WO2008102227A2

WO2008102227A2 - Ventilation system and method for controlling multiple air to air heat exchanger with temperature sensors

Info

Publication number: WO2008102227A2
Application number: PCT/IB2008/000340
Authority: WO
Inventors: Jan Lietaer
Original assignee: Jan Lietaer
Priority date: 2007-02-22
Filing date: 2008-02-15
Publication date: 2008-08-28
Also published as: BE1017474A7; WO2008102227A3

Abstract

The present invention is related to a method and system for controlling the air flow to a plurality of heat exchanger units (1,2,3,4) installed in a building, each heat exchanger unit being arranged to allow a inflowing air flow (6) from the exterior of the building to the interior, and an outflowing air flow (5) from the interior to the exterior, wherein heat can be exchanged between said air flows, characterized in that for at least one unit the outgoing air flow rate is different from the ingoing air flow rate, said flow rates being controlled on the basis of the energy contained in the air present in different areas (A, B, C, D) of the building, said energy being determined on the basis of temperature measurements.

Description

VENTILATION SYSTEM AND METHOD FOR CONTROLLING SUCH A SYSTEM

Field of the Invention [0001] The present invention is related to a method for operating and controlling a ventilation system in a building, and to the ventilation system as such.

State of the art [0002] Today, ventilation systems are often incorporated into a building concept. These systems are designed to provide a supply of fresh outdoor air to the rooms of the building, in order to replenish stale and odorous air inside the house. [0003] In the context of so-called ^Λpassive houses', i.e. houses with extreme low energy consumption, ventilation systems are playing an even more prominent - role. In a passive house, the aim is to minimize heating costs by recovering the heat from outgoing air. To this aim, it is known to use a central heat exchanger, where outgoing air heats up the fresh outdoor air before the latter is distributed to the different rooms. In this heat exchanger, two ventilators are present, for blowing air in opposite directions. These ventilators are designed for maintaining the in -and outgoing flows equal to each other, in order to maintain a constant pressure in the house. Such ventilation systems are referred to as 'central balanced' systems. [0004] Other systems are known which comprise several heat exchangers, for example one heat exchanger in each room that has an outside wall. This solution avoids the complex ducting arrangement required for the central system. These Mecentral' systems are however also working in a 'balanced' mode, i.e. in each of the heat exchangers, the in -and outgoing air flow rates are equal to each other, in order to maintain the air pressure in each room essentially constant.

[0005] A problem of centralised as well as decentralised balanced systems is the difficulty of controlling such a system. Any influence (e.g. sunshine heating up one room, a door being left open, ... ) may lead to one room overheating or cooling down too much. In the case of an overheating room, too much high energy inner air is exhausted through the heat exchanger, and thereby lost.

Aims of the invention

[0006] The present invention aims to provide a ventilation system which is easier to regulate, while keeping energy losses to a minimum.

Summary of the invention

[0007] The present invention is related to methods and systems as described in the appended claims. [0008] The invention is firstly related to a method for controlling the air flow through a plurality of heat exchanger units installed in a building, each heat exchanger unit being arranged to allow simultaneously an inflowing air flow from the exterior of the building to the interior, and an outflowing air flow from the interior to the exterior, wherein heat can be exchanged between said air flows, characterized in that for at least one unit the outgoing air flow rate is controlled to be different from the ingoing air flow rate, the flow rates through said unit being controlled on the basis of the energy contained in the air present in an area of the building to and/or from which said unit is arranged to supply and/or extract air, said energy being determined on the basis of temperature measurements. Specific embodiments of the method of the invention are described in the combination of claim 1 with one or more of the dependent claims 2 to 10. [0009] The invention is further related to a ventilation system for air ventilation in a building, comprising a plurality of heat exchanger units, installed in connection with external walls of said building, and arranged for allowing heat exchange between simultaneous outgoing and ingoing air flows out of and into said building, each heat exchanger unit being further equipped with a plurality of temperature sensors, arranged to measure temperature changes taking place in said outgoing and/or ingoing air flows, characterized in that said system comprises a control means, adapted to control the air flow rates through said heat exchanger units, on the basis of said temperature measurements, in such a manner that in at least one heat exchanger unit, the outgoing air flow rate is different from the ingoing air flow rate. Specific embodiments of the system of the invention are described in the combination of claim 11 with one or more of the dependent claims 12 to 20.

