A data center cooling system, comprising: a plurality of computer racks that support a plurality of heat-generating devices; a plurality of cooling units each arranged adjacent to a computer rack and comprising: at least one fan arranged to circulate air from an open front side of the computer rack adjacent a human-occupiable workspace through the rack and to an open back side of the computer rack adjacent the cooling unit; at least one cooling coil adjacent the open back side of the computer rack and arranged to receive heated air circulated from the computer rack and cool the heated air; a control valve configured to control a volumetric flow of cooling fluid circulated to the cooling coil; and a unit controller communicably coupled to the control valve and configured to modulate the valve based on: a variance between the actual approach temperature and an approach temperature set point of the cooling unit, wherein an approach temperature is a difference between a leaving air temperature of that particular cooling unit and an entering cooling fluid temperature of that particular cooling unit, wherein the unit controller (145, 608) is configured to adjust the approach temperature set point based on a signal representing a variance between the actual value of the open percentage of the control valve and a desired value of the open percentage of the control valve and to further modulate the control valve based on the adjusted approach temperature set point. a plurality of computer racks that support a plurality of heat-generating devices; a plurality of cooling units each arranged adjacent to a computer rack and comprising: at least one fan arranged to circulate air from an open front side of the computer rack adjacent a human-occupiable workspace through the rack and to an open back side of the computer rack adjacent the cooling unit; at least one cooling coil adjacent the open back side of the computer rack and arranged to receive heated air circulated from the computer rack and cool the heated air; a control valve configured to control a volumetric flow of cooling fluid circulated to the cooling coil; and a unit controller communicably coupled to the control valve and configured to modulate the valve based on: a variance between the actual approach temperature and an approach temperature set point of the cooling unit, wherein an approach temperature is a difference between a leaving air temperature of that particular cooling unit and an entering cooling fluid temperature of that particular cooling unit, wherein the unit controller (145, 608) is configured to adjust the approach temperature set point based on a signal representing a variance between the actual value of the open percentage of the control valve and a desired value of the open percentage of the control valve and to further modulate the control valve based on the adjusted approach temperature set point. at least one fan arranged to circulate air from an open front side of the computer rack adjacent a human-occupiable workspace through the rack and to an open back side of the computer rack adjacent the cooling unit; at least one cooling coil adjacent the open back side of the computer rack and arranged to receive heated air circulated from the computer rack and cool the heated air; a control valve configured to control a volumetric flow of cooling fluid circulated to the cooling coil; and a unit controller communicably coupled to the control valve and configured to modulate the valve based on: a variance between the actual approach temperature and an approach temperature set point of the cooling unit, wherein an approach temperature is a difference between a leaving air temperature of that particular cooling unit and an entering cooling fluid temperature of that particular cooling unit, wherein the unit controller (145, 608) is configured to adjust the approach temperature set point based on a signal representing a variance between the actual value of the open percentage of the control valve and a desired value of the open percentage of the control valve and to further modulate the control valve based on the adjusted approach temperature set point. a variance between the actual approach temperature and an approach temperature set point of the cooling unit, wherein an approach temperature is a difference between a leaving air temperature of that particular cooling unit and an entering cooling fluid temperature of that particular cooling unit, wherein the unit controller (145, 608) is configured to adjust the approach temperature set point based on a signal representing a variance between the actual value of the open percentage of the control valve and a desired value of the open percentage of the control valve and to further modulate the control valve based on the adjusted approach temperature set point.