Hot Water Storage

Immersion heaters

With electric central heating or when solid-fuel heating is not operating, hot water can be provided by an immersion heater which screws into this boss.

A direct cylinder is used where the immersion heater alone provides hot water. Some solid-fuel appliances use a direct cylinder only when the heating system, which provides hot water as part of its function, is not in use.

This may be operated, using a time clock, to heat water at chosen periods.


Immersion heaters are now nearly always 3kW wattage rating. Because of the high current rating, these should be connected via a separate power circuit, using a cable of 4mm2. Immersion heaters have thermostatic devices allowing temperature settings to be between 49°C and 82°C. For domestic purposes a satisfactory setting is around 60°C.


There are two types of heater – the dual and the single – stage coil. Hot water stratifies and rises and the water at the crown of the cylinder is always hotter than that lower down.

Water stratifies to the extent that there can be a marked difference in temperature over a very small distance.

The two-stage heater will either heat the water in the crown for light draw-off use, such as washing up and washing, or heat the bulk of the water for major use, such as baths.

The immersion heater length should be about two-thirds of the depth of the cylinder. The heater screws into the boss. The threads are sealed by winding PTFE tape round the thread against direction of turn, or by using hemp string and plumbing nontoxic compound.

Next, unscrew the sealing plate from the boss, and insert the heater. Then, tighten this against its sealing washers using a large wrench. Do not overtighten as this might rupture the cylinder. These are usually made from thin copper which, though overall structurally strong, are vulnerable at connection points.

Where hot-water is part of an oil or gas-fired heating system, function controls are usually used to operate switching of heating and hot water.

Electric storage heating systems, operating on off-peak electricity, can be used to heat the domestic water overnight on this cheaper tariff; heat loss can be boosted at full rate where necessary.

Good connections

Cylinder connections are all of similar, male-iron thread type. These consist of the hot-water draw off, taken from the crown of the cylinder, and the cold feed from the cold-water storage cistern, made to a connection about a quarter of the way up the cylinder.

The primary hot-water connections are located on the opposite side of the cylinder. The flow circuit connection is about a quarter of the way down from the top of the cylinder and the return one an equivalent distance from the bottom.

Cold supply is usually provided by 15mm tube. The draw-off water needs are usually met by a 22mm tube but sometimes 28mm pipe is required where there is greater hot-water demand.

Room for expansion

To allow for hot-water expansion, a 22mm vent pipe is taken by an offset fitting from the crown of the cylinder and brought up to and curved over the loft cold-water cistern, above the water.

This pipe is a safety precaution, for though hot water normally expands up the vent pipe for about 1-1.5m, it seldom vents into the cistern, unless excessively high temperatures are inadvertently attained.

Keeping hot

Sometimes taps are some distance from the cylinder, leaving a large amount of cold water in the pipes, which first has .to be drawn off. This can be avoided by means of a system of secondary installation. A normal draw-off connection is fitted, with an added supply circuit positioned above this and returning below it. When water is drawn from the distant hot tap it breaks the secondary circulation system.


The cylinder is most usually and best placed in an airing cupboard, where heat dissipated is put to use. However, excess loss of heat is expensive and the cylinder should be insulated. This is best done with one of the proprietary jackets – made of foam, glass-fibre or mineral wool. These jackets are ‘tailored’ to fit the size of cylinder and normally are fitted, quickly and simply, with straps, around the cylinder body.

Once the cylinder is filled with water, it will be very heavy. It should be stood on stout pieces of timber, set across the direction of the floor joists. Allow space beneath, for air to circulate, otherwise condensation may occur under the dished base.

A gate valve should be incorporated in the cold-water feed near the cylinder and a drain cock fitted to enable the cylinder to be drained down if necessary.

A terminal on some cylinders enables a drain cock to be screwed in. Otherwise, the cold-feed pipe to the cylinder is a good position. An MT pattern angle coupling can be fitted at the point where the pipe enters the cylinder.

Where the cylinder makes provision for a drain cock, the small sealing plate must be removed.

It is important that drain points are fitted where they can be quickly and conveniently reached. Taps in supply or draw-off pipework may be either compression or capillary ended. These are connected in the same way as other fittings.

10. November 2011 by admin
Categories: Featured, Handyman Tips | Tags: , , , , , , , , , | Comments Off on Hot Water Storage


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