An expansion tank in a closed water system serves which primary purpose?

• A. Provide a place to add makeup water
• B. Allow for changing volume of water when heated and cooled
• C. Maintain a static head (constant pressure) on the system, due to the weight of the water
• D. None of the above

Answer: (C)

Expansion tanks keep pressure stable in a closed water system by buffering changes in the water’s volume as temperature changes. When water heats, it expands and would push system pressure higher; the expansion tank provides an air-filled space that compresses to absorb that extra volume, keeping the overall pressure near the design value. This steady pressure is the static head you feel throughout the system, determined by the weight of the water column but moderated by the tank’s air cushion. Makeup water isn't the primary purpose of the tank; its main function is to prevent pressure spikes by accommodating water expansion and maintaining a consistent pressure level.