Typical Piping Arrangement for Direct Return System

Unit Operating Limits 

Airflow

The minimum recommended airflow for proper system staging and temperature control is 35% of nominal airflow. However, airflows below this point can be utilized. Use appropriate minimum settings on the VAV boxes to ensure that the self-contained unit is not operating in a surge condition. Continuous operation in a surge condition can cause a fan failure. Check with your local office for confirmation of acceptability of low airflow operating conditions.

Fixed pitch sheaves are used on the Trane unit. Air balancing adjustments are accomplished by obtaining alternate fixed pitch sheaves available from the factory.

Waterflow

Use 3 gpm/ton for maximum unit capacity and efficiency. However, 2.5 or 2 gpm/ton may yield significant pumping energy and piping cost reductions. These reduced water flows may have only a 1-2% impact on the self-contained unit capacity and efficiency. Specific waterflow limits are included in the data tables in this catalog.

Sensors — Location And Wiring Requirements Static Pressure Sensor

Locate the static pressure sensor about two-thirds to three-fourths of the way down the longest duct run. Stay approximately 10 duct diameters downstream and 2 duct diameters upstream of any major interferences, turns or changes in duct diameter.

Pneumatic tubing lengths between the sensor and regulator should not exceed 250 feet for 1/4-inch OD or 500 feet for 3/8-inch OD tubing.

Choose a static pressure set point at the lowest level that meets customer/code requirements for air quantity and air quality. A lower static pressure setpoint reduces fan energy requirements and improves acoustic performance.

Wiring Requirements For Self-Contained Optional Sensors

Wiring requirements for the zone temperature remote reset, supply air  reset, and morning warmup optional sensors are all NEMA CLASS Il. This is economical low voltage (30 volts maximum) wiring.