to specify (I believe) that in trying to enlarge the mortises, the company damaged certain opening elements (perforation of rooms, knocks on the glazing beads), not to mention the absolutely crooked shape of the entrances " redesigned ". Even if you can't see them behind the cover, it still looks a bit messy. So I had no hesitation in asking for compensation in addition to bringing them up to standard.

I do not think the cut VMC some of the time: the model I chose consumes less 15 W average. By cons, something to make me say that the air passing through the entries is insufficient: when one comes back from work, there is a smell of smoke in the living / dining room. I imagine that the chimney flue (open fire, even if shutter door) creates a pull factor to compensate for the relative tightness of the rest of the room. Simple hypothesis. One day I put a small wood stove to improve the thing but is not there yet ...

Anyway, thank you Did for your comments full of common sense, again and again.

[dedeleco]

Okay, we are not all the same !!!
However:

4 people, including 2 1/2 Cheesy Grin portions on 140m². Nothing special.
4 showers of 5 minutes each, with water flowing intermittently, or 20 minutes. In total, 2 hours a day at the bathroom, morning and evening included (2x15mn / person)

I lived in a similar house on 135 m2 in 1974 with 5 = 2 + 3 grown children, more than 20 years ago without any problem of condensation, nor any limit on the number of showers for my children !!!!
There was no VMC and no double glazing and the ventilation systems according to the standards of the time were closed in winter due to the cold and the leaks of the windows and doors well sealed with seals !!!

Also with a child under 2 and a half servings, I do not understand how you have a humidity problem, more airing every day, which I did not do at all, completely outside of my concerns epoch !!!!

We normally sweat in particular my wife sweats especially of the feet and hands (and therefore rapid destruction of shoes) much more than me !!!
We sweat what we drink and eat, about 1 to 2 liters per day, but the difference between people is not huge, like 10 to 20 liters (except in intense efforts like 100km of hills by bike with passes of repeated 500m, in great heat (32 ° C) and again I reach only 3 to 4 liters max, to see what I drink on the return !!!! after such efforts).

Also I do not understand and for you in your home, I would look for other causes and solutions than CMV alone, in particular other hidden sources of humidity.
Especially in the Gard little humid in principle.

I remind you that the humidity is vicious and that I found that I could easily make a mistake on the causes, without a thorough scientific analysis on the precise indices !!!

In most of the houses where I live, the CMV is almost eliminated, except on certain days of continuous rain at temperatures above 10 ° C, with 100% outdoor humidity which is found inside unchanged, and which then condenses easily !!!
But in this extreme case, the VMC is not effective, because it brings in air at almost 100% humidity by not changing its T, if outside it is not cold at all !!
If in the Gard, in autumn or spring, on certain days, this is your case: outside 100% humidity with T outside close to 20 ° C, close to T inside, (or even above T inside on hot days and humid) then the VMC is not a solution because it brings in a lot of air which stays inside at 100% humidity and which condenses an enormous quantity of water proportional to the air flow of the VMC !!
If the inside is colder than outside, by bad luck after a sudden heating outside, or with an air conditioning even working even little, then it is the catastrophe by condensation inside all the water vapor coming from outside! !
In France it is rare (although I have already seen it some days) but in the USA, it is catastrophic with outside 100% humidity in air at more than 30 to 37 ° C and inside air conditioned at less than 30 ° C (if not unlivable) because the houses (often wooden) are saunas and rot completely with deadly molds !!!

So you have to understand the physics of condensation, even with VMC to avoid bad luck days (rare in France, but 2 months a year in the USA east coast and tropical countries !!)