[0010] The invention is thus related to a control mechanism for decentralised ventilation including heat recuperation, arranged so that the in and outgoing air flows in each heat exchanger unit are no longer equal to each other, but wherein the energy loss or gain is optimized for the totality of the rooms in a building. When the outer air is excessively cold, this can be done by exhausting less air from rooms containing energy-rich air than is being introduced into said energy-rich rooms, whereas from rooms having low-energy air, more air is being exhausted than introduced. When the outer air is too hot, the system is inverted, thereby keeping the rooms in the building cool. Brief description of the figures

[0011] Fig. 1 illustrates a decentralized balanced ventilation system as known in the art.

[0012] Fig. 2 shows the building of figure 1, equipped with a ventilation system according to the invention.

[0013] Fig. 3 is a schematic representation of a heat exchange unit usable in the invention, and different locations of temperature sensors [0014] Fig. 4 illustrates a possible way of connecting the temperature sensors according to the invention.

[0015] Fig. 5 illustrates an embodiment wherein one room is provided with a forced outflow. [0016] Fig. 6 illustrates an embodiment wherein one room is provided with an ^Λin-room circulation' capability.

[0017] Fig. 7 and 8 illustrate further examples of modes of operation according to the invention.

[0018] Fig. 9 shows a building used as the basis of a simulation exercise.

Detailed description of the invention

[0019] In a ventilation system controlled by the method of the invention, the rooms that are to be ventilated are provided with a plurality of heat exchange units, for example one in each room that is bordered by an exterior wall, wherein the heat exchanger is installed in connection with such an external wall, so that air flow through the heat exchanger takes place from the inside of the house to the external environment and vice versa. A heat exchange unit can be a known apparatus, comprising two ventilators and a means such as plates or ducts, for exchanging heat between the opposing air flows. Heat exchange units as described in DE202005011482U may for example be used in the system of the invention. [0020] The difference with existing decentral balanced ventilation technology lies in the fact that the in and outgoing flow rates of each unit are not controlled to remain equal to each other. The control of these flow rates is preferably such that the total energy loss in the building is minimised or maximized if the building is too hot. When there are differences in ^• temperature and moisture inside the building, this will result in a difference between the in and outgoing air flow rates in each unit. In general, the invention leads to a more economical use of energy rich air (= humid and hot) , compared to energy poor air. Due to the difference between in and outgoing air flow rates in each unit, air flows will be generated between rooms in the building. These internal air flows will be directed such that they will contribute to establishing desired temperatures. [0021] Figure 1 shows an example of a building having two rooms A and B, each room being equipped with a heat exchanger unit (1,2 respectively), installed in an exterior wall of the building. The air flows shown are in balance, therefore this system is controlled according to the prior art. [0022] Figure 2 illustrates the control mechanism of the invention applied to the same building, wherein 2 spaces are ventilated by 2 unbalance units. In figure 2, the situation is such that the outer air is cold, and that room B contains high-energy air, and room A contains low- energy air. The energy of the air can be higher as a consequence of a higher temperature and/or higher moisture content. To maintain a maximum of energy inside the house, the control is such that less energy-rich air flows out of room B, and more energy-poor air flows out of room A. The difference in air flow rates in both units will generate a flow between the different rooms, through an opening (e.g. a door) between A and B. The controller means used in the invention can find this optimum via an algorithm based on temperature measurement alone.

