Room temperature control loop

When fume hoods open and close in a VAV laboratory, the resulting changes in supply air flow can disrupt the room temperature. The thermal power sequence is designed to reduce that disruption by calculating and applying a compensating change in the supply air temperature. The calculation is based on a constant room temperature, which is assumed to be the comfort heating setpoint.

In other modes, other temperature sequence applies and separate heating and cooling setpoints are supported.

Room temperature feedback

The room temperature PID output is a percentage that goes above and below zero. The value represents a heat transfer rate to the room by the supply air terminal.

Supply air flow control loop

Supply air temperature control loop maps the desired supply air temperature to the supply air temperature range configured for the terminal. It is transmitted to the terminal as percentage. The heating coil function maps that percentage back to temperature to calculate the supply air temperature setpoint. The supply air temperature PID calculates the valve opening needed to drive the measured supply air temperature to the setpoint.

Calculate supply air temperature heating and cooling setpoint

The discharge temperature setpoint comes from scaling the feedback value up or down according to the current supply airflow rate. Typically, it scales up because the actual flow is less than the maximum heating or cooling flow. If the flow rate is low, it takes a greater temperature difference to deliver the same heating or cooling power.

Calculate supply flow setpoint for cooling and heating

When cooling is required, the value of the temperature loop is negative and is limited by the supply flow minimum and maximum. The output cooling demand, in percent, is then calculated and mapped to the cooling airflow limits.

When heating is required, the value of the temperature loop is positive and is limited by the supply flow minimum and maximum. The output heating demand, in percent, is then calculated and mapped to the heating airflow limits.