[LazyPup]

Water reaches maximum density at 39degF and if either heated or cooled it expands. Typically a 40gallon water heater will expand the water by approximate 1/2 gallon of volume when heated from the cold supply temperature to the desired temperature of 125degF.

In a typical municipal water supply system the expansion can be absorbed by a slight backflow through the supply line back to the municipal water main, but in instances where we must install a Pressure Reducing Valve and/or backflow preventer the water cannot backflow so we must provide additional expansion capability in the system by means of an expansion tank at the water heater.

Modern expansion tanks have an internal membrane that separates the air from the water so they require no maintenance.

Older expansion tanks such as yours are fundamentally just a hollow tank which has air in it at standard atmospheric pressure when installed. Air will compress whereas water will not so as the water distribution pipes are filled water enters the bottom of the tank and compresses the air upwards maintaining equal pressure between the water and air. This then leaves a small chamber of air at the top of the tank which can further compress to absorb the thermal expansion.

There are a number of problems associated with the older type tanks that have a direct air/water contact.

The most noticable effect is that the air can slowly be absorbed into the water and carried out of the tank leaving an insufficient charge of air in the tank. When this occurs you close the valve on the water line at the bottom of the tank, then open the bleeder valve to drain all the water out of the tank. This will restore a full charge of air. You then close the bleeder and open the valve on the water line and as the water enters the tank from the supply it will once again compress the air upwards and maintain equilibrium with the water pressure.

Another major concern with the older type of tanks is that the air chamber is constantly moist and in the presence of oxygen in the air the tanks rust internally. These type of tanks should be inspected at least annually. Pay very close attention to weld areas such as where the end caps are welded to the tank or any side seams that may be welded. Pin hole leaks will most often occur at the weld joints.

While generally not a problem on chlorinated municipal water supplies, if your water is not chlorinated there is a potential for Biofilm to form in this type of tank. Biofilm is a known health risk and in instances where you have a non-chlorinated water supply it would be a good idea to consider replacing the old style expansion tank with a modern tank equiped with an internal membrane.

Typically if there is a problem with an expansion tank it will be demonstrated by
1. Water hammer in the piping system
2. water leaking from the water heater Temperature & Pressure relief valve.