Capital Planning & Budget

hvac

 

Reheat System Control

Constant volume reheat systems are known to be "energy wasters" because of the concept of total cooling at the air handling unit (source) and, depending on space conditions, total reheating with the use of duct mounted reheat coils in all branch ducts year round.

Although desirable to turn off reheat pumping systems during the cooling (summer) season, certain space requirements and high humidity conditions sometimes do not permit this strategy, and require the "standard" continuous 55 degree F discharge temperature setpoint with reheat and dehumidification control. Spaces where this may be necessary include Library Archives, Music Instruments, Scientific Instruments, etc.

With the introduction of direct digital controls, however, reheat energy consumption can be reduced while still operating such a system.

Digital control reset can adjust the supply air cooling coil discharge to a higher level condition to minimize reheat requirements. Here are two methods to accomplish discharge air reset:

  1. Room control - Providing there is a sensor located in each room served by a reheat coil, cumulate all room sensor signals and allow the coolest room to reset the discharge sensor within reasonable limits e.g. 62-69 degrees F. The range of readjustment must not exceed requirements of the room with the greatest cooling requirements. This same principle can be applied to a pneumatic control system.
  2. Return air control - Provide a single sensor in the return air duct which will, in effect, be sensing the "average" temperature of all rooms served by reheat coils. This single sensor will reset the discharge sensor within reasonable limits e.g. 62-69 degrees F. The same principle can be applied to a pneumatic control system.

With digital control systems as they relate specifically to computerized Energy Management Systems (EMS) or Campus Automation Systems (CAS), additional strategies can be employed to further reduce the consumption of energy.

During the entire year, it stands to reason that if an air handling system is OFF, then the reheat pump can also be OFF since there is no flow of air through the reheat coils to transfer the heat to. Requirements include start/stop points for each air handling system fan motor and for the reheat pump motor. Through software, simple interlocks can be written so that whenever the air handling systems are OFF, the reheat pump will be OFF. Chances are good that the air handling units already have start/stop control points, therefore the relatively simple addition of a reheat pump motor start/stop point is the only requirement to accomplish this strategy.

There are other factors which can be worked into the "equation", for example:

  1. Most often the reheat system water temperature is controlled at a constant setpoint e.g. 180 degrees F. It is possible, with additional sensors and controllers, to reset this water from outside air temperature with a low limit, e.g. OA = -20 degrees F, HWS = 180 degrees F (and) OA = 60 degrees F, HWS = 130 degrees F.
  2. There may be time periods during the spring and fall seasons when the reheat pump can be OFF even though the air handling systems are ON. Requirements for this strategy are more involved. For example, if the outside air temperature and humidity are known, as well as the indoor temperature and humidity of each building, then the enthalpy (the measure of heat content) difference can be calculated to determine if a reheat pump could be ON or OFF. This enthalpy measurement however, is more suitable for mixed air-economizer control strategies to be covered in a separate writing.