[0023] Figure 3 shows a schematic view of a heat exchanger usable in the invention, installed in a wall 8. This unit has an outflowing path 5 and an inflowing path 6. In the heat exchange unit 7, the two flowpaths pass in each others vicinity (for example through adjacent tubes or between plates), thereby allowing the opposite air flows to exchange heat. The heat exchanger can also be a rotating heat exchanger or any other type of heat exchanger which allows simultaneous in and out flow. Different types can be used in the same building. The unit 7 comprises ventilators (not shown) to drive the respective air flows. [0024] According to the invention, at least two temperature sensors are installed in connection with each heat exchange unit. According to a first embodiment, two sensors 10,11 are placed in the outgoing flow path 5, before and after the unit (see figure 3a) . According to a second embodiment, two sensors 12,13 are placed before and after the unit, in the ingoing flow path 6 (figure 3b) . In these two embodiments, the sensors are able to measure the temperature drop in the outgoing or respectively the ingoing flow path.

[0025] Preferably however, four temperature sensors (10,11,12,13) are installed in the in -and outgoing flowpaths, and arranged to measure both temperature drops simultaneously (figure 3c) . According to a further embodiment, temperature sensors 10 and 12, arranged for measuring the temperature of air flowing into the heat exchanger on both sides of the wall, are not placed in the air flow, but adjacent to it. This arrangement ensures a correct temperature measurement, even when the direction of the air flows is inadvertently reversed, for example under conditions of strong wind.

[0026] Figure 4 shows a possible arrangement of the temperature sensors 10 to 13, in a Wheatstone bridge circuit. Any other known arrangement for connecting the sensors and obtaining the temperatures can be used. [0027] According to the invention, a method is provided for controlling the in and outgoing air flow rates in the heat exchange units, on the basis of temperature measurements, such as obtained from the sensors as described above. Both flows are controlled while continuing to flow simultaneously in the two directions.

[0028] The following formulations will be used in the present description :

• Temperature measured by sensor 10 : T_Int = Temperature of outgoing flow 5, measured at the interior of the building. This corresponds to the interior temperature. • Temperature measured by sensor 11 : T_Ext_out = Temperature of outgoing flow 5, measured at the exterior of the building.

• Temperature measured by sensor 12 : T_Ext = Temperature of ingoing flow 6, measured at the exterior of the building. This corresponds to the exterior temperature.

• Temperature measured by sensor 13 : T_Int_in = Temperature of ingoing flow 6, measured at the interior of the building. [0029] On the basis of these temperatures, a 'Quality value (Q_val) ' is calculated. This can be for example one of the following formulas : Ql = T _ Int -T _ Ext _ out Q2 = T _ Int _in-T _ Ext

T Int T Int in

Q3 =

T _Int + T _ Ext_ out T_Int_ in + T_ Ext

T _ Ext _ out T _ Ext

T _ Int + T _ Ext_ out T _ Int_ in + T _ Ext

T Int -T Ext out β5 =

T _Int _ in -T _ Ext

Other mathematical formulas are possible for the quality value, however they should all reflect the relative enthalpy richness of the inside air compared to that of the other rooms. In other words, the Quality value should continuously increase or decrease with increasing interior temperature T_Int.

[0030] As explained later, according to an embodiment, a desired temperature T_set can be defined by the user for each unit. In this case, the quality value

, , _Λ _ T Set - T Ext out can also be Qβ , with Q6 = -^= = = .

T_Int_in-T_Ext

[0031] The unbalance of a particular heat exchanger can be quantified by the ^unbalance ratio' , hereafter called the ^ΛDB' value. DB is equal to :

-1 for the status wherein the outgoing flow rate is zero (complete inflow)

0 for the status wherein outgoing flow rate is equal to ingoing flow rate (balance)

+1 for the status wherein the ingoing flow rate is zero (complete outflow) . [0032] In between these values, DB is calculated with the formulae:

Ingoing flow = Vmaster (1-DB) Outgoing flow = Vmaster (1+DB) Vmaster is the flow rate that is needed for normal ventilation of the rooms that are ventilated with the unit, e.g. based on the dimensions of the room. The ingoing and the outgoing flow is equal to the Vmaster when the unit is in balance. For example DB= -0.6 results in an ingoing flow of 1.6 times the Vmaster , and an outgoing flow of 0.4 the Vmaster.

