Terminal Reheat
Terminal Reheat
The design is peak air and peak cooling capacity.
This system is composed of a central, constant volume fan located on the cold deck which delivers conditioned air at a constant cooling supply air temperature to each room's terminal unit. A reheat coil is located in each room's terminal unit to reheat the supply air when necessary. The return air is drawn from a common return air plenum; the perimeter and the interior rooms return air is assumed to be uniformly mixed before being seen by the central cooling coil.
System Simulation
A constant volume of air is delivered at a constant cooling supply air temperature to each room regardless of what the room temperatures are this hour. The admittance of this constant stream of cold air will depress the room temperature downward in the rooms not at their design cooling condition.
If the drift temperature falls below the heating thermostat in a particular room, the room reheat coil is activated to maintain the room temperature at the heating thermostat setpoint for that hour. Since the cooling coil is not controlled by the cooling thermostat (unless a supply air reset control is specified), it is advisable to lock cooling out rather than setting the cooling thermostat up during periods of non-operation.
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply fan is controlled in time clock fashion, i.e., if the schedule reads 80% for a particular hour, the fan will operate at full rpm for 80% of the hour and remain off the rest of the hour. Since the supply fan delivers conditioned air to the space, reducing the percent it can operate during a given hour will cause the system to possibly lose control of the room temperature and humidity. If the fan is not allowed to operate long enough, the room temperature will be allowed to drift, the amount dependent on the magnitude of the unmet space load.
3. Specifying a supply air reset control will cause the cooling supply air temperature to be reset upward based on the worst case cooling room. This, in turn, will cause the room relative humidity to drift. The heating supply air temperature cannot be reset since the reheat coil must respond to the room thermostat. The preheat coil will, however, shift it's control point upward proportional to the amount that the cooling supply air temperature is reset upward.
4. Reset Schedules. Only the cold deck may be reset by outside air reset schedules. Unless a reset schedule is input by the user, the program will assume a constant cooling supply air dry bulb for all outside air conditions.
Application Notes
● This system, when used without a supply air reset control, will provide the best humidity control at part load when compared to all other standard system types. The trade-off is that it will also consume the most energy.