AHU controllers were used extensively for controlling air handling units, especially for complex units requiring more I/O interfaces than the UNT. Many FX-PC controller models can replace an AHU, but we recommend using the FX-PCA3611 because of its onboard I/O complement. If the FX-PCA3611 controller does not have enough onboard I/O to meet the replacement application requirements, then add FX-PCX Expansion I/O Modules.