HVAC Hydronic Heating Systems 4 — Questions and Answers
Question 1: What type of pipe fitting is used on hydronic systems to allow thermal expansion without stressing the pipe or boiler connections?
- Union fitting
- Dielectric union
- Expansion loop or offset (Correct answer)
- Slip coupling
Correct answer: Expansion loop or offset
Expansion loops, offsets, or expansion joints are built into hydronic piping runs to absorb the linear expansion of pipes as they heat up, preventing stress on fittings and equipment.
Question 2: What is the purpose of a dielectric union in a hydronic system?
- To allow pipe movement from thermal expansion
- To prevent galvanic corrosion between dissimilar metals (Correct answer)
- To reduce noise from water hammer
- To isolate a section of pipe for servicing
Correct answer: To prevent galvanic corrosion between dissimilar metals
Dielectric unions contain an insulating bushing that electrically isolates dissimilar metals (such as copper and steel), preventing the galvanic corrosion that occurs when different metals contact each other in water.
Question 3: A technician reads 22 psi static pressure at the boiler on a cold system. After firing the boiler, pressure rises to 28 psi. This indicates:
- A system leak is developing
- Normal system operation with thermal expansion (Correct answer)
- The pressure relief valve is about to open
- The expansion tank has lost its pre-charge
Correct answer: Normal system operation with thermal expansion
A 6 psi rise from cold fill pressure to operating pressure as water heats is typical and indicates normal thermal expansion being managed by a properly functioning expansion tank.
Question 4: What glycol concentration is typically recommended for hydronic antifreeze protection to -20°F?
- 15% propylene glycol
- 30% propylene glycol
- 50% propylene glycol (Correct answer)
- 70% propylene glycol
Correct answer: 50% propylene glycol
A 50% propylene glycol solution provides freeze protection to approximately -26°F and burst protection to well below -60°F, making it suitable for -20°F protection with a safety margin.
Question 5: Which instrument is used to measure the flow rate through a hydronic circuit during balancing?
- Manometer
- Psychrometer
- Flow meter or ultrasonic flow meter (Correct answer)
- Anemometer
Correct answer: Flow meter or ultrasonic flow meter
Flow meters, including ultrasonic clamp-on models or inline turbine types, measure the actual water flow rate in GPM needed to verify and balance hydronic circuits.
Question 6: What is 'short cycling' in a hydronic boiler, and what commonly causes it?
- The circulator turning on and off rapidly due to thermostat bounce
- The boiler firing briefly and then shutting off repeatedly due to oversizing or low mass (Correct answer)
- The zone valves opening and closing faster than the system can respond
- Rapid pressure fluctuations from a waterlogged expansion tank
Correct answer: The boiler firing briefly and then shutting off repeatedly due to oversizing or low mass
Short cycling occurs when an oversized boiler quickly satisfies the aquastat or thermostat demand and shuts off, only to turn on again moments later, reducing efficiency and increasing wear.
Question 7: In a hydronic system, what does the term 'pumping away from the point of no pressure change' mean?
- The circulator should always push water away from the boiler
- The circulator should be positioned to discharge away from the expansion tank connection (Correct answer)
- Pump discharge pressure should overcome all system head losses
- The pump should be located on the return line only
Correct answer: The circulator should be positioned to discharge away from the expansion tank connection
When the circulator is located so that it pumps away from the expansion tank connection point, it adds pressure throughout the system, preventing air infiltration and cavitation.
HVAC Hydronic Heating Systems 4
What type of pipe fitting is used on hydronic systems to allow thermal expansion without stressing the pipe or boiler connections?