How expansion tanks work
When water is heated in a heating system, it expands. In order to safely account for this increase in water volume, an expansion tank is necessary. The expansion tank’s air cushion allows water to expand and contract naturally. This relieves the pressure of the heating system. When water cools it leaves the expansion tank and returns to the system.
Expansion tanks have a specific volume of air in them. When hot water is forced into the tank, the air is compressed and water is given a place to go. Old expansion tanks faced the problem of keeping air in the tank. As water entered the expansion tank, air was displaced. This problem was solved with diaphragm-type tanks. Synthetic diaphragms separate the air in the tank from the water and greatly reduce the air loss.
Symptoms that an expansion tank is taking on excess water:
- Water is dripping from the pressure relief valve.
- The lower part of the tank, where the air would normally be cool, is warm to the touch because of the expanded hot water.
A way to repair an expansion tank:
- Allow the tank to cool by turning off the boiler and closing the water shutoff valve. In order to get an accurate reading of the air pressure on the diaphragm, the hot water side pressure must be at 0 PSI.
- Monitor the pressure of the expansion tank by attaching a pressure gauge to its air-inlet valve.
- If air is necessary, add more air with a pump until the gauge reads the default manufacturer’s pressure of 12 PSI (for Extrol, Fill-Trol and Radiant Extrol models).
- Restart the system by opening the water supply valves.
- Turn the boiler on.
- Allow 1-2 hours for the system to recharge and then recheck the expansion tank.