The short answer
Water expands when heated. In a closed system (one with a check valve or pressure-reducing valve at the meter), that extra volume raises pressure. A thermal expansion tank holds it. A tankless heater stores very little water, so it expands much less than a 50-gallon tank; local plumbing code and the inspector decide whether the installation needs one.
In the catalog
EFIELD TANK001 Expansion Tank, EFIELD TANK002 Expansion Tank and Eastman 60023 Expansion Tank.
Products mentioned
Questions readers ask
Does a tankless water heater need an expansion tank?
It holds little water, so thermal expansion is small, but some closed systems and local codes still call for one. Ask the installer or inspector.