What are the differences between a single flow ventilation and a double flow CMV?

What does VMC mean?

First of all, the term VMC means "controlled mechanical ventilation" because this technical equipment uses fans to suck in and blow air into a dwelling.

The CMV meets the principle of general and permanent ventilation: the ventilation must allow to renew the stale air of the whole dwelling all year long, 24 hours a day, in particular thanks to the fan which allows to control the flows whatever the climatic conditions.

In which rooms to extract stale air?

Whether it is for a single flow or a double flow ventilation, the principle is to extract the stale air in the so-called "humid" rooms, i.e. the bathroom, the toilets, the kitchen, the laundry room and in general all the rooms where there is a water point.

In which rooms should you put grilles on the windows or blow in fresh air?

New air should enter living rooms, such as bedrooms, the living room, an office. With a single flow CMV you put ventilation grilles on the windows of these rooms, whereas with a double flow ventilation you blow tempered air via ventilation vents and a dedicated network.

The air is to be blown into the living rooms, such as bedrooms, living room, office.

Note that there are also single flow systems by insufflation called VMI (mechanical ventilation by insufflation). The new air is pulsed by the fan in the living rooms and escapes through grilles in the wet rooms. This system is used in cases where it is preferable to put the house under overpressure (for example in the presence of radon).

The single flow CMV

Single-flow controlled mechanical ventilationgenerally consists of permanently drawing in stale air from damp rooms.

Depending on the models of single flow ventilation the suction rate can vary according to the humidity level present in these rooms this is called a humidity sensitive CMV, unlike the self-regulating CMV which sucks in at a constant rate.

How does the air circulate with a single flow CMV?

As the single flow ventilation sucks the air in and discharges it outside, this will create a depression in these damp rooms. It is therefore necessary to let in fresh air somewhere, which is why it is necessary to put ventilation grilles on the windows in living rooms such as the living room, bedrooms or an office, to compensate for the depression generated by the suction by letting in fresh air through the window grilles.

The whole house is then ventilated by sweeping: the air flow will go from the living rooms to the nearest wet rooms through the under-door vents.

The problem with single flow ventilation is that the air entering the house will be at the temperature of the outside air, which means that in winter you will have very cold air entering your house or flat generating thermal discomfort near the grilles. This air is heated by your heating, to be then sucked by the vents present in the wet rooms in order to be evacuated outside at indoor temperature thus with a continuous loss of calories in winter.

How to regulate the extract air flow rate

The adjustable hygro ventilation will allow to limit the extracted airflow to limit the energy loss. The extracted airflow will be regulated according to the humidity level of the humid rooms by using humidity sensitive vents.

The extracted airflow will be regulated according to the humidity level of the humid rooms by using humidity sensitive vents.

The incoming airflow can also be regulated according to the humidity level via humidity sensitive air inlets. The air inlets can be placed on the mortise of the joinery (hole that is made in the window to let in outside air) or in the masonry which reduces the entry of outside noise into the house.

The air inlets can also be regulated according to humidity levels via humidity sensitive air inlets.

According to the technique used for the extract units and the grilles on the windows. We can differentiate the systems in single flow ventilation into three categories,

• self-controlled ventilation,
• hygro A ventilation
• hygro B ventilation
.

To learn more about the difference between these ventilation units you can see the corresponding article: What is the difference between hygro A and hygro B single flow auto adjustable ventilation?

What is the air intake rate with a Single Flow?

Depending on the number of rooms in the dwelling, the regulation imposes an air intake flow rate of 15 to 30 cubic metres per hour for the toilet and bathroom, and 30 to 120 cubic metres per hour for the kitchen. The regulations do not necessarily take into account the surface area or volume of the dwelling, which can create aberrations, particularly in the case of a large dwelling with few rooms or a small dwelling that is very partitioned.

Moreover, humidity sensitive single flow ventilation units will regulate the airflow according to the humidity level but not according to the level of chemical compounds (VOCs)and CO2 present in the air. This means that in winter when the weather is very nice and cold and the air is generally dry, the airflow rate will be at its minimum.

Yet this is when we are going to be locked in our homes because of the cold outside. As you are going to be locked in your house without opening the windows, you will increase the CO2 and VOC levels in your indoor air.

This will increase the CO2 and VOC levels in your indoor air.

