Rheem Technical Service Bulletin: Thermal Expansion

The Dangers of Thermal Expansion in Closed Water Systems

Water expands when heated. In a typical 40-gallon water heater, heating water from cold to its thermostat setting can increase its volume by approximately 0.53 gallons. This expanded volume must be accommodated.

Modern plumbing systems, often equipped with backflow preventers and check valves, create 'closed systems' where expanded water cannot return to the municipal supply. This confinement leads to dangerous pressure build-up within the water heater and connected piping. Repeated cycles of high internal pressure can stress tank welds and fittings, potentially weakening the system. The critical relationship is: water + heat + pressure + closed system = potential explosion!

This pressure increase can activate the Temperature and Pressure (T&P) relief valve, causing it to release hot water. However, persistent high pressures, even if below the T&P valve's emergency threshold, can accelerate tank leakage and reduce the water heater's lifespan. A common sign of thermal expansion issues is the T&P valve releasing about one cup of water per ten gallons of heater capacity during each heating cycle.

Solution: Thermal Expansion Tanks

The most effective method to manage thermal expansion is to control the pressure generated by heated water, keeping it within safe operating limits. This is achieved by installing an expansion tank.

A Rheem THERM-X-GUARD thermal expansion tank features a sealed compressible air cushion. This tank provides a dedicated space to absorb the additional volume of expanded water. When water is heated, the expansion tank accommodates the increase. When hot water is used, the tank returns the stored water to the system.

Diagram: Water Heater System Components

A diagram illustrates a water heater setup, showing key connections:

• Hot water outlet to fixtures
• Gas line
• Venting (to local code)
• Union fitting
• Isolation shut-off valve
• T&P discharge line
• Cold water inlet

A thermal expansion tank is also depicted as an integrated component.

Understanding Temperature and Pressure (T&P) Valves

T&P valves are essential safety devices that protect against dangerous over-temperature and over-pressure conditions. Without a functioning T&P valve, a water heater tank could rupture or explode, posing severe risks to property and personal safety.

Rheem water heaters are tested to withstand a pressure of 300 PSI, with a working pressure limit of 150 PSI. The T&P valve is designed to open if the internal pressure exceeds 150 PSI or if the water temperature reaches 210° F. The valve will remain open until the unsafe condition is resolved.

Regular maintenance is crucial: T&P valves should be inspected and tested every six months. A helpful mnemonic for testing is to perform it during daylight saving time changes.

Pressure vs. Temperature Graph

A graph titled "Pressure vs. Temperature increase in closed piping system" visually demonstrates the pressure escalation in a closed system as water temperature rises. The Y-axis shows pressure in PSI (0-600), and the X-axis shows temperature in °F (65-100). A red line labeled "WATER" indicates that pressure increases sharply with temperature, with the T&P valve activating around 150 PSI.

(Chart courtesy of Amtrol MC#4090 (9/05))

An example illustrates this: heating water from 70°F by less than 20°F in a closed system can raise pressure to 150 PSI, activating the T&P valve.

Tank Integrity and Expansion Tank Sizing

Rheem water heater tanks are built and tested for a 300 PSI test pressure and a 150 PSI working pressure. Any visible bulging of the tank's bottom or top indicates it has been subjected to pressures exceeding 300 PSI, which voids the warranty.

Proper sizing of an expansion tank is critical for its effectiveness. Always consult the specific manufacturer's specifications for guidance.

Image: Water Heater with Expansion Tank

An image depicts a water heater equipped with an expansion tank. The expansion tank is connected to the plumbing, typically near the cold water inlet, to manage thermal expansion.