The term expansion tank refers to a device that absorbs excess pressure in a closed plumbing system caused by thermal expansion. Tankless water heaters heat water on demand and do not store hot water, which influences whether an expansion tank is necessary. This article explains when an expansion tank is required for a tankless setup, how thermal expansion works in modern homes, and key considerations for code compliance and installation best practices.

Do Tankless Water Heaters Require Expansion Tanks

In most residential applications, a tankless water heater does not require an expansion tank. Because there is no stored hot water, the potential for uncontrolled thermal expansion within the heater itself is limited. However, the need for an expansion tank depends on the overall plumbing system design. If the home has a closed municipal supply with a pressure-reducing valve, or a backflow preventer, thermal expansion can occur elsewhere in the system, making an expansion tank desirable or required by code. Some manufacturers may recommend or require one in specific installation configurations, so always consult the unit’s installation manual.

How Thermal Expansion Works in a Plumbing System

Thermal expansion occurs when water is heated and expands in volume. In a typical closed plumbing system, this expansion increases pressure unless there is a way to absorb the additional volume. In a tank-type water heater, the stored hot water is what primarily creates pressure, and an expansion tank is commonly used to mitigate this pressure. In a tankless system, the absence of a storage tank reduces the amount of heated water mass available to expand, but expansion can still occur if the system includes a backflow preventer, check valves, or recirculation lines that confine water within a closed loop. If such devices trap hot water in a confined portion of piping, a dedicated expansion tank can help prevent pressure spikes that could stress fittings, valves, or the water heater connections.

When An Expansion Tank Is Beneficial for Tankless Installations

Closed water supply with backflow preventer or pressure-reducing valve: These devices can trap expanding water, making an expansion tank prudent to prevent excessive pressure.
Recirculation systems: If the home uses a hot-water recirculation loop, expansion could occur within the loop rather than at the heater, increasing the value of an expansion tank.
High overall system pressure: In regions with higher incoming water pressure, or where a pressure regulator is set to elevated levels, an expansion tank can provide a buffer against spikes.
Local code or manufacturer requirements: Some jurisdictions or burner manufacturers specify an expansion tank for certain tankless models, especially when installed on a closed loop.

Other Considerations For Installation And Code Compliance

Consult the installation manual: Always check the manufacturer’s guidance for expansion tanks or pressure requirements specific to the model you install.
Check local codes: Some municipalities may require an expansion tank on any new construction with a backflow preventer or closed system, regardless of the heater type.
Pressure concerns: If the system operates at high pressures, an expansion tank can help protect fixtures, connections, and the unit’s heat exchanger from stress.
Sizing and placement: If an expansion tank is recommended, select the correct size based on the system’s water volume, temperature range, and static pressure. Place the tank on the cold-water supply near the heater or at the point of entry for the closed-loop portion of the system.
Maintenance: Expansion tanks require inspection of the air charge and proper precharge pressure to function correctly. A failing expansion tank can be as problematic as having none.

Practical Guidance For Homeowners

Homeowners evaluating whether to install an expansion tank with a tankless heater should assess three factors: the presence of a backflow preventer or pressure-reducing valve, the existence of a hot-water recirculation loop, and local code requirements. If any of these elements indicate a closed or semi-closed system, consulting a licensed plumber for a system-specific assessment is wise. In many cases, the absence of a storage tank reduces the likelihood that an expansion tank is necessary, but not always. A professional assessment ensures safety, reliability, and code compliance.

Frequently Asked Questions

Do all tankless water heaters require an expansion tank? No. Most do not, but certain configurations and local codes may necessitate one.
Can thermal expansion cause problems without an expansion tank? Yes, in a closed system, expansion can stress pipes and fittings; an expansion tank mitigates this risk.
Where should an expansion tank be installed? Typically on the cold-water supply line near the water heater or at the point where the system becomes closed, as specified by code and manufacturer.
How do I determine the right size for an expansion tank? A plumber can calculate based on system volume, expected temperature rise, and local pressure; common residential tanks range in capacity and precharge pressure.

In summary, while a tankless water heater often does not require an expansion tank, certain system configurations—especially closed supply lines, recirculation loops, or elevated incoming pressure—can benefit from one. Always align with manufacturer guidance and local codes, and consult a licensed professional to confirm the best approach for a specific home.