Controls and Thermostats Flashcards
7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Controls and Thermostats flashcards as text
What is the typical main air supply pressure used in a pneumatic HVAC control system?
Answer: 15–20 psi
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.
How does a Variable Frequency Drive (VFD) control motor speed in HVAC applications?
Answer: By varying the electrical frequency supplied to the motor
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.
What is two-position control in HVAC?
Answer: Simple on/off control where the output is either fully on or fully off
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.
What is the purpose of Demand-Controlled Ventilation (DCV)?
Answer: Vary outdoor air ventilation based on actual occupancy using CO2 sensors
DCV uses CO2 sensors as a proxy for occupancy levels and modulates outdoor air ventilation accordingly, preventing over-ventilation of lightly occupied spaces.
What steady-state control problem does the Integral (I) term in PID control eliminate?
Answer: Steady-state offset or residual error from setpoint
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.
What is the primary function of a Building Automation System (BAS)?
Answer: To centrally monitor and control multiple building systems including HVAC, lighting, and security
A BAS (also called BMS or EMS) provides centralized monitoring, control, and data logging for all major building systems, enabling coordinated operation and energy optimization.
Why do smart and WiFi thermostats typically require a 'C' (common) wire?
Answer: To provide continuous 24V AC power for their electronics, displays, and wireless communication
Smart thermostats require the C wire to complete the 24V AC circuit and draw continuous power for their microprocessors, touchscreens, and WiFi radios, which conventional thermostats did not need.