When the heating system is first set up, all the pipes are filled with air. The expansion tank is completely filled with air as well. You can think of the expansion tank as a large dead-end part of the heating system. When water is introduced, the pipes will be filled with water, but air will be trapped in the expansion tank. This is what we want. The expansion tank typically has some water that comes into the bottom of it, but the majority of its volume remains filled with air.
As the boiler operates, the air in the tank is compressed and the tank may be more than half filled with water when the system is hot.
The air eventually is lost from the conventional expansion tank. It sometimes leaks out slowly through the air valve, but the majority is dissolved into the water over time.
When the tank is filled with water, it is said to be
. The system loses its shock absorber. When the boiler comes on, the pressure in the system will rise quickly. If all goes well, the relief valve operates and water leaks out through the discharge pipe from the relief valve. If the relief valve is missing, inoperative or its discharge is obstructed, a dangerous high pressure situation can develop.
You will notice the relief valve discharging and eventually notice the expansion tank is waterlogged. Restoring the air in the tank is done as follows:
The isolating valve on the pipe connecting the heating system to the expansion tank is closed.
A hose is usually connected to the fitting on the drain valve, and the drain valve is opened.
The air inlet valve (usually located near the top of the tank) is also opened.
The water runs out of the drain line and air is introduced through the air inlet.
When the tank is empty
the hose is removed from the fitting,
the drain valve is closed,
the air inlet is closed, and
the isolating valve is opened.
Water from the heating system makes its way up into the bottom of the tank, compressing the air.
The system is ready for operation again.
Many modern expansion tanks have a rubber (neoprene) diaphragm that separates the air and the water. The system works exactly the same, but the flexible rubber diaphragm keeps the air from being dissolved into the water. These systems rarely become waterlogged and are therefore more convenient.
There is some controversy about where the expansion tank is best located. In most cases, it is on the hot-water supply distribution pipe coming off the top of the boiler. (Conventional tanks on closed systems have to be above the top of the boiler.) If the tank is connected to the return side of the boiler, the system will still work just fine, maybe better.