HVAC Controls and Thermostats 1 — Questions and Answers
Question 1: What type of thermostat uses a bimetallic strip as its primary sensing element?
- Electronic thermostat
- Pneumatic thermostat
- Mechanical thermostat (Correct answer)
- Digital thermostat
Correct answer: Mechanical thermostat
Mechanical thermostats use a bimetallic strip that physically bends with temperature changes to make or break electrical contacts.
Question 2: What is the 'deadband' in thermostat operation?
- The maximum allowable temperature swing
- The temperature range between heating and cooling setpoints where neither system operates (Correct answer)
- The time delay before a system restarts
- The differential between supply and return air temperature
Correct answer: The temperature range between heating and cooling setpoints where neither system operates
The deadband is the neutral zone between heating and cooling setpoints that prevents simultaneous heating and cooling operation.
Question 3: What does DDC stand for in building automation and HVAC controls?
- Dual Digital Control
- Direct Digital Control (Correct answer)
- Dynamic Data Control
- Distributed Device Control
Correct answer: Direct Digital Control
DDC (Direct Digital Control) uses microprocessors to directly monitor and control HVAC equipment using digital signals and algorithms.
Question 4: Which control type continuously modulates output between 0% and 100% in proportion to the deviation from setpoint?
- Two-position control
- Floating control
- Proportional control (Correct answer)
- Step control
Correct answer: Proportional control
Proportional control continuously varies output based on how far the controlled variable deviates from setpoint, unlike simple on/off control.
Question 5: What is the primary purpose of a heat anticipator in a mechanical thermostat?
- To speed up the heating cycle
- To prevent temperature overshoot by shutting off the furnace slightly early (Correct answer)
- To increase system energy efficiency
- To lower the heating setpoint automatically
Correct answer: To prevent temperature overshoot by shutting off the furnace slightly early
The heat anticipator pre-heats the thermostat's sensing element to signal the furnace to shut off before the setpoint is fully reached, preventing overshoot.
Question 6: In HVAC controls, what does the term 'setpoint' refer to?
- The maximum temperature the system can achieve
- The desired target value that the control system works to maintain (Correct answer)
- The outdoor design temperature for load calculations
- The pressure differential between supply and return ducts
Correct answer: The desired target value that the control system works to maintain
A setpoint is the target value — usually temperature — that the control system continuously works to achieve and maintain in the conditioned space.
Question 7: What type of sensor is most commonly used in modern electronic thermostats to measure temperature?
- Bimetallic strip
- Mercury bulb
- Thermistor or RTD (Correct answer)
- Bourdon tube
Correct answer: Thermistor or RTD
Modern electronic thermostats use thermistors (NTC/PTC) or RTDs (Resistance Temperature Detectors) because their electrical resistance changes predictably with temperature.
HVAC Controls and Thermostats 1
What type of thermostat uses a bimetallic strip as its primary sensing element?