At Eccotemp, we’re often asked whether an expansion tank is needed when installing a tankless water heater. The short answer is: No, expansion tanks are not required for tankless water heater installations.
Why Not?
Expansion tanks are typically used with traditional tank-style water heaters to manage thermal expansion — the natural increase in water volume as it heats up. Because tank-style heaters store a large amount of hot water at once, the water expands and needs somewhere to go. The expansion tank provides that extra space, helping to prevent excess pressure in your plumbing system.
How Tankless Is Different
Tankless water heaters, unlike storage tanks, do not hold water. They heat water on demand, only when a faucet or appliance is in use. Since water is not being continuously stored and reheated, there’s no significant thermal expansion occurring inside the unit that would require an expansion tank.
Code Compliance and Plumbing Requirements
In most residential applications, plumbing code does not require an expansion tank with a tankless system, unless there is a closed-loop plumbing system with a backflow preventer or pressure-reducing valve. Even in those rare cases, the expansion tank would be protecting the plumbing system as a whole — not the tankless heater itself.
Summary
✅ Expansion tanks are not needed for tankless water heaters
🔥 Tankless units heat water only as needed — no stored water = no thermal expansion
🛠 Installation is simpler without the added component of an expansion tank
📘 Always check local plumbing codes, but in most cases, it's not required
If you have questions about your specific setup or need installation guidance, our support team is here to help.