Reversed circulation is the flow of water around the primary circuit, going in the opposite direction to the intended direction of flow. It is a situation that can arise in domestic hot water systems which do not have pumped primary flow and return pipes and no temperature controls over the water being heated. Water in the hot storage vessel slowly reaches the same or nearly the same temperture as that in the boiler. For example; if a solid fuel back boiler has been alight all day and not much water is drawn off from the hot storage vessel the temperature of the water will continue to rise until the end of the day the return pipe temperature is very nearly the same as the flow pipe temperature and as a result the circulation of water, due to convection currents, tends to cease. During the night as the fire dies down and goes out should the heat loss in the flow pipe be greater than in the return pipe the water will cool quicker and become less dense and as a result fall and push the hotter, lighter water back up to the return pipe.
This reversal of the circulation will continue until the water completly cools. If during this period of reversed circulation the fire is relit and hot water will rise up the flow pipe from the boiler and meet the decending cooler water resulting in very loud banging and knocking noises within the system, which will continue until normal circulation is resumed. As a preventive against reversed circulation, thermal insulation material is placed around the primary flow pipe and the return pipe is left uninsulated, this should ensure that the flow pipe is always warmer than the return.
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