This will therefore increase indoor air pollution and yet it is in this situation that you will renew the air in your home the least. Look for the mistake.

But of course, if we reason only on the fact that we suck in air from humid rooms, humidity sensitive CMV may seem adequate because it will increase its intake airflow when the humidity level of the air increases by taking your shower or cooking.
From my point of view, the humidity sensitive single flow ventilation system aims to protect the building by avoiding condensation while limiting energy losses as much as possible but does not protect the health of the occupants.

The self-regulating ventilation system protects more the health of the inhabitants but at the expense of comfort and energy savings. Indeed, these systems cause large heat losses in winter.

The self-regulating ventilation system protects more the health of the inhabitants but at the expense of comfort and energy savings.

Does single-flow CMV filter the air?

No, because the air intake grilles have no particle filters, which of course lets in pollutants from the outside air such as dust or pollen.

Does the single flow CMV recover the energy in the air?

No, because the air sucked in that has been heated by your heating system is of course extracted from the house without passing through an exchanger, unlike Double flow ventilation.
If you have understood the functioning of the Simple Flux CMV, it means that you have just understood that you are permanently bringing in cold air into your bedrooms and living room, which you are trying to heat, only to be sucked back in and injected outside continuously!

The energy loss is therefore significant. In addition, you will create discomfort due to cold air or hot air in the summer that enters your home permanently, without being filtered.

The energy loss is therefore significant.

What are the advantages of the single flow CMV?

The main advantage of single flow ventilation is its low cost because on average a single flow ventilation installation installed by a professional, usually the electrician, will cost you between 1000 and 1500 unlike double flow ventilation which can go from 4000 to 12 000 depending on the performance of the central unit, its options and the equipment put in place (type of network, silencer, enthalpy exchanger, Canadian well, etc..)
. But we should add the cost of heating to have a real operating cost.

Dual-flow ventilation

How does a dual-flow ventilation system work

Double flow controlled mechanical ventilation manages the incoming and outgoing airflow mechanically via low consumption motors. This is referred to as supply air and extract air, or fresh air and stale air.

In which rooms to extract stale air?

The outgoing air flow always corresponds to the flow drawn in from damp rooms such as single flow ventilation, i.e. damp air is drawn in from the bathroom, kitchen and toilet.

In which rooms to blow fresh air?

The incoming air flow is blown in via vents present in living rooms such as the living room, bedrooms or an office. As the fresh air is blown in mechanically, it is not necessary to put ventilation grilles on the windows of the living rooms. This avoids degrading the performance of the window by drilling holes in the sash and frame to fit an air intake grille!

When you think about it, it is still a shame to degrade the airtightness of a window by one or two grid openings of about twenty centimetres, whereas the purpose of the joints of the latter (double or triple compression joints) is to avoid air infiltration and therefore draughts! This is a bit of a backwards world and yet this is what happens in 95% of new buildings or renovations. The remaining 5% concern houses equipped with double flow ventilation or simply houses that have not been equipped with CMV.

What is the temperature of the supply air?

The main advantage of double flow ventilation is therefore to keep your building envelope airtight to avoid any heat loss. No more holes in your beautiful windows that cost you so much! The air mechanically blown into the living rooms is a so-called "tempered" air because this air will recover the calories from the stale air when it passes through the heat exchanger integrated in the double-flux ventilation.

The stale air drawn in from the kitchen, bathroom and toilet, which is warm and humid air, will therefore give back its calories to the new air as it passes through the exchanger. This is how you avoid energy losses by ensuring a continuous and tempered air renewal.

The temperature of the incoming air is always a few degrees lower than the outgoing air, but at least it is not at the outside temperature! For example if it is -5° outside and 20° inside and you have a good heat exchanger with efficiencies above 90%, you will easily be able to blow in air at 18°.

The calculation of efficiencies depends on the temperature of the incoming air.

The calculation of the efficiencies depends on many parameters and the method used. It is also possible to install an electric heating coil or to place an air/air or air/water exchanger upstream to preheat or cool the incoming air. In this case you will need to install what is called a hydraulic or air-source heat exchanger.

How much air exchange rate for a double flow?

Since the double flow ventilation unit is equipped with two motors, you can, when putting this DF CMV into service, adjust the suction flow rate and the insufflation flow rate in order to balance the installation so that there is the same flow rate of sucked in air as of insufflated air with the help of a flow meter.

