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1.
The water supply temperature from the boiler to MIXIT is above 50 degrees C.
2.
If there is a sensor error whereby the appropriate preheating cannot be determined by sensor
values.
The time out value is 1200 seconds. If this value shall be changed Grundfos service needs to be contacted.
If the supply temperature is below the 50 degrees C and the coil could not be preheated inside the
predefined time limit, MIXIT will remain in the preheat state as this could be a sign of a missing heat supply,
and frost damage could occur at the coil if the fans and dampers are released anyway.
4.3.2
Signal diagram of AHU
Depending on local geographical requirements, the topology of the AHU may be of a different design from
case to case. For example, heat recovery may happen using regenerator wheels if there are not large
requirements for keeping clean and dirty air streams separated. If there is a nearly constant requirement
for dehumidification and economizer loop may be fitted to the AHU etc.
Common for all is that heat recovery is needed and there are coils to control the temperature leaving the
AHU. It is for this temperature control MIXIT can be used.
In Europe air handling units often have a structure like the one shown in Figure 11. In the figure sensors and
control loops relevant for control of the air temperature leaving the AHU are shown, and sensors and
control loops relevant for controlling the pressure have been omitted.
Figure 11: Air handling unit with control blocks.
The air handling unit in Figure 11 is of a type where heat recovery happens in a crossflow heat exchanger
with separated air streams. Thus, the air supply from the AHU into the zone is entirely fresh air that is
preheated in a crossflow heat exchanger. The amount of preheat is controlled by operating the bypass
dampers by which the amount of outdoor air though the heat exchanger is controlled.
In scenarios where the heat recovery cannot deliver sufficient heat, the heating coil of the AHU is operated
to deliver additional heat to the outdoor air stream. Likewise, heating via the heating coil is needed if the
crossflow heat exchanger needs defrosting. In that case, the outdoor air is bypassed around the crossflow
