DRAWING TWO ORIGINAL DESIGN The original design logic was typical of many systems designed and installed in the past sixty years. A return air controller modulated the heating valve (V1) and the chilled water cooling valve (V2) in proper sequence as the means of controlling the occupied space temperature. The heating valve closed completely before the cooling valve allowed any chilled water flow through the cooling coil. This part presented no problem relating to the system logic. A mixed air controller modulated the mixing dampers (D1) and (D2) to maintain a constant 58°F upstream of the heating coil. An outside air high limit controller overrode the mixed air controller, preventing free cooling when the outside air temperature exceeded 70°F. A minimum positioning relay (MPR1) assured at least minimum ventilation when the fan system was active. The outside air high limit could be upgraded to an enthalpy function, comparing the total cooling load of the outside air to the total cooling load of the return air. This loop selects the air stream with the lesser load reducing the workload on the cooling coil. The minimum positioning relay could be upgraded to a carbon-dioxide sensor or VOC (Volatile Organic Compound) sensor better addressing the occupied space contaminant load. The mixed air design caused the "free cooling" to be active all the time the fan ran. The 58°F mixed air design intended to address the free cooling requirements when the space required its maximum amount of cooling. At all other times heat had to be active to compensate for the over-cooling of the mixed air. This is similar to opening the windows in your house and then turning on your furnace to compensate for the cold air coming through the windows. If you just closed the windows, the furnace would be required at a lesser rate. Drawing two illustrates the overlap of free cooling and heating. The total BTU quantity of unwanted free cooling had to be matched with an equal BTU quantity of heating only to address this problem with the control logic. Additional heat was required to address the normal building heat losses of the occupied space. 8.70
RkJQdWJsaXNoZXIy ODY1ODQy