If you consult a professional and they don't talk about commissioning and balancing the system, you can skip it and look for another professional to install your dual flow ventilation, even if they tell you they never do it It's easy to realise that a heating system is working well but if nobody checks the ventilation system how can you realise that it's actually working as it should?

When commissioning, you can also set the air change rate of your home which is expressed in volume per hour. Depending on the project and the region, the air exchange rate of your home will vary between 0.3 and 1 volume per hour. This means that if you start with a renewal of 0.5 volumes per hour, you will have renewed all the air in your house in 2 hours. This may seem like a lot, but it's what you need to keep the air in your home healthy. I invite you to read this article which details the commissioning procedure and the adjustment of the flow rates.

Does the double flow CMV dry out the air?

The main disadvantage of double flow ventilation is that it can dry out the air that is blown into your home. Indeed by passing in the heat exchanger the humidity present in the outside air can condense (don't forget the preferably dry siphon on your installation).At the exit of the exchanger the low humidity level of the air blown in can then generate a feeling of discomfort for the occupants! But this physical problem will only occur on entry-level double flow units not equipped with an enthalpy exchanger.

This is an important point in the choice of your double flow ventilation and of course this option is not present on the first price double flow ventilations. Double flow ventilation with a rotary exchanger avoids this problem but it is a vast debate between static plate exchangers and rotary exchangers that we will not develop in this article.

Does double flow ventilation filter pollens and particles?

Yes, and this is another advantage over the single flow! Indeed, you have filters at the level of the exchanger to filter the outside air of the particles and pollens but also to filter the stale air of your housing. The purpose of these filters is to avoid the clogging of the heat exchanger but especially to avoid the introduction of particles present in the outside air.

How often should you change the filters in a Double Flux CMV?

They say that the filters should be cleaned every 3 months and changed every year but it depends on your geographical location and also the season. For example, in the spring, if you live in the country, you have a much higher pollen count in the air than someone who lives in the city. But in the city you will have a much higher level of particulate matter related to dust and exhaust than in the country.

If you don't change the filters, you will reduce the airflow rates, and possibly even degrade the performance of your central air unit motors. Entry-level models will not compensate for this loss of airflow, unlike the more elaborate dual-flow CMVs, which will compensate for this clogging using pressure sensors. This is known as "constant flow rate motors"

How much do the filters in a dual-flow system cost?

The cost of the filters is also something to consider when choosing your plant. Very often entry-level machines have higher filter costs than high-end machines. This can represent a budget ranging from 50 to several hundred euros per year.
Filter replacement is very important to ensure good airflow and to avoid over consumption of electricity and possible damage to the motors. The motors of the new generations of double-flow ventilation are normally low consumption.

In conclusion, what to choose between a single flow ventilation and a double flow ventilation?

We unhesitatingly recommend double flow ventilation. It ensures real comfort on a daily basis by ensuring the quality of the air in your home, but also by ensuring energy savings thanks to the recovery of energy from the stale air.

Moreover, your children will spend more than half of their lives sleeping in their bedrooms, and you yourself will spend a third of your life in your bedroom. That's why it's important not to neglect air renewal in these rooms. You will say, as 95% of people do, that there is no point because you sleep with the window open or you open it every morning. But why do you need to do this? Have you ever wondered why your body tells you to do this? Simply because you need fresh air because you are "suffocating."

If you feel the need it is because you are already craving fresh air every morning. So imagine your child after his nap or 12 hours of sleep locked in his little bed?

You can take the example of your body becoming dehydrated and calling out for water by creating the sensation of thirst ... if you are thirsty it is because you are already lacking water. So now that you have realised this point, are you sure you want to be short of oxygen all your life and thus have your sleep disrupted?

The cost of a dual flow installation is higher than a single flow ventilationbut does your health come at a price? How much do you value the quality of your sleep on your daily performance? Isn't your child's comfort for many years paramount to avoid asthma, coughs, or any other pathology?

Maybe it's better to buy a slightly cheaper kitchen, downsize your garage, and reduce the size of your pool to help ensure that you and your family have clean air for the long years you'll be living in your future home.

"As you will have understood, I am in favour of the Double flow ventilation, and every morning when I get up I enjoy finding that my bedroom has been ventilated without the need to open the window... Madam is not cold, and I can breathe!

If you have any questions or comments, please feel free to leave them below.