HVAC Hydronic Heating Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a compression tank (expansion tank) in a closed hydronic system?
- To increase system pressure during operation
- To absorb the expansion of water as it heats up (Correct answer)
- To separate air from the circulating water
- To provide additional water volume when needed
Correct answer: To absorb the expansion of water as it heats up
The compression tank absorbs the increased volume of water that occurs as it heats, preventing dangerous pressure spikes in the closed system.
Question 2: In a primary-secondary hydronic system, what is the purpose of the secondary loop?
- To supply domestic hot water
- To allow zone control without affecting primary loop flow (Correct answer)
- To increase overall system efficiency
- To provide redundancy if the primary pump fails
Correct answer: To allow zone control without affecting primary loop flow
Secondary loops allow individual zones to operate at different flow rates and temperatures without disrupting the flow in the primary distribution loop.
Question 3: Which type of hydronic terminal unit uses a fan to distribute heat from the hot water coil to the room?
- Baseboard convector
- Radiant panel
- Fan coil unit (Correct answer)
- Radiant floor tubing
Correct answer: Fan coil unit
Fan coil units (FCUs) use an integral fan to force air across the hot water coil, providing faster heat distribution than passive convectors.
Question 4: What does the term 'delta T' refer to in hydronic system design?
- The difference between supply and return water temperatures (Correct answer)
- The pressure differential across the circulator pump
- The temperature rise across the boiler heat exchanger
- The difference between indoor and outdoor design temperatures
Correct answer: The difference between supply and return water temperatures
Delta T (ΔT) is the temperature difference between the supply and return water in the system, typically 20°F in standard hydronic designs.
Question 5: A hydronic system is experiencing 'ghost flow' through a zone that is supposed to be off. What is the most likely cause?
- Oversized circulator pump
- Failed zone valve not fully closing (Correct answer)
- Air locked in the zone circuit
- Undersized expansion tank
Correct answer: Failed zone valve not fully closing
Ghost flow occurs when a zone valve fails to close completely, allowing hot water to bypass the zone thermostat control and heat an unintended area.
Question 6: What is the recommended minimum water velocity in hydronic piping to prevent air pockets from forming?
- 0.5 ft/sec
- 1.5 ft/sec (Correct answer)
- 3.0 ft/sec
- 5.0 ft/sec
Correct answer: 1.5 ft/sec
A minimum velocity of about 1.5 ft/sec is needed to carry entrained air bubbles along to the air separator rather than allowing them to collect in high points.
Question 7: Which piping arrangement in a hydronic system ensures equal flow and pressure drop through all terminal units?
- Series loop
- One-pipe system
- Reverse-return system (Correct answer)
- Direct-return system
Correct answer: Reverse-return system
In a reverse-return system, the total length of supply plus return pipe is equal for every terminal unit, resulting in balanced flow without extensive manual balancing.
HVAC Hydronic Heating Systems 2
What is the primary purpose of a compression tank (expansion tank) in a closed hydronic system?