The expansion tank stores extra heat transfer fluid (oil) and
permits expansion of the oil in the system when heated.
It also serves as a cold-oil seal which keeps only relatively cool
oil in contact with the air. The oil expands into the tank, from
the heater through the two-inch pipe which is extended to the top
of the tank. When the oil contracts, it flows from the tank into
the heater through one-inch heat trap, which also prevents hot oil
circulation between the heater and the expansion tank. The pipe
plug in the trap is a clean-out for the trap, should it ever
Operating Oil Level
The operating oil level as seen in the glass gauge should be high
enough to permit the float switch to remain "on" when the oil is
cold (low) and yet not high enough to overflow when hot (high).
It is desirable to start the heater with the oil level low enough
to allow for expansion of the oil as it heats.
Usually after the system is brought up to operating temperature,
a level in the upper half of the expansion tank is satisfactory.