[0033] According to a first embodiment of the method of the invention, the in and outgoing flow rates in each heat exchange unit are adapted (i.e. the DB value is adapted) in order to make the quality values equal for every heat exchange unit. This control mechanism is preferably performed as follows :

1. measurement of the temperatures (2 or 4) in each unit

2. calculation of the quality value for each unit

3. adjusting the air flow rates of the in and outgoing air flows in each unit, preferably by producing an output signal which is a control signal for the speeds of the ventilators in the in and outgoing paths of each heat exchanger unit

[0034] The output signal is such that the difference between the measured quality values is reduced. The above steps are repeated at regular intervals, for example 3minutes. For example the mean value and standard deviation of the quality values is calculated at each measurement step and the output signal is aimed at minimizing the standard deviation from this mean value. [0035] If for example, one room is significantly hotter than the other rooms (e.g. because it is situated on a side with a lot of sunshine) , then the quality value for the heat exchanger of that room will be higher than the other quality values. The control mechanism will then reduce the outgoing flow rate from the hot room, in order to preserve enthalpy rich air present in that room. A surplus of outside air will flow into the room, causing an overpressure in said room, and a re-distribution of air in that room to other rooms, through doors or other openings. Preferably, when the method of the invention is applied, doors between rooms are provided with openings to let air pass from one room to another, even when the door is closed. The re-distribution of air will continue until the temperature in the room is sufficiently diminished. The method of the invention thus allows an unbalance to exist in individual heat exchangers during a period of time, during which the temperatures are changed. The end state can be a state in which all heat exchangers are in a balanced condition, but this is not necessarily the case. It is also possible that at least some of the heat exchangers will settle into a continued state of unbalance (different flow rate in opposing directions, but constant flow rates in time) . This may be the case for example when one room receives more sunlight than other rooms and thereby consistently heats up more than the other rooms, or when a desired temperature is set for each unit (see further ) . In certain circumstances, it may also be necessary to maintain a number of units in so-called ^Λblowthrough' mode, wherein the flow in one direction is essentially 100% and the flow in the opposite direction is essentially zero (DB is +1 or -1) . As explained later, this is mainly applied in a number of units when desired temperatures are set in the different rooms of the building..

[0036] The way in which the flow rates are adjusted can be chosen in different ways (e.g. increase outgoing flow rate while maintaining ingoing flow rate equal, or other ratios of changes to both flow rates), without departing from the scope of the invention.

[0037] According to a further embodiment, the control is performed in order to maintain the sum of all in and outgoing flow rates egual to zero (i.e. sum of DB values is zero) . In other words, when the flow rates are all expressed as positive numbers, the sum of all outgoing flow rates is equal to the sum of all ingoing flow rates.

The advantage of this is that no large over or underpressure is built up in the house, with minimum energy loss as a consequence.

[0038] Two preferred ways of obtaining this result are possible :

- by changing the in and outgoing flow rate always by the same amount

- by changing the flow rate as a function of the total flow rate through the unit, i.e. depending on the intended flow for the unit (Vmaster) .

The condition 'sum of DB = 0' is preferably an additional condition imposed on the ventilation system, in addition to the condition of equal quality values.

[0039] According to a further embodiment, a desired temperature is set for each unit : T_set (being the temperature which is desired in the room where the unit is installed) .

[0040] In the latter case, a distinction is preferably made between units that are in ^parallel' mode, and units that are in ^exchanger' mode. Exchanger mode is defined as the status wherein the interior temperature T_Int is lying between the exterior temperature T_Ext and the T_set value. In all other cases, the exchanger is in ^parallel' mode. In the exchanger mode, the unit is controlled so that the changes to the interior temperature are minimised. When a unit is in the parallel mode, the control is such that the interior temperature approaches the desired temperature as quickly as possible. This can be done by increasing the air flow into these rooms.

