Ventilation and Heating
Ventilation and Heating
This system is created with the Unit Ventilator zone forced air piece of equipment. A separate unit heater, including air handler and heating coil, is located in each room. The design heating supply air temperature is assumed to be 95 F unless overridden by user input on the zone sizing tab of the system properties.
The supply fan follows the main heating fan schedule in a time-clock fashion, bringing in outside air through the main heating coil according to the ventilation schedule.
When the room drift temperature rises above the room heating thermostat, the heating coil is de-activated, allowing the space temperature to drift upward. Since the supply air will be at the return/outside air dry bulb temperature (plus fan heat), scheduling outside air into the space will temper this effect to some degree.
When the room drift temperature drops below the room heating thermostat, the heating coil is modulated to produce a supply air dry bulb that will bring the room temperature up to the heating thermostat temperature.
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 is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main heating 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. Supply Fan Controls. 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 heating coil to modulate at a higher supply air temperature during the percent of the hour the fan is available. 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.
Application Notes
● If the ventilation air is brought in through a separate ventilation system, a zone direct DOA should be used. Economizer controls are not available to the standard UV system; however, opening and shutting of windows can be simulated (even if there are no external walls) by specifying a dry bulb economizer type and scheduling the economizer available during occupied hours.
● If the heating fan is to be cycled during unoccupied hours, enter fan cycling on the availability manager tab of the system properties.
● Internal rooms (rooms with no walls, roofs, etc.) should be provided with adequate outside air if internal loads are scheduled; otherwise, the room temperature may go out of control.
● Since the ventilation air brought into the space will create "positive" building pressure, it is suggested that infiltration be scheduled opposite the outside air schedule, e.g., for outside air scheduled at 40% for a particular hour, the infiltration schedule should read (100-40) = 60%.