» Air Conditioning Unit Changeover Control - The ACX2

The ACX2 auto changeover unit is designed for air conditioning applications where two air conditioning units are being used on a duty/standby basis to provide cooling for technical areas such as server rooms and computer rooms. A/C units are frequently sized on a 'run' and 'standby' basis allowing continued operation in the event of run unit failure, or to provide extra cooling on very hot days. This requires that an operator is aware that the run unit is faulty, or that the temperature is rising, and that the standby unit needs switching on manually. If the area is unmanned, this could be a problem.

The ACX2 auto changeover is a small compact and economical electronic changeover unit receiving 'fault inputs', from both air conditioning units and from an optional independent over temperature sensor. The ACX2 interfaces with each AHU's start/stop circuit or fire shutdown link via its no-volt interface relays and controls both AHUs, allowing the run unit to operate, while holding the standby unit locked-out.

The selection of which is the 'run' and which is the 'standby' unit, and for how many hours the 'run' unit should operate before a duty share cycle changeover occurs, is user programmable. The 'run' unit operates until the changeover (1 to 999 hours), at which point the ACX2 stops the 'run' unit and starts the 'standby' unit. This air handling unit in turn runs until the changeover point is reached again, when the cycle repeats.

In the event of a fault with the duty unit, the ACX2 automatically stops the duty AHU and starts the standby AHU. If however the standby unit also becomes faulty, both AHU units are instructed to operate to provide cooling while waiting for repair (as the fault could be minor; filter blocked). In the event of a fault or power failure to the ACX2 unit, both relays failsafe to closed, allowing both air conditioning units to be started and run manually.

To provide additional temperature protection, the ACX2 interfaces with an optional remote temperature sensor. If the sensor detects a temperature rise above the pre-set level, the ACX2 instructs both A/C units to start running to give maximum cooling to the room. A non-volatile memory stores the 'time until next changeover' and the 'selected time between changeover'. In the event of a power failure, the 'time to next changeover' continues running from where it stopped at the point of power failure to ensure that 'even time running' continues.