[0041] When several rooms have an interior temperature which is not between the T_set and T_ext values

(i.e. units that are in parallel mode), the method can provide one or more units to switch to the ^Λblowthrough' mode, in which the units have an essentially 100% flow in one direction (out or in) . The preferred operation in this respect is to have a total outflow in rooms where the temperature is farthest from the T_set value, and total inflow in rooms where the temperature is closest to the T_set value. [0042] A more detailed description of the preferred embodiment involving a T_set value, is given hereafter. The quality value as described above is calculated only for those heat exchanger units which are in the ^exchange' mode. This quality value (e.g. one of the examples Ql to Q6) is then called ^ΛQual_exchange' . When a heat exchanger is in the parallel mode, a different quality value λQual_parallel' is calculated. Qual_parallel can be calculated as : T_set-T_int, but other formulas are possible, as long as they reflect the relative ^λtemperature error' of the rooms compared to one another. [0043] Another value used in this embodiment is the Vmaster value, defined above, as the flow that goes to a unit if DB=O.

[0044] According to this embodiment, the following sequence of steps is performed, in every control cycle of the system :

- For each unit, the two or four temperatures as described above are measured, and the units are divided into ''parallel' and ^exchange' units. - the Qual_Exchange and Qual_Parallel values are calculated.

- Next, a first correction on the DB value of the units who are in exchange modus is done in such way that the Qual_Exchange values become more equal.

All of the ^parallel mode' units are then put in throughblow mode, which is preferably done according to :

- If the amount of units in parallel is more than half the total amount of units, all the units in parallel modus are put into complete unbalance, having a DB -1 or 1 depending on if their Qual_Parallel is larger or smaller than the average of Qual__Parallel

- If the amount of units in parallel is less than half the total amount of units, then all the units that are in parallel are put into complete unbalance wherein those who have an interior temperature larger than the T_set have a DB =-1, the others have a DB =+1

Preferably, a further correction is done as follows : - All the DB values of units in Exchange modus are shifted by a second correction so that their sum of all DB is zero .

Finally : - The flows of all the units are modified so that they reflect their corrected DB value (first and/or second correction). The out going flow becomes Vmaster (1+DB), the ingoing flow Vmaster (1-DB) [0045] According to an embodiment, all measurements of the exterior temperature in each unit, are replaced by a single measurement (by a single sensor) .

[0046] According to an embodiment, a minimum and maximum level is imposed on the in and outgoing flow rates for each heat exchanger unit. This option can be used to ensure a minimum ventilation in each room.

[0047] The method of the invention can be applied in combination with other control systems, such as so-called ^Λdemand controlled' system, which may be a system where ventilation is actuated on the basis of a measurement of C02 levels. This combination may lead to better results than can be obtained by combining balanced systems with Memand-controlled' systems. By using the method of the invention, the energy loss can be further reduced.

[0048] Figure 5 shows the application of the method of the invention in a building comprising 3 rooms, wherein room C is a room with forced outflow. Air from this room can only flow to the exterior of the building but not to other rooms. In other words, the ingoing flow and outgoing flow of the heat exchanger unit 2 are connected to different rooms of the building. In room C this causes a guaranteed outflow of air, which can be useful when odours or bacilli need to be contained (e.g. toilets or bathrooms) .

[0049] Figure 6 shows an application in a two-space building, wherein room B is an ^Λin-room circulation' space. In this embodiment, the heat exchanger is equipped with a duct 9 that guides the incoming air towards the vicinity of the opening (e.g. a door). This can also be achieved by accelerating the incoming air, and possibly by sending it along the ceiling. In this way, the air flowing from room B to room A is always pure. [0050] Figure 7 shows a case wherein room A is too hot, and the rest of the rooms is too cold, while the outer air is cold. The method of the invention will result in a higher inflow rate in room A. As a consequence, an air flow will be generated from the hot room to the rooms that are too cold. This is only possible when the source room A is not sensitive to odours, or when the room that is too hot is provided with internal circulation. This can happen on a sunny winter day, with one room catching an excess of sunlight and thereby being susceptible to overheating.

