4.7.4 2-pipe fan coil unit with electric heater
In 2-pipe applications with electric heater, the thermostat controls a valve in
heating/cooling mode with changeover, heating only, or cooling only plus an
auxiliary electric heater.
Cooling only is factory-set (P001 = 1) with enabled electric heater (P027).
In cooling mode, the valve receives an Open command if the acquired temperature
is above the setpoint.
The electric heater receives an On command if the acquired room temperature
drops below "setpoint" minus "dead zone" (= setpoint for electric heater) while the
electric heater is enabled (P027 = On).
"Setpoint for electric heater" is limited by parameter "Maximum setpoint for Comfort
mode" (P016).
In heating mode, the valve receives an Open command if the acquired temperature
is below the setpoint. The electric heater is used as an additional heat source when
the heating energy controlled by the valve is insufficient.
The electric heater receives an On command, if the temperature is below "setpoint"
minus "setpoint differential" (= setpoint for electric heater).
Remote enabling/disabling of the electric heater is possible via input X1, X2 or U1
(RDG2..KN)/X3 (RDG2..T) for tariff regulations, energy savings, etc..
Input X1, X2, or U1/X3 must be commissioned accordingly (P150, P153 and
P155). See Multifunctional input, digital input [➙ 115].
The electric heater can also be enabled/disabled via bus. (RDG2..KN)
Do not assign the function to a local input X1, X2 or U1 if "Enable electric heater"
input is used via bus.
● With a DC 0...10 V fan, On/Off control for the electric heater can be selected by
setting P203 = 4. The electric heater must be connected to outputs Q2
(RDG26..KN), Y2 (RDG20..KN).
● The electric heater starts with a delay of 15 seconds, to ensure the fan supplies
sufficient air flow to dissipate the heat (also applies to applications with DC
control of the electric heater).
● CAUTION! If the fan is disabled, the electric heater is not influenced
and may still run.
● To avoid overheating of the electric heater, the thermostat guarantees at least
fan speed medium (Auto fan speed: value in the middle of Vmin (P357) – Vmax
(P359), manual fan speed: P358) if the electric heater needs to be energized.
We generally recommend controlling the electrical heater via one external relay.
This applies when the application is covered by RDG20..KN (max current output on
the triac is 1 A), but also for application with RDG26..KN where the current is lower
than the max load supported by Q2.
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