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Control-side limitation of the air temperature after the electric heater
The control of the electric heater has always to take place so, that the air temperature after the
electric heater never exceeds the permitted air temperature in the unit (40°C, if not differently spec-
ified in the technical data). This item must be especially observed, when the air handling unit is
only operated with a partial air flow (e.g. in times of reduced use of the building).
As the heat output of an electric heater is provided very quickly and with ON-OFF operation in
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the full amount, arises a significant risk of overheating the unit, which results in damaging sev-
eral components, especially when the unit is operated with low airflows.
For this purpose, the air handling unit has to be equipped with a supply air sensor, which di-
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rectly measures and monitors the air temperature generated by the electric heater.
The control engineering must be used to ensure that the heating power of the electric heater is
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controlled so, that the temperature remains within the permitted air temperature in the unit.
Avoid overheating of AHU components by residual heat of electric heater
In order to avoid excessive heating of components by residual heat of electric heater control must
ensure that fan motor keeps on running for at least 5 min after cut off of electric heater! By the use
of an enabling contact (see Figure 121) the control engineering has also to ensure, that the elec-
tric heater can only start its operation when the fan is running.
If in case of main power failure (for example lightning stroke) this automatic run cannot be en-
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sured, AHU may become damaged by the residual heat of electric heater.
In order to avoid damages, an uninterrupted power supply is recommended.
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If AHU is not operated by an uninterrupted power supply after every main power failure an AHU
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inspection is requested as indicated in chapter 9 (Maintenance).
Safety concept
Electric heater power supply is to be equipped with 2 contactors in serial connection
The 2 safety thermostats must be connected such to protect the AHU both, by contactors in power
supply and by software of control. In case of failure contactors must immediately cut off electric
heater from power supply
The 2 safety thermostats are to be connected in serial connection
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The 2 safety thermostats are equipped with manual reset.
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After triggering the reason for stopping must be detected and eliminated before reset of the
thermostat!
Thermostat 1(Figure 118 and Figure 119)
Position of thermostat body: fastened on electric heater at connection side, accessible by re-
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moving the electric heater access panel.
Triggering temperature: pre-set – value must not be changed.
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Sensor position: between heating bars.
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Function: alarm stop in case of over temperature because of low airflow.
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Figure 118: Thermostat with
cover cap on the reset button
Thermostat 2 (Figure 120)
Cover cap on reset button
Reset button
Figure 119: Thermostat with
uncovered reset button
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