[0051] Figure 8 illustrates a mode of operation according to the invention, wherein several rooms in the building need to be cooled or heated, this being possible by means of outer air having the right temperature. In the case shown, this is done by unit 1 having almost 100% inflow and unit 2 having almost 100% outflow. This mode can be useful for cooling the building at night. [0052] The invention is equally related to a ventilation system as such, said system comprising a plurality of heat exchanger units equipped with temperature sensors as described above, and a control means for controlling the air flow rates in said units in the way described above. The control means may be a central control unit, connected to every heat exchanger in the system, or every heat exchanger may be equipped with a control unit, so that all units react as a multiple agent system communicating with one another, preferably using a bidirectional data connection. The system of the invention may be provided with heat exchange units and additional means as described in any of the embodiments described hereabove. The system can comprise a single heat exchanger in connection with one or more rooms (i.e. one in each of said rooms) , but it can also comprise several heat exchangers in connection with one (e.g. larger) room. [0053] The method of the invention allows to obtain an energy efficiency rise of 10 to 15% compared to existing methods. The method and system of the invention is particularly useful in combination with a passive house. The system allows also to maintain desired temperature differences in different parts of the building using different T_set values. This is something that is not possible with the current technology.

Simulations

[0054] Figure 9 shows the ground plan of a house used in a simulation, the results of which are shown in table 1. Every room comprises an odour source 101,102,103, which emits 1 ^Λodour unit' per hour. The end equilibrium is calculated, in the case of a balance system and of an unbalance system according to the invention.

The above shows that the unbalance state created by the method of the invention can obtain a better air quality than a balanced state. This unbalance can persist in case of a continued heat in our outflow e.g. an open window or an open sun protection.

Claims

1. A method for controlling the air flow through a plurality of heat exchanger units (1,2,3,4) installed in a building, each heat exchanger unit being arranged to allow simultaneously an inflowing air flow (β) from the exterior of the building to the interior, and an outflowing air flow (5) from the interior to the exterior, wherein heat can be exchanged between said air flows, characterized in that for at least one unit the outgoing air flow rate is controlled to be different from the ingoing air flow rate, the flow rates through said unit being controlled on the basis of the energy contained in the air present in an area (A, B, C, D) of the building to and/or from which said unit is arranged to supply and/or extract air, said energy being determined on the basis of temperature measurements.

2. The method according to claim 1 wherein for each unit a first and/or a second set of temperature measurements are performed : • the first set (10,11) being :

T_Int = interior temperature,

T_Ext_out = Temperature of outgoing flow (5), measured at the exterior of the building,

• the second set (12,13) being - T_Ext = exterior temperature,

T_Int_in = Temperature of ingoing flow (β) , measured at the interior of the building said method comprising the steps of :

• measuring for each unit said temperatures, • calculating for each unit a quality value on the basis of said temperatures, said quality value being a measure of the energy of the air present in said area of the building, • adjusting the air flow rate of said ingoing and/or outgoing air flow in each unit, so that the difference between said quality values decreases.

3. The method according to claim 2, wherein the quality value is one of the following:

Ql = T_ Int-T_ Ext_ out Q2 = T _Int _ in-T _ Ext

T Int T Int in β3 =

T _Int + T_ Ext _ out T_Int_ in + T_ Ext

24 = T_ Ext _ out T Ext

T _Int + T _ Ext _ out T Int in + T Ext

T T _ I Tnntt-—T T_ E F.xytf _ o nuutt

T _Int _ in-T __ Ext

4. The method according to any one of claims

1 to 3, wherein said air flow rates are further controlled so that the sum of all inflowing flow rates into the building equals the sum of all outflowing flow rates out of the building.

5. The method according to any one of claims

2 to 4, wherein the interior and exterior temperatures are measured in the air flow towards said heat exchanger units.

6. The method according to any one of claims

2 to 4, wherein the interior and exterior temperatures are measured outside the air flow towards said heat exchanger units .

7. The method according to any one of the preceding claims, wherein for each heat exchanger unit a desired temperature (T_set) is set, and wherein said air flow rates are controlled so as to maintain the temperature of the area of the building in connection with which said heat exchanger unit is installed, as close as possible to said desired temperature.

