Since the zone becomes too warm, a controller is required that disables the heater when a setpoint is reached. We will implement a hysteresis controller with a setpoint of 295.15 +/- 1K (21-23°C). A temperature sensor will measure the zone air temperature.
The heater modulation level should be set to 1 when the heater is on and to 0 otherwise. Furthermore, the pump should only circulate water when the heater is on.
The figure below shows the air temperature after the controller is added.