An Expansion Tank is installed on systems where water is heated to store the volume of water created by thermal expansion without a dangerous pressure increase. Water cannot be compressed, yet when water is heated its volume increases.
In a building’s water supply system many times the small volume increase simply pushes its way back towards the city water supply without any problems. But there are times when there may be a check valve, backflow preventer, pressure reducing valve or, other device which creates a closed system that does not allow this volume of water created by the thermal expansion to flow back towards the water supply. When there is a closed system the thermal expansion from the water being heated in the water heater can cause a dramatic pressure rise in the buildings water supply. Typically with a proper functioning T&P Valve on the water heater this will result in the discharge of a small amount of water from the T&P Valve, about a coffee cup full.
An Expansion Tank is a cylinder that is connected to the water supply of the water heater between the valve for the water heater and the water heater itself. The Expansion Tank attaches on one end with a male threaded connection and inside the cylinder there is a rubber bladder that is filled with air, pressurized to match the water supply pressure. Air unlike the water can be compressed, and when thermal expansion causes an increase in water volume in a closed system the water will force its way into the Expansion Tank without a drastic increase in pressure on the building’s water system. There are Expansion Tanks made for potable and non-potable water, when installing an Expansion Tank on a water heater a Potable Water Thermal Expansion Tank should be used.