Characteristics of the building
The considered building is used as office headquarters and its rooms are air conditioned by means of fan-coil units. Given its geographical location, during spring and autumn there is a considerable difference in temperature between early morning and central hours of the day, and that prompted the engineer to propose a plant with a four-pipe hydraulic distribution system, capable of feeding the fan-coil heat exchange coils with a choice of heated or cooled water. This choice also allows serving different environments with different exposure.
Temperature regulation function
The office automation is achieved through a KNX system and includes several functions, such as temperature control. For this purpose, two different ekinex devices are used:
- EK-EQ2-TP KNX thermostats equipped with single room temperature regulation function;
- EK-HC1-TP KNX actuators equipped with command actuator function for fan-coil units. These devices are capable of controlling the ventilation group as well as the electrothermal drives on the adduction valves of hot and cold fluids.
In order to ensure quiet operation and significantly reduce power consumption, the engineer chose to use fan-coil units equipped with BLDC (Brushless DC) electric motors controlled by an inverter drive directly mounted on the unit. This technical solution allows a continuous modification of the airflow by means of a 0-10 V control signal generated by the ekinex fan-coil actuator EK-HC1-TP. This choice guarantees an optimization of the response of the system when there is a variation of the thermal loads; this variation can be caused by either external (such as outdoor temperature or solar irradiation) or internal factors (such as people presence or electrical equipment heat dissipation) and is therefore compensated by continuously changing heating or cooling power.
Single room or zone temperature regulation
According to the division of spaces chosen by the architect, every room or zone is equipped with an ekinex room thermostat EK-EQ2-TP acting as temperature controller. The setpoint is set by a central supervision system connected tothe KNX network; it is possible to adjust the setpoint locally in a -3 … +3 K range in order to allow the people inside the room to adapt the actual temperature to their individual perception. Both the setpoint and the actual measured temperature are sent cyclically on the bus to feed the supervision page. The room temperature control is carried out by a PI (Proportional-Integral) controller integrated inside the room thermostat. Thecontroller is also equipped with two PWM (pulse width modulation) outputs for heating and cooling, separated from each other by a neutral zone. These outputs, through the fan-coil actuator, control the hot (V1) and the cold (V2) circuitvalve of the fan-coil unit.
The graph shows the connection between the control value received from the room thermostat and the command signal sent by the actuator to the inverter drive of the brushless motor.
The 4-pipe configuration allows both heated and cooled fluids to be constantly available in every room; the changeover between heating and cooling conductionmodes is automatic and based on users’ needs and internal conditions. For example, in the technical office with exposure on the west side, the system is set on heating during early morning, then automatically switches to coolingduring the central hours of the day.
In order to avoid the air to blow at an unpleasant temperature, both in heating and cooling mode, the fan-coil actuator does not start the ventilation groupuntil the fluid inside the heat exchange coil has not reached a proper temperature value. This function is useful during the first activation or after long inactivity periods and is carried out thanks to a temperature sensor (T1)mounted on the heat exchange coil of the fan-coil unit.
An open window during air conditioning causes a waste of energy; for this reason, contacts are used to detect if a window is open or closed. When a window is opened, the room thermostat causes the fan-coil actuator to automaticallyswitch to Building Protection operating mode, closing down the valves and stopping the ventilation group. This mode remains active as long as the window is open; when the window is closed, the actuator automatically returns to theprevious operating mode. A configuration parameter allows differentiating a short or a long opening; only the latter recalls the energy saving function.
While in cooling mode, condensation water collected in condensation basin is discharged by the activation of a discharge pump. When the water level inside the basin reaches a safety threshold, a level switch (C2) sends the warning to the fan-coil actuator and simultaneously a dedicated communication object changes state; this communication object is used in logical OR with other similar objects to activate a binary output giving permission to start the discharge pump.
Thanks to the presence of two digital outputs not used for temperature control, the device is also suitable for switching on/off two lighting devices (K1, K2).This opportunity avoids the need to install a binary output dedicated for lighting only and allows optimizing the overall cost of the automation system. In this case, the command is sent on the bus via ekinex FF Series pushbuttons.
All fan-coil units are fitted with a filter which holds the suspended dust before the air is blown inside the room; in order to make the periodic maintenance operations (i.e.: cleaning and replacing) easier, the fan-coil actuator is equipped with an operation hour counter which increments its own value when the ventilation is running. When the programmed time between two consecutive maintenances is reached, the device displays (both locally on the room thermostat display and centrally on the building supervisor) the need for filter replacing.