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84 VAV-PRC011M-EN

DDC Controls

of heating capacity to the heating load, resulting in more stable temperature control. The heating

minimum airflow setpoint is enforced during reheat.

The amount of reheat supplied is dependent on both the degree that the space temperature is

below the active heating setpoint and the time that the space temperature has been below the

active heating setpoint. If not already off, reheat de-energizes when the zone temperature rises

more than 0.5°F (0.28°C) above the heating setpoint.

The Stage 1 “on” time is proportional to the amount of reheat required. For example, when 50%

of stage 1 capacity is required, reheat is on for 90 seconds and off for 90 seconds. When 75% of

stage 1 capacity is required, reheat is on for 135 seconds and off for 45 seconds. When 100% of

stage 1 capacity is required, reheat is on continuously.

Stage 2 uses the same “on” time logic as stage 1 listed above, except stage 1 is always energized.

For example, when 75% of unit capacity is required, stage 1 is energized continuously, and stage

2 is on for 90 seconds and off for 90 seconds. When reheat is de-energized, the cooling minimum

airflow setpoint is activated. Caution: Care should be taken when sizing electric heaters. Discharge

air temperatures should not exceed between 100°F and 110°F, with a temperature between 85°F and

95°F being optimal for space temperature control. If too hot of air is delivered to the space through

ceiling-mounted diffusers, and then leaves the space through ceiling-mounted return-air grilles,

the buoyancy of this hot air will tend to cause some of the air to bypass from the supply-air diffusers

to the return-air grilles, resulting in uneven air distribution and possible comfort complaints. To

prevent stratification, the warm air temperature should not be more than 20°F (6.7°C) above zone

air temperature. (See Diffuser, “D”, section for additional application details).

Single-Duct: SCR Modulation of Electric Heat

SCR is a heat controller that controls a single stage electric heater proportional to an analog signal.

The analog output signal is proportional to the amount of reheat required. Typically with SCR heat,

the heater is turned on and off on a very short cycle time to provide proportional control of heat

output. This allows for closer matching of heating capacity to the heating load, resulting in more

stable temperature control.

When the space temperature drops below the active heating setpoint, the air valve open to the

Airflow Setpoint Reset Minimum Local Heat and the SCR controls the electric heater to maintain

space temperature at the active heating setpoint. SCR control is dependent on the degree that the

space temperature is below the active heating setpoint and the amount of time that the space

temperature has been below the active heating setpoint.

If the discharge air temperature reaches the Discharge Air Temperature Design Setpoint, the air

valve opens further and modulates between Airflow Setpoint Reset Minimum Local Heat and

Airflow Setpoint Reset Maximum Local Heat to maintain space temperature at the active heating

setpoint, while the SCR controls the electric heater to maintain discharge air temperature at the

Discharge Air Temperature Design Setpoint. If the air valve reaches Airflow Setpoint Reset

Maximum Local Heat, the SCR controls the electric heater to maintain space temperature at the

active heating setpoint, while the air valve remains at Airflow Setpoint Reset Maximum Local Heat.

Reheat de-energizes when the space temperature rises more than 0.5°F (0.28°C) above the heating

setpoint. When reheat is de-energized, the cooling Airflow Minimum Setpoint is enforced.

Ventilation Control

Ventilation control enhances the usability of Trane DDC controllers in more select applications that

require measurement of outside air (ventilation). Ventilation control is designed for use with

constant volume single-duct VAV units which modulate the primary damper and associated reheat

to maintain an average constant discharge air temperature. The reheat is modulated to provide

discharge air temperature consistent with AHU supply air temperature (typically 50º–60ºF). This is

critical to ensure that ASHRAE Standard 62.1 ventilation standards are attained, consistently

maintained, and monitored. When connected to a Tracer™ building automation control system,

trend logging, remote alarming, etc. is available. In fact, the Trane Tracer Control System can

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