Thermal behaviour of the air inlet
Manage the temperature at the sensor level for the inlet to work in any season
The position of the shutter of the humidity sensitive air inlet is directly determined by the value of the relative humidity measured at the level of the sensor. However this value can be different from the one measured in the centre of the room in which the air inlet is installed, because for the same absolute humidity, the relative humidity varies according to the temperature. It is then easy to understand that the temperature at the level of the sensor of the air inlet has an essential impact on the relative humidity read. It is thus fundamental to control this temperature, in order to control the position of the shutter whatever the climatic conditions (indoor and outdoor temperature, indoor and outdoor relative humidity) and to adapt it directly to the indoor emission of humidity.
The importance of a "good" thermal coefficient
The thermal coefficient CT is determined by the formula: Tsensor = CT x (Tindoor - Toutdoor)
Where T = temperature in oC
Several years of research made it possible to determine and control an ideal value of coefficient CT. With a coefficient CT between 0.25 and 0.32, the Aereco humidity sensitive air inlets offer a great amplitude of modulated flow for every season, and are ready to react to the lowest emission of interior moisture. In winter, in an unoccupied room where the relative humidity (RH) is low, the shutter is in closed position, but it is ready to react and to open for any emission of humidity. More than 25 years of experiment enable Aereco to obtain a perfect control of the industrial reliability and perenniality of this thermal coefficient.
Consequences of a too high thermal coefficient
With a higher thermal coefficient (TC > 0.32) due to a bad sensor insulation against the outdoor air, the sensor temperature would be too low in winter. So it would read a too high humidity rate which would generate a too high air inlet opening, even when the humidity rate is low in the room. Its modulation capacity would be then very reduced, even null. That would increase the thermal losses, in particular by the crossing flow, and would decrease the air quality in the occupied rooms (airflow would not be distributed according to the needs). In addition, a too low sensor temperature would cause a big increase of the hysteresis (difference between both ways of the opening-closing/humidity curve), which then would not make it possible to determine a unique shutter position for a known relative humidity rate (see example on last graph).
Comparison of various products according to standard EN 13141-9
EN 13141-9 standard, which defines the humidity sensitive air inlets test method, requires measurements known as isothermal (indoor and outdoor air with the same temperature) and non isothermal (colder outdoor air) in order to evaluate the thermal coefficient impact on the operation of the humidity sensitive air inlets. As shown on the graph below, the Aereco humidity sensitive air inlets curve (model presented: EHA2 5-35) is simply shifted towards the left (lower relative humidity needed for the opening) when the outdoor air temperature is lower (10oC, blue curve). This way, a great modulation amplitude and a similar operation to isothermal operation (grey curve) is kept even with the low average indoor relative humidity in winter:
When the air inlet thermal coefficient is too high (badly isolated sensor) as it is the case in the test of the humidity sensitive air inlet hereafter (other brand), the non isothermal operation at outdoor temperature of 10oC (blue curve) does not allow anymore real airflow modulation according to the indoor RH. The sensor temperature is too low and this increases the RH it reads in comparison with the actual RH at the centre of the room. The air inlet is therefore almost permanently open, and does not control anymore the airflow. This phenomenon is accentuated as the outdoor temperature decreases.
