The HAX-53R auxiliary plug in card allows control of the Auxiliary Relay on the HTC5 or HTC3 series controllers. This relay would normally be used for optional switching of the supply air fan but with the addition of the HAX53R it can be used as an extra Temperature control stage. Terminals 10, 11 & 12 on the controller can now be used as an extra stage of HEATING, COOLING or the control of the third compressor on triple compressor A/C units. The HAX53R has LED indication of Relay status and incorporates user adjustable Start Point and Differential potentiometers.