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Outer room temperature control loop
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Air inlet temperature control loops via heat recovery
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Setpoint for air inlet temperature control with coil
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Coordinating balance between heat regeneration and heat injection with mixing loop
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Defrosting of heat recovery system
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Local coil control loops (controls valve position)
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Safety functions:
o
Coil preheat in cold conditions
o
Coil frost protection
o
Valve anti stick functionality
o
Valve position monitoring
4.3.3
Signal diagram of AHU with MIXIT configured with the heating coil application
Integration of MIXIT with an AHU controller can be done with a few or more datapoints, depending on the
level of required reliability around the control and the number of features in MIXIT that are used.
Figure 12 shows an integration corresponding to the one in Figure 11, where only start/stop signals and
setpoints are sent to the MIXIT unit. There is no read back of the set datapoints. The object numbers are for
BACnet but the corresponding Modbus register are listed in Table 5.
If MIXIT is in the coil preheat state or the in frost protection state the “
Temperature control enabled
”
at
BI,1 and which is also a physical relay signal is not set, indicating that MIXIT is in a different state than
normal temperature control. Hence, appropriate action may be taken regarding the fans and dampers
setting, for example to help mitigate frost damage of the coil. The frost protection in the figure happens by
a temperature switch with manual reset. This can be used exclusively or alongside the build in frost
protection function in MIXIT.
Figure 13 shows the same system, but where setpoints and on/off signals are read back, and the flags for
finished preheating of the coil (BI,1) and frost risk detected (BI,7) are used for appropriate sequencing with
the rest of the AHU control, e.g. Figure 9 see about a possible implementation.
The read back of the set datapoints has the advantage that the AHU controller can check that the set
datapoints were registered by MIXIT. An alternative is to send all datapoints with a fixed interval as it is
unlikely a transfer will fail continuously, but this gives no assurance that integration has happened
correctly.
In the figure shows the use of BO,4 as a way of signaling frost risk to MIXIT via BACnet. Again the Mobus
registers corresponding to the BACnet example in the figure are listed in Table 5.
4.3.3.1
Use of air temperature sensor
In both Figure 12 and Figure 13 a signal splitter at the air temperature is shown. This may be avoided by
connecting the air temperature sensor only to MIXIT and reading the air temperature for the AHU
controller via MIXIT, e.g. see Figure 5. In a future software release it will be possible to send the air
temperature to MIXIT via the fieldbus, hereby avoiding physically to connect an air temperature sensor to
MIXIT.
For the current software version MIXIT can be integrated with the AHU controller without the air
temperature sensor by using the radiator heating application as sketched in section 4.4.
