Secondary circulation.

Expansion-pipe supplying the hot-water system, so that it can be left open without allowing the water to overflow. In practice the expansion-pipe is usually terminated by a bend, the mouth of which is just above the overflow of the tank, so that, if foam and bubbles rise through it, as they are likely to do with some waters, the drip will be carried safely away.

With a "pressure system" of hot-water supply, in which the boiler takes water directly from the street main, without the intervention of a tank, secondary circulations are impracticable, as the expansion-pipe necessary for clearing them of air would continually overflow; so that, with a system of this sort, a preliminary flow of cold water from the hot-water pipes is unavoidable.