CSLB C-36 Plumbing ContractorPlanning and EstimatingEasy
A plumbing contractor is planning the installation of a new water heater in a commercial building. The building requires an expansion tank to be installed with the water heater. What is the primary purpose of an expansion tank in a closed-loop hot water system?
- ATo prevent backflow of hot water into the cold water supply.
- BTo absorb thermal expansion of water and prevent excessive pressure buildup.
- CTo increase the hot water delivery pressure to fixtures.
- DTo filter sediment and impurities from the incoming water supply.
Show answer & explanationAnswer & explanation
Correct answer: B. To absorb thermal expansion of water and prevent excessive pressure buildup.
When water is heated, it expands. In a closed-loop system (common with check valves or pressure-reducing valves on the main supply), this expansion can cause dangerously high pressure. An expansion tank provides a cushion to absorb this increased volume, protecting the system.
Why the other options are wrong
- A. Backflow prevention is handled by backflow preventers or check valves.
- C. Pressure boosting is typically handled by separate pumps, not expansion tanks.
- D. Water filtration is handled by dedicated filters, not expansion tanks.
Hot Water Expansion Tank
A pressurized tank installed in a closed hot water system to absorb the increased volume of water due to thermal expansion, preventing excessive pressure.
- Required in closed-loop systems (e.g., with PRVs or backflow preventers).
- Protects water heaters, pipes, and fixtures from high pressure.
- Contains an air bladder that compresses as water expands into the tank.
Memory trick: Hot water expands, tank gives it room to dance.