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Plumbpedia™ - How Does A Water Heater Expansion Tank Or Valve Work?

  Plumbpedia™ - How Does A Water Heater Expansion Tank Or Valve Work?

 When water is heated it expands. For example, water heated from 90°F to a thermostat setting of 140°F in a 40 gallon hot water heater will expand by almost one-half gallon. This is because when water is heated, its density decreases and its volume expands (see below). Since water is not compressible, the extra volume created by expansion must go someplace. During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. Hence, system pressure increases.

Heated Water Expands...

Temperature vs Density
 Thermal expansion of water in a closed plumbing system can create a number of annoying and potentially dangerous problems. These include: the build up of unusually high pressure in a system (even when a pressure reducing valve is installed); pressure surges; and the chronic or continuous dripping of a temperature and pressure (T&P) relief valve.

 More serious problems can also occur due to thermal expansion. When dangerous pressures are built up in a water heater, internal parts may fail such as the internal flues, fittings or water connections. If a flue way collapses it can lead to the potential release of toxic gases, such as carbon monoxide into living spaces. Thermal expansion can also lead to a ruptured or distorted hot water heating tank and may void the manufacturer's warranty.

 Plumbing codes require that thermal expansion control be addressed in plumbing systems. A temperature and pressure relief valve is not considered a thermal expansion device. This is because when water is allowed to continuously drip from the T&P relief valve, minerals from the water can build up on the valve, eventually blocking it. This blockage can render the T&P valve useless and potentially lead to hot water heater explosions.

 If your water heater does not have an expansion tank or expansion valve, it is because the water supply from your provider is allowing any expansion that occurs to be absorbed back into their system. If your expansion tank or valve has been removed because it was leaking or never replaced if you changed your water heater, you should consider having it replaced immediately. You are at risk of damaging your water heater, and the possibility of an explosion.

Water Containment vs Water Relief Solutions
Expansion Tank    Water Containment solutions allow for thermal expansion while containing thermally expanded water in the plumbing system. Thermal expansion tanks are considered water containment devices. These products require no installation of discharge lines or drains.
Expansion Valves    Water Relief solutions discharge thermally expanded water at a pressure setting that is below the setting of the water heater's temperature and pressure relief valve. These products, such as an expansion valve, must be piped to a suitable drain or discharge location.

Expansion Tanks - How a Diaphragm Expansion Tank Works
 When water is heated in a closed system it expands. Water is not compressible, therefore, the additional water volume created has to go someplace. When an expansion tank is installed the excess water enters the pre-pressurized tank (figure 1). As the temperature and pressure reaches its maximum, the diaphragm flexes against an air cushion (air is compressible) to allow for increased water expansion (figure 2). When the system is opened again or the water cools, the water leaves the tank and returns to the system.

Tank 1
  figure 1. As the water temperature increases, the expanded water is received by the tank.
Tank 2
  figure 2. As the water and pressure reaches its maximum, the diaphragm flexes against the air                 cushion (air is compressible) to allow for increased water expansion.

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