Controlco Blog

The challenge: increase the operational efficiency of a 3-story medical office building in Oakland.

The approach: apply outdoor air reset to the constant supply air temperature setpoint.

The problem: the 25-year-old staged-cooling rooftop unit is controlled by a simple Honeywell W7100 discharge air controller. A retrofitted DDC controller on the unit monitors and modulates a VFD added to the fan, but does not stage the unit or otherwise control the discharge temperature. The W7100 controller has remote setpoint terminals available, but requires a 0-500 ohm resistance.

The solution: with a 0-10Vdc modulating output available on the Auto-Matrix DDC controller, an ACI transducer (the DRN-3.1 voltage-to-resistance transducer) allows for remote control of the supply air temperature. The 0-10Vdc signal is scaled by the DRN3.1 across 0-500ohms, which the W7100 controller sees as 40-90F. Now the supply setpoint can be altered manually by a service technician, or can be altered automatically with an outdoor air temperature reset algorithm.

The results: the HVAC service contractor is happy with the increased functionality, and the customer will enjoy increased building comfort and energy efficiency, even from their aging AC unit.