Hot Water Storage Systems
Domestic hot water is an essential amenity in any home. Its provision may be linked with a central-heating system, or heat may be provided by an independent source, such as immersion heater. There are several types of hot-water cylinders and of immersion heaters, which can meet every situation. Once installed, lag a cylinder to prevent unnecessary loss of heat.
In older houses, hot-water facilities are often provided by a square galvanized storage tank, heated by a solid-fuel boiler in the kitchen, possibly supplemented by an immersion heater. These arrangements rarely include satisfactory heating and are relatively costly to run.
Modern practice is to heat water through a hot-water cylinder, usually made of copper, though glass-fibre and stainless-steel cylinders can be used. The cylinder water is usually heated by the house central-heating system or by means of an immersion heater.
A standard cylinder is 915 x450mm in circumference. The bottom of the cylinder is concaved or ‘dished’ to give strength. The top is domed or ‘crowned’ to help to prevent air locks. The capacity of such a cylinder is usually adequate for the domestic hot-water needs of a three-bed-roomed home. For bigger homes and families, and situations where there is a greater demand for hot water, larger cylinders can be used.
Direct and indirect
There are two main forms of cylinder – the direct and the indirect. In the direct cylinder, the circulating hot water is heated, drawn off and then replenished by fresh storage water. The indirect cylinder has an inner jacket, called a calorifier, through which hot water circulates to heat the surrounding draw-off water. These two waters do not co-mingle.
Another version of the indirect cylinder is the self-priming type. The primary and secondary waters are separated by an air bubble. The water in the cylinder must be kept below boiling point, or the bubble will disperse, and the waters merge. It will then be necessary to allow the water to cool and contract so that the bubble can re-form.
The circulating hot water, when it is part of a central-heating system, is purged of air which lowers the risk of scaling and corrosion. ‘Live’ water contains a high proportion of oxygen. When this is purged of air, the ‘dead’ water is unlikely to give problems.
Circulating hot water can be distributed by what is called natural or gravity circulation or it can be pumped. Gravity circulation works on the principle that molecules of hot water expand, allowing expansion and distribution of hot water to take place. This requires pipework possessing a minimum bore of between 22mm and 28mm.
In the process of circulation, the molecules of the cooling water contract and return to the heating source, where the water is reheated and re-cycled.
When a pumped system is employed to circulate the hot water, 15mm or 22mm pipe can be used, dependent on the capacity of the hot cylinder. This method is called a pumped primary. It is essentially part of a central-heating system and to allow this to work most effectively and economically, a special type of indirect cylinder is needed.
This is called a high-recovery cylinder, which heats and reheats water very quickly. A conventional cylinder will heat and recycle water from cold in about an hour; the high-recovery version takes only about half this time.
A self-priming cistern may not be used on a pumped-primary system, as the pump action would release the separating air bubble.
A direct cylinder can be adapted by a special element, to convert it to a high-recovery unit. It is then suitable for use on hot-water pumped primary circuits. The element fits in the immersion-heater boss.
10. November 2011 by admin
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