8. The method according to claim 7, wherein

_{^ r} ■ i ■ ^ . ^ π_s ^τ Set - T Ext out the quality value Qo is applied, with Qo = — = = = .

T_IntJn-T_Ext

9. The method according to claim 7 or 8, comprising the steps of : - For each unit, measuring the two or four temperatures as described in claim 2, and dividing the units into 'parallel' and 'exchange' units, wherein 'exchange' units are defined as units wherein the interior temperature T__Int is lying between the exterior temperature T_Ext and the T_set value, the remaining units being 'parallel' units.

- For each 'exchange' unit, calculating the Qual_Exchange value wherein Qual_Exchange is a measure of the energy of the air present in the area of the building where the unit is installed,

- Adjusting the air flow rates of the 'exchange' units, in such a way that the Qual_Exchange values of said units become more equal to each other.

- Putting the 'parallel' units in a 'throughblow' mode, wherein air flow takes place substantially in one direction only.

10. The method according to claim 9, wherein for each 'parallel' unit, the ^ΛQual_parallel' value is calculated, being a value which reflects the difference between the interior temperature and the T_set value, and wherein the following is performed :

- If the amount of 'parallel' units is more than half the total amount of units, all the 'parallel' units are put into 'throughblow' mode, having essentially total inflow or total outflow, depending respectively on if their

Qual_Parallel is larger or smaller than the average of Qual Parallel, - If the amount of ^parallel' units is less than half the total amount of units, all the ^Λparallel' units are put in ^throughblow' mode, wherein those who have an interior temperature T_int larger than T_set are set to essentially total inflow, the others to essentially total outflow.

11. A ventilation system for air ventilation in a building, comprising a plurality of heat exchanger units (1,2,3,4), installed in connection with external walls (8) of said building, and arranged for allowing heat exchange between simultaneous outgoing and ingoing air flows (5,6) out of and into said building, each heat exchanger unit being further equipped with a plurality of temperature sensors (10,11,12,13), arranged to measure temperature changes taking place in said outgoing and/or ingoing air flows, characterized in that said system comprises a control means, adapted to control the air flow rates through said heat exchanger units, on the basis of said temperature measurements, in such a manner that in at least one heat exchanger unit, the outgoing air flow rate is different from the ingoing air flow rate.

12. The system according to claim 11, wherein said control means is a central controller unit.

13. The system according to claim 11, wherein said control means comprises multiple controllers, each controller arranged in connection with a heat exchanger unit.

14. The system according to any one of claims 11 to 13, wherein each heat exchanger unit is equipped with at least two temperature sensors :

- a first set of sensors (10,11) arranged to measure the interior temperature (T_int) , and the temperature (T_ext_out) of outgoing flow (5) , measured at the exterior of the building, and/or - a second set of sensors (12,13) arranged to measure the exterior temperature (T_ext) and the temperature (T_int__in) of the ingoing air flow (6), measured at the interior of the building.

15. The system according to claim 14, wherein each heat exchanger unit is equipped with said first and said second set of sensors (10,11,12,13).

16. The system according to claim 14 or 15, wherein the sensors (10,12) arranged for measuring the interior and exterior temperature are arranged in the air flow towards said heat exchanger units.

17. The system according to claim 14 or 15, wherein the sensors (10,12) arranged for measuring the interior and exterior temperature are arranged outside of the air flow towards said heat exchanger units.

18. The system according to any one of claims 11 to 17, wherein at least one heat exchanger unit is equipped with a means (9) for guiding the incoming air directly towards the vicinity of an opening (e.g. a door) between the area where said unit is installed and a neighbouring area of said building.

19. The system according to any one of claims 11 to 18, wherein at least one heat exchanger unit is installed in such a way that the ingoing flow path leads into a first area of the building, and the outgoing flow path leads out of a second area of the building, said first and second areas being separated by a wall.

20. The system according to any one of claims 11 to 19, wherein said control means is arranged for controlling said air flows according to the method of any one of claims 1 to 10.