page 13
exchanger and the heating coil takes over the entire heat load. In that case, the heating coil may be
exposed to frosty air why a coil frost protection routine is needed.
Finally, in periods where the outdoor air is hotter than the discharge air, and cooling is needed in the zone,
the crossflow heat exchanger cools the supplied outdoor air and any additional needed cooling happens by
a cooling coil.
4.3.2.1
The air handling unit controller
An example of the structure of an AHU controller is also shown in Figure 11. The nomenclature of the
sensor and control signals are the following.
Signal name
Description
T
A
Supply air temperature
T
O
Outdoor air temperature
T
Z
Zone air temperature
T
S
Water supply temperature to mixing loop
T
R
Water return temperature from coil
T
F
Water supply temperature to coil
T
Z
reference
Reference for zone temperature
T
A
reference
Reference for supply air temperature
damper ratio
Control signal for amount of heat exchanger bypass
valve position
The position or rotation of a valve depending on settings in the valve motor, the
signal may be proportional to the opening degree
setpoint
The temperature setpoint of a control loop
on/off
On/off signal for a particular control loop
Table 4: Nomenclature for AHU controller (in figure 1).
The blocks of the AHU controller have the following functions:
•
zone temp controller:
Calculates the reference for supply air temperature bases on the discharged
air stream temperature and the outdoor temperature.
•
sequencing and defrost:
Decides which control loop runs and by which setpoint. Priority for air
heating is via the heat recovery system if such is present. It also controls defrosting of the heat
recovery system.
•
preheat temp controller:
Controls the air temperature leaving the heat recovery by setting the
damper ratio for the fresh air stream passing through the heat recovery system. For AHUs where
the heat recovery happens via a heat wheel, it sets the speed of the heat wheel.
•
coil preheat routine:
Purges the heating coil with warm water at startup before the fans are turned
on and the dampers towards outside air are opened. This is done for comfort reasons and to
prevent coil from freezing in cold conditions.
•
air temp heating:
Controls the air temperature leaving the heating coil by setting the
position/rotation of the valve of the mixing loop. Heating via the coil has second priority when a
heat recovery system is present in the AHU.
•
air temp cooling:
Controls the air temperature leaving the cooling coil by setting the
position/rotation of the valve of the mixing loop. Cooling via the coil has second priority when a
heat recovery system is present in the AHU.
4.3.2.2
Summary of control functions
In headlines the controller of the AHU does the following task (Air pressure and CO2 control excluded).
