New HVAC26A, the CA20 replacement

Original APAC CA20

HVAC26A controller is an advanced microprocessor based controller for 2 stage air conditioning units. It is a direct pin and size compatible replacement for the CA20 and comes preloaded with compatible control software. The use of latest FLASH based microprocessor technology has meant that additional functionality has been included whilst keeping costs down. This has also resulted in a design utilising fewer components, improving reliability.

To reduce wiring, all but the compressors are directly controlled by on board relays and most connections are plug in for ease of manufacture and service. As much of the control wiring as possible has been included on the board to further reduce manufacturing labour.

To prevent incorrect wiring, all connectors on the PCB are have differing numbers of pins or connector style, hence no two connectors are the same.

The controller is part of an intrinsically safe overall system. The safety interlock sensors/switches act directly by interrupting the 24 VAC power to the various control outputs, regardless of the microprocessor control. A relay on each stage bypasses the low-pressure switch contact to prevent nuisance fault tripping during defrosts. On-board fusing is also provided for both the control circuits and the thermostat 24 VAC supply.

As an improvement on the CA20, the HVAC26A has an integral 2 line by 16 character LCD display that is programmed to show operating status information and fault status. Two buttons provided on the board allow service personnel to access status and fault history information. Faults are remembered, even if the power to the controller is interrupted and can be read out by a technician to aid in fault finding. The fault history can also be cleared so that only the latest fault history is available at the next service visit.

The board has four option links to allow selection of the defrost time, enable lead/lag operation and enable system speed up to aid in fault finding.

In addition, the board has an expansion port to allow for future expansion interfaces, such as a BMS interface, data logging, internet access, extra I/O etc.

The HVAC26A has the following features:

• Direct acting fault chain, independent of the the electronic controls
• Electronic sensing of all fault chain faults
• On board LCD display for displaying unit operating status, faults and out door coil temperatures
• Outdoor coil temperature sensor fault detection
• Last 4 fault trips are retained in non volatile memory for readout by service personnel (fault log can also be erased)
• Low standby power
• High quality, fault tolerant, over specified relays ensure long service life
• On board fusing of all external circuits including thermostat circuit
• Adjustable defrost times
• Defrost sensor short/open circuit detection.
• Lead/Lag operation
• Speed up link
• MODBUS communications
• Expansion BUS for add on peripherals
• Operating software is field upgradable
• Custom variants of operating software can be accommodated

For menu type applications, the user can choose to edit and store the operating parameters in the UI or on the master controller.

We have published some guides to help with troubleshooting this controller:

Block Diagram | Troubleshooting | Operating Manual

The software version in the controller determines its behaviour. Both CA20 and HVAC26A controllers show the software version number when power is switched on. If you need an identical replacement, take note of the version number that's displayed then order one of the units listed below. They all have the same price.