CSLB C-20 HVAC ContractorHeating SystemsMedium
In a closed hydronic heating loop, what is the primary purpose of the expansion tank?
- ATo boost water pressure for distant zones
- BTo accommodate the volumetric expansion of water as it heats, preventing excessive pressure buildup
- CTo filter sediment from the circulating water
- DTo remove air bubbles from the system
Show answer & explanationAnswer & explanation
Correct answer: B. To accommodate the volumetric expansion of water as it heats, preventing excessive pressure buildup
Water expands in volume as it is heated in a closed hydronic system; the expansion tank provides a cushioned air space (often via a diaphragm) to absorb this expansion and keep system pressure within safe limits.
Why the other options are wrong
- A. Circulator pumps, not expansion tanks, provide pressure/flow to zones.
- C. Sediment filtering is handled by strainers or dirt separators.
- D. Air removal is the job of air separators/vents, not the expansion tank.
Hydronic Expansion Tank
A closed-system component containing a cushioned air or diaphragm chamber that absorbs the volume increase of heated water, preventing dangerous pressure rise.
- Water expands roughly 4% in volume when heated from 40°F to 200°F
- Diaphragm-type tanks separate air charge from system water
- Undersized tanks cause relief valve weeping or nuisance pressure trips
Memory trick: Hot water swells, the tank absorbs and quells.