HVAC Controls and Thermostats 3

Question 1

What is the typical main air supply pressure used in a pneumatic HVAC control system?

Accepted Answer

15–20 psi

Answer

Pneumatic control systems typically operate with a main compressed air supply pressure of 15–20 psi, which is used to actuate pneumatic controllers, actuators, and dampers.

Question 2

How does a Variable Frequency Drive (VFD) control motor speed in HVAC applications?

Accepted Answer

By varying the electrical frequency supplied to the motor

Answer

A VFD controls motor speed by varying the frequency of the AC electrical supply to the motor, following the affinity laws to reduce fan or pump energy at part load.

Question 3

What is two-position control in HVAC?

Accepted Answer

Simple on/off control where the output is either fully on or fully off

Answer

Two-position (on/off) control switches equipment fully on when the controlled variable falls below setpoint and fully off when it rises above, with no intermediate states.

Question 4

What is the purpose of Demand-Controlled Ventilation (DCV)?

Accepted Answer

Vary outdoor air ventilation based on actual occupancy using CO2 sensors

Answer

DCV uses CO2 sensors as a proxy for occupancy levels and modulates outdoor air ventilation accordingly, preventing over-ventilation of lightly occupied spaces.

Question 5

What steady-state control problem does the Integral (I) term in PID control eliminate?

Accepted Answer

Steady-state offset or residual error from setpoint

Answer

The integral term accumulates error over time and continuously adjusts the output until the steady-state offset is eliminated and the controlled variable reaches exactly the setpoint.

HVAC Practice Test

HVAC Practice Test

HVAC Controls and Thermostats 3