This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Every system for hot water heating should be connected with an expansion tank placed at a point somewhat above the highest radiator. The tank must in every case be connected to a line of piping which cannot by any possible means be shut off from the boiler. When water is heated, it expands a certain amount, depending upon the temperature to which it is raised and a tank or reservoir should always be provided to care for this increase in volume.
Expansion tanks are usually made of heavy galvanized iron of one of the forms shown in Fig.. 33 and 34, the latter being used where the head room is limited. The connection from the heating system enters the bottom of the tank and an open vent pipe is taken from the top. An overflow connected with a sink or drain pipe should be provided. Connections should be made with the water supply both at the boiler and at the expansion tank, the former to be used when first filling the system, as by this means all air is driven from the bottom upward and is discharged through the vent at the expansion tank. Water that is added afterward may be supplied directly to the expansion tank where the water line can be noted in the gage glass. A ball cock is often arranged to keep the water line in the tank at a constant level.
The size of the expansion tank depends upon the volume of water contained in the system, and the temperature to which it is heated. The following rule for computing the capacity of the tank may be used with satisfactory results.
The square feet of radiation divided by 40 equals the required capacity of the tank in gallons.