306A Water Heating Systems 2 — Questions and Answers
Question 1: What is the difference between a direct-vent and a power-vent water heater?
- There is no difference
- A direct-vent uses sealed combustion with intake and exhaust through the wall; a power-vent uses room air for combustion and a fan to exhaust through the wall (Correct answer)
- Direct-vent is electric; power-vent is gas
- A power-vent uses natural draft; a direct-vent uses forced air
Correct answer: A direct-vent uses sealed combustion with intake and exhaust through the wall; a power-vent uses room air for combustion and a fan to exhaust through the wall
A direct-vent water heater draws combustion air from outside through a dedicated intake pipe and exhausts through an adjacent pipe (sealed combustion). A power-vent uses room air for combustion and a motorized fan to force exhaust gases horizontally through a sidewall vent.
Question 2: What type of venting material is required for a high-efficiency condensing gas water heater?
- Type B metal vent
- Stainless steel chimney liner
- PVC or CPVC plastic vent pipe (Correct answer)
- Masonry chimney only
Correct answer: PVC or CPVC plastic vent pipe
High-efficiency condensing water heaters produce cool, acidic exhaust (below the dew point) that would corrode metal venting. PVC or CPVC plastic vent pipe is used because it resists the acidic condensate.
Question 3: What is the efficiency advantage of a tankless (on-demand) water heater over a standard storage tank heater?
- Tankless heaters have lower purchase costs
- Tankless heaters eliminate standby heat loss because water is heated only when flowing through the unit (Correct answer)
- Tankless heaters have larger storage capacity
- Tankless heaters use less electricity for ignition
Correct answer: Tankless heaters eliminate standby heat loss because water is heated only when flowing through the unit
Tankless water heaters eliminate standby heat loss — the energy wasted keeping a tank of water hot 24 hours a day. They heat water only when a fixture is opened, achieving energy factors of 0.82 to 0.96 compared to 0.58 to 0.70 for standard tank heaters.
Question 4: What is the minimum flow rate typically required to activate a tankless water heater?
- 0.1 L/min
- 0.5 L/min (0.13 gpm)
- 2.0 L/min (0.5 gpm) (Correct answer)
- 5.0 L/min (1.3 gpm)
Correct answer: 2.0 L/min (0.5 gpm)
Most tankless water heaters require a minimum flow rate of approximately 2.0 L/min (0.5 gpm) to activate the heating element or burner. Flow rates below this threshold will not trigger the unit.
Question 5: What maintenance task should be performed annually on a gas water heater to ensure safe operation?
- Replace the thermostat
- Inspect the flue and vent connector for proper draft, check the T&P valve, drain sediment from the tank (Correct answer)
- Replace the entire burner assembly
- Repaint the exterior
Correct answer: Inspect the flue and vent connector for proper draft, check the T&P valve, drain sediment from the tank
Annual maintenance includes inspecting the flue and vent connector for blockage or corrosion, testing the T&P relief valve operation, and draining sediment from the tank bottom to maintain efficiency and safety.
Question 6: What causes 'stacking' in a storage-type water heater?
- Sediment buildup at the bottom of the tank
- Repeated short draws of hot water that cause the top of the tank to reach dangerously high temperatures while the thermostat at the bottom remains satisfied (Correct answer)
- Multiple heaters connected in series
- Ice formation in the vent pipe
Correct answer: Repeated short draws of hot water that cause the top of the tank to reach dangerously high temperatures while the thermostat at the bottom remains satisfied
Stacking occurs during repeated short draws (such as frequent handwashing) where the cold replacement water entering the bottom keeps the thermostat calling for heat, while the already-hot water at the top continues to get hotter, potentially reaching dangerous temperatures.
306A Water Heating Systems 2
What is the difference between a direct-vent and a power-vent water heater?