What expansion tank means
An expansion tank is a small tank with an air cushion that absorbs the extra volume of water as it heats, in a closed water heater or boiler system.
Where it matters on a plumbing job
Dripping relief valves and pressure spikes often trace to a missing or waterlogged expansion tank. Check the tank's air charge on water heater calls, and quote a new tank with any heater install where the system is closed.
What the model code says
- A device to control pressure is required where thermal expansion pushes pressure downstream of a pressure-reducing valve above its setting. (IRC 2021, Chapter 29)
Check which code edition your building department enforces for expansion tank before you quote.
What it costs
Koira's water heater installation cost guide puts a typical job at $1,350, with most between $890 to $2,400. The guide shows its sources and what moves the price.
Work it out
- Cost guide: Water heater installation cost
- Licensing: Plumbing license rules by state
- Tools, templates and guides: Free plumbing resources
Sources: 2021 International Residential Code, Chapter 29, Water Supply and Distribution (ICC). Checked October 9, 2026.