Water is a poor conductor of heat. It cannot be heated by conduction or by radiation. If heat is applied to the top of a vessel of water, but slight rise of temperature will result. Water must be heated by circulation or convection, and to cause the water to circulate the heat must be applied at the lowest part of the containing vessel.

If heat is applied to the bottom of a vessel of water, the water immediately begins to circulate. The water directly above where the heat is applied is heated by conduction, expands in bulk, consequently becomes lighter. It is then displaced by the cooler and denser water surrounding it, which in turn becomes heated and is displaced by the surrounding water; thus establishing local circulation of the water inside of the vessel.

If in place of a vessel of water a U-shaped tube, Fig. 117, be used and the ends of the loop connected at the top, as shown in the illustration, the water will rise in the leg of the tube to which the heat is applied, and will descend in the other leg to replace the ascending column of water. This establishes a continuous movement of the entire volume of water in the tubes in the direction of the arrows.

This movement is known as circulation in a circuit. That is what occurs when water in a storage tank or range boiler is heated from a waterback or water heater.

The velocity of circulation in a circuit depends upon the temperature to which the water is heated and the height of the circuit.

Thus with a hot fire and a high loop the velocity of flow would be much greater than with the same loop and a slow fire or with a hot fire and a low loop. The chief cause retarding circulation is friction, therefore short radius bends, contracted waterways, small pipes and unreamed pipe ends should be avoided when installing hot water supply systems.