Controlled Mechanical Ventilation and its heart: the energy exchanger

In a previous article I explained to you the reasons why the mechanical ventilation system wins the comparison with the simple manual opening of the windows to obtain an adequate exchange of air inside our home.

We therefore learned that our VMC system, to carry out this air renewal, performs two simple operations at the same time:

• extracts the stale air from the rooms to expel it outside the home
• draws in new air from outside the house to be introduced into the rooms

This task is entrusted to what is called the "ventilation unit" (the main machine and also the most expensive element of the system), which is the red colored element in the following diagram:

Inside the machine there are two simple fans that work in two opposite directions:

1. The first fan sucks the stale air from some inhabited rooms and pushes it outside the house, through the “ocher” piping system.
2. The second fan works in the opposite direction, sucking clean air from the outside of the house to push it towards the other inhabited areas, through the “cyan” colored piping system.

ATTENTION: the two air flows cross (and I stress they cross, they do not mix!) inside the ventilation unit. Let's see how these two air flows intersect:

The two streams of air, as shown in the image above, cross inside an object which is called an "energy exchanger".

How does the exchanger work?

Try to imagine many channels, of very small section, separated from each other by very thin walls, which are followed by the air in two opposite directions. Here ... the warmer air flow, through the thin walls, transfers the heat to the cooler air flow.

Let's try to better understand how this heat exchange takes place in the two seasons, that is, in summer and winter:

What happens in the summer?

In the summer season, 26° air is taken from inside the house (the air conditioner is working) to be expelled outside. This air crosses, inside the recuperator, the air flow coming from the outside of the house which is for example at 35° (we are in summer!). Through the walls of the recuperator, the expelled air flow cools the incoming one, so that the new air introduced into the rooms does not have a temperature of 35° but a more convenient temperature, for example 28°.

Yes .... I know ..... 28° is greater than 26° and therefore the new air we put in the house is warmer than the one we extract. But we must focus on the electricity consumption of our air conditioner, which will simply have to cool the introduced air to 28°. The alternative would be to open the windows, let in new air at 35°, which must be cooled by our air conditioner with a much higher electricity consumption! It is on this temperature difference that the energy savings that can be obtained with a VMC system can be understood.

What happens in the winter?

In the winter season, the air at 22° is taken from inside the house (the heating is working) to be expelled outside. This air crosses, inside the recuperator, the air flow coming from the outside of the house which is for example at 0° (we are in winter!). Through the walls of the recuperator, the expelled air flow heats the incoming one, so that the new air introduced into the rooms does not have a temperature of 0° but a more convenient temperature, for example 19°.

Yes .... I know ..... 19° is less than 22° and therefore the new air we put in the house is colder than the one we extract. But we must focus on the consumption of our heating system, which will simply have to heat the air introduced to 19°. The alternative would be to open the windows, let in new air at 0°, which will have to be heated by our heating system with a much higher consumption! Also in this case, based on this temperature difference, it is possible to understand the energy savings obtainable with a VMC system.

ATTENTION: the temperatures shown in the examples described above are temperatures that derive from the degree of performance (efficiency) of the energy recuperator, that is, from the material with which it is built. There are many ventilation units on the market, which can be equipped with different types of recuperators: the higher the efficiency of the recuperator ... the greater the energy recovery from one air flow to another. But that's another story ... which maybe we'll talk about in a future article!

And at this point ... I already imagine the next question:

What happens on cold summer days and hot winter days?

It can happen a particularly hot day in winter, when the outside air temperature is not at 0° but for example at 23°! Just as it can happen a particularly cool day in summer (perhaps after a storm), when the outside air temperature is not 35° but for example 25°! These are the cases in which the air taken from outside the home does not need to be "preheated" or "pre-cooled", as it already has an adequate temperature. In cases like these it would not be convenient to make the two air flows cross inside the recuperator, because the exchange of energy would return to our disadvantage. And therefore the ventilation unit avoids crossing through a simple stratagem called "bypass": that is, the flow of stale air being extracted is diverted to prevent entry into the recuperator, for direct expulsion towards the outside of the house. The bottom of the image above is worth more than 1000 words.

Please note. Source of some images: web