How a Double Check Valve Works in a Storage Water Heater

Electric storage water heaters are designed with several features to ensure they operate safely under the pressures of heating water in a closed system. Let’s explore the mechanisms and principles behind these safety features:

1. Double Check Valve and How It Works

A double check valve, often referred to as a check valve or non-return valve, is integrated into a storage water heater in Singapore to prevent the reverse flow of hot water from the heater back into the cold water supply line. This double check valve is crucial for several reasons:

Safety: It prevents hot water from entering the cold-water system, which could lead to scalding injuries.

Water Quality: It avoids contamination of the cold-water supply by keeping the two systems separate.

How it Works: The double check valve allows water to flow in only one direction—into the heater. It typically consists of a flap or a ball that rests against a seal. When water flows towards the heater, it pushes the flap or ball open. When the flow stops or reverses, the flap or ball returns to its seat, blocking the flow back into the supply line.

2. Pressure Increase When Heated Up and Constrained Inside the Fixed Volume Tank

When water is heated, its volume expands. In a closed tank, this expansion leads to an increase in pressure. According to the ideal gas law (though water isn’t a gas, the principles of pressure, volume, and temperature interrelation are similar), if the volume is fixed and temperature increases, the pressure must also increase. This phenomenon is particularly significant in water heating systems because water expands by approximately 4% when heated from room temperature to near its boiling point.

3. Function of a Pressure Relief Valve

The pressure relief valve is a critical safety device on a water heater. It is usually preset to a specific pressure limit to prevent the pressure inside the tank from exceeding safe levels, which could otherwise lead to tank rupture or explosion.

Common Settings in Singapore: The valve is typically set to open at 8 bar (about 116 psi), which allows it to relieve excess pressure safely and prevent damage or failure of the water heater.

4. Incoming Water Pressure Set at 50 psi

The incoming water pressure for most residential settings in Singapore is set around 50 psi. This is a sufficient pressure to ensure efficient flow and operation of water fixtures but is low enough to be safely managed by standard plumbing components and devices.

5. Pressure Increases When Heated Up, A Double Check Valve Prevents Backflow

When water is heated in a closed system like a storage water heater, the pressure increase depends on the amount of thermal expansion that occurs. Since water expands when heated (approximately 4% from room temperature to 100°C), the pressure in the tank can increase significantly. If not properly managed, this could add substantial pressure to the system. However, with a functioning expansion tank or pressure relief valve, this pressure is controlled to prevent any damage or excessive pressure in the water supply system.

6. Hot Water Backflow Occurs Without A Double Check Valve

Without a double check valve mechanism, there is a potential for the expanded, heated water to flow back into the cold water line when pressure in the heater becomes higher than the incoming water pressure. Whether it would reach as far as the kitchen sink tap depends on the configuration and pressure dynamics of the plumbing system. Generally, in a typical residential setup, the probability of hot water from the heater backflowing all the way to the kitchen sink is low, especially if there are intermediate fixtures or branches. However, the absence of a backflow preventer is unsafe and contrary to good plumbing practices.

Each component—the double check valve, the pressure relief valve, and the pressure settings—plays a crucial role in ensuring the safe operation of storage water heaters. These features work together to manage the dynamics of heated water and maintain system integrity, preventing potential hazards.