Tankless water heaters, also known as on-demand systems, operate differently from traditional storage tanks. While they typically demand less space and can provide endless hot water, safety features remain essential. This article examines whether a pressure relief valve is required on tankless units, how these valves function, installation considerations, and maintenance best practices to ensure reliable and safe operation for a U.S. household.

Understanding Pressure Relief Valves

A pressure relief valve (PRV), also called a temperature and pressure relief valve (T&P valve), is a safety device designed to prevent excessive pressure or temperature within a water heater or plumbing system. If pressure or temperature climbs above safe levels, the valve releases water to lower the pressure, reducing the risk of a rupture or explosion. On conventional storage tanks, PRVs are standard equipment and typically required by building codes. The question for tankless systems centers on whether similar protection is necessary and how it is achieved.

Tankless Water Heaters And Safety Valves

Tankless water heaters heat water on demand as it flows through the unit. Because these systems do not store hot water, some assume they do not need a PRV. However, safety considerations still apply. The primary safety concerns for tankless systems include thermal expansion, pressure buildup in the hot water line, and backflow from connected fixtures or devices. In many installations, aPRV is not built into the unit itself, and compliance depends on local codes and the full plumbing design. Some manufacturers provide internal thermal relief features or recommend external safeguards, while others may require a PRV or backflow prevention valve as part of the overall system.

Do They Need A Pressure Relief Valve

In general, a tankless water heater does not always require a dedicated PRV on the unit itself, but an appropriate pressure and temperature protection strategy is still necessary. Key considerations include:

Local Codes: Building and plumbing codes vary by jurisdiction. Some require a T&P valve or an external pressure relief device when installing certain tankless models or in specific configurations, especially where thermal expansion could occur.
Thermal Expansion: If the home uses a closed plumbing system with a backflow preventing valve, thermal expansion can cause pressure rise. An expansion tank or a PRV may be required to accommodate this expansion.
System Configuration: In some installations, a remote PRV or an inline relief valve at the point of use may be recommended to protect the hot-water loop and connected appliances.
Manufacturer Instructions: Always consult the installation manual for the specific model. Some manufacturers include built-in safety features, while others rely on external devices.
Water Supply Condition: Hard water buildup and mineral scale can influence valve function and heat transfer. Proper filtration and maintenance support safe operation.

Installation Considerations

When planning a tankless water heater installation, consider integrating safety devices that align with code requirements and best practices:

Expansion Tank: If the home has a closed municipal water supply, an expansion tank helps absorb water volume changes and reduces pressure buildup in the distribution system.
Backflow Prevention: A backflow prevention device safeguards the potable water supply and can interact with PRV requirements. Proper placement is essential.
Outdoor vs Indoor Units: Indoor installations may require a PRV or venting considerations to handle pressure scenarios, while outdoor units have different exposure factors.
Accessibility: Any relief device must be accessible for inspection, testing, and replacement as needed. Obstructions can render a safety valve ineffective.
Drainage: If a T&P valve is installed, ensure a proper drain path to a safe location to handle any discharge during valve operation.

Maintenance And Best Practices

Regular maintenance and inspection help ensure the safety features function correctly. Consider the following best practices:

Annual Inspection: Inspect any installed expansion tanks, PRVs, or related valves for corrosion, leaks, or signs of wear.
Test Procedures: If a relief valve is present, follow manufacturer guidelines for testing. Do not block or bypass the valve during testing.
Water Quality: Use appropriate water treatment or filtration to reduce mineral buildup that can impede valve performance and heat exchanger effectiveness.
System Pressure Checks: Periodically verify that the system pressure remains within safe operating ranges as specified by the unit’s manual and local codes.
Professional Evaluation: Schedule a licensed plumber to review the entire installation after any changes to the plumbing system or when upgrading equipment.

Common Scenarios And Solutions

Several typical situations illustrate how safety strategies work in practice:

Closed-Loop Systems: In homes with a closed loop, an expansion tank is often the simplest way to control pressure fluctuations, potentially reducing the need for a PRV on the heater itself.
High Water Pressure: If incoming water pressure exceeds safe limits, a pressure-reducing valve at the main water supply or a whole-house filtration system with a pressure regulator can prevent stress on the tankless unit.
Thermal Shock Risks: Rapid changes in water temperature can cause stress on internal components. Proper venting, correct gas or electric connections, and correct flow rates mitigate this risk.

Key Takeaways

For homeowners considering a tankless water heater, the need for a pressure relief valve hinges on local codes, the presence of thermal expansion, and the overall plumbing configuration. Essential actions include reviewing the installation manual, confirming code compliance, and ensuring expansion and relief strategies are in place. A properly designed system, with or without a dedicated PRV on the heater, protects both household plumbing and occupants from pressure-related hazards while preserving the efficiency and longevity of the tankless unit.