Reversing Changeover for Heating & Cooling | rickardair

Reversing Changeover for Heating & Cooling

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The RICKARD Reversing Changeover facility ensures that the VAV diffuser controls the temperature accurately in both heating and cooling mode i.e. when the central system is supplying either warm or cold air.


When the system switches from cooling to heating mode, the changeover sensor detects the increase in supply air temperature and switches the direction in which the actuator operates.  This means that when the system is in cooling mode, the diffuser will drive open as the room temperature increases, whereas in the heating mode the diffuser will close as the room temperature increases.


Converting a standard RICKARD master diffuser to incorporate changeover functionality is as simple as plugging in the supply air/changeover temperature sensor. Every master controller is pre-activated.  This temperature sensor must be fitted in such a way that it senses the primary air temperature being supplied to the diffuser.

NOTE: Slave diffusers receive a control signal from the master diffuser and therefore do not require nor should they be fitted with a changeover sensor. It is also important that a slaves changeover sensing is turned off on the MLM application. Failure to do so will result in a zone not operating correctly. Only one changeover sensor should be activated per zone i.e. the master.

If a re-heater is fitted to the neck of the diffuser, care must be taken to ensure that, the changeover sensor is installed in such a way that it is not affected by radiant heat from the heater. Every changeover sensor is labeled “Please install supply air sensor max distance from diffuser” to ensure this.

The controller compares the primary air and set point temperature. Whenever the supply air temperature exceeds the set point temperature by one degree Celsius, the control action is reversed and switches to heating mode. Cooling mode is re-instated when the primary air temperature falls one degree Celsius below set point temperature.


Should a VAV air diffusion unit be fitted with a re-heater, the heater will be proportionally energized between 0.5°C and 1.5°C below setpoint temperature, regardless of which mode the controller is in.  Effectively, a re-heater will only be energized at Minimum Supply Air Status in the cooling mode and at Maximum Supply Air Status when in the heating mode. This control logic is extremely energy efficient from a Green Building perspective.