is a control scheme that allows an airside system to modulate the supply air temperature based on outside air temperature, worst-case room demand, or a combination of the two. When enabled, the temperature of supply air is increased, which allows for reduced compressor energy or reheat energy, but also increases fan energy in a VAV system. When supply air temperature reset is based on outside air temperature, the supply air temperature can be increased as the outside air temperature decreases, allowing for more economizer operation. This is done in a linear fashion as shown in Figure 5 13. Figure 5 13 based on outside air temperature 62 supply-air- temperature setpoint, F 61 60 59 58 57 56 55 54 40 50 60 70 80 90 outdoor dry-bulb temperature, F can also be based on the worstcase space load demands. If the worst-case space does not require conditioned air at the design supply air temperature, the temperature can be reset upward. 5 50 System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN
The two individual strategies can also be combined so that the supply air temperature is reset based on both outside air temperature and worst case room (Figure 5 14). Figure 5 14 based on worst case room and outside air 65 64 63 62 supply-air- temperature setpoint, F 61 60 59 58 57 56 55 54 53 52 supply-air-temperature reset by demand 40 50 60 70 80 90 outdoor dry-bulb temperature, F Application considerations Cooling loads that are being served by higher supply air temperatures often require more air and increased fan energy. When the outdoor air temperature is less than the supply temperature, the compressors can be shut off and an airside economizer can provide the needed cooling. The use of warmer supply air can result in higher space humidity levels, especially during humid weather. Related reading Rooftop VAV Systems Applications Engineering Manual (Trane literature number SYS-APM007-EN) CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 51
Sample scenarios Table 5 1 Sample Scenarios Scenario 1 (page 52) Scenario 2 (page 53) Scenario 3 (page 53) Topic based on worst case room based on outside air temperature based on outside air temperature and worst case room loads Note: For further discussion of creating utilization schedules, refer to Creating schedules on page 6 120. For further discussion of creating alternatives, refer to Creating alternatives on page 6 114. Scenario 1. based on worst case room. 1 In Create Systems, select the Options tab for the system to which supply air reset is to be applied. Click the Advanced Options button. 2 Select a utilization schedule that accommodates supply air temperature reset. A custom schedule can be applied. 3 (Optional) Enter the maximum reset temperature difference. Note: When the utilization schedule reads 1% or greater, supply air temperature reset will be enabled. 5 52 System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN
Scenario 2. based on outside air temperature. See Figure 5 13 for a graph of this strategy. 1 Uncheck Use system default outside air reset to input a new reset schedule. 2 Enter the appropriate values for the air reset schedule. Scenario 3. based on outside air temperature and worst case room loads. See Figure 5 14 for a graph of the strategy. 1 Select an appropriate utilization schedule or create a custom schedule. 2 Uncheck Use system default outside air reset to input a new reset schedule. 3 (Optional) Enter a maximum reset differential. 4 Enter the appropriate values for the air reset schedule. Additional items 1 TRACE will not use the supply air temperature reset fields during the design simulation. CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 53
2 Do not specify a reset supply air temperature lower than the design supply air temperature. 3 System types that have room-controlled cooling coils controlled via the room thermostat cannot use supply air temperature reset. Examples of these systems include, but are not limited to: single zone, fan coils, packaged terminal air conditioner, and watersource heat pump. 5 54 System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN