Sensor Installation Handbook Preface 3rd edition - Siemens ...

Practical tips
84
Basic circuit:
If the air temperature downstream of the heat exchanger falls below the
preset temperature, then:
■ the supply/extract fans switch off
■ the outside air/exhaust air dampers close
■ the heating coil valve opens
■ the internal pump switches on
■ the frost alarm trips.
The frost alarm is self-locking, to prevent the system from being switched
on again automatically by the frost thermostat. The alarm must be acknowledged
locally (remote acknowledgement is prohibited).
To prevent overheating inside the unit, the heating coil valve is regulated
by the frost protection thermostat while the frost alarm is active.
Tips for avoiding nuisance frost alarms:
The main cause of a frost alarm is a drop in temperature. It is possible, how -
ever, for a frost thermostat to trip even when the heat supply is adequate.
This may be due to one of the following circumstances:
a) Changes in load, such as a change from one fan-stage to another,
or a heat recovery system coming on/off load
b) Temperature stratification
c) System start-up after a shutdown
To prevent the frost thermostat from tripping in situations where the temperature
is adequate, the following basic circuits have proven effective:
■ Preventive frost protection thermostat
■ Stabilised start function
■ Setpoint-regulated start-up control