Excepting in cases where the hot-water cylinder or cistern is deliberately left bare so that heat from it may be used for airing clothes or warming a room, there are obvious advantages in retarding the cooling of its contents. A certain amount of fuel will be saved, and water which has once been well heated will keep hot a long time. If the jacketing is very thoroughly done, and no water be drawn off, the water may retain a high temperature even for days.
The smaller a cistern is, the more quickly does it part with its heat, since the radiating surface is larger in proportion to its contents. For example, a cubical tank 24 inches on the side, holding about 50 gallons of water, has 24 square feet of surface – say I square foot per gallon; whereas a tank 3 feet on the side, holding 160 gallons, has 54 square feet of surface, or less thansquare foot to the gallon, and therefore would hold its heat longer than the smaller tank.
The hottest water in a cistern is always at the top, where it may almost have reached boiling point though the water at the bottom is nearly cold. So radiation will be greatest at the top and decrease towards the bottom, and the upper part of a tank therefore requires insulation most.
If the cistern is square-cornered and close to an angle of the walls, the two sides next the walls can be well insulated by stuffing cotton-wool, wool, hair, or other fibrous material into the spaces between the walls and cistern. This will imprison air which, next to complete absence of air, is the best of insulators if unable to move. Special care will be needed round the edges to avoid leakage.
The remaining sides and top, or all the sides, if they are accessible, can be jacketed with boiler felt,hich or more thick, or, failing this, with old rugs or carpets with a long nap, the nap turned inwards. The covering is held up against the sides by bands tied very tightly round top and bottom. If a pipe comes in the way, a slit must be cut, and enlarged to the size of the pipe in the right place.
To make a really neat job, and at the same time improve the insulation, a wooden casing of match-boarding may be constructed to fit outside the soft covering, an opening with cover being arranged opposite the manhole.
An alternative, which gives good results, and is easy to carry out, is to use asbestes for the jacket. From 5-10 lb. Of asbestes meal for every square foot of surface will be needed, according to the thickness of coating desired. (The bottom cf the cistern may be left out of the calculation, as very little heat escapes through it.)
To prevent the walls being splashed during operations, pin paper on them; and spread a dust sheet or two on the floor.
Before application of the asbestes begins, the cistern should be heated until it is as hot as the hand can bear; but it must not be allowed to become hotter than this. Then make a thin wash of asbestes and water, and brush the cistern – which is assumed not to be painted – over with it.
The meal should be spread out and all large lumps put aside to soak. The One meal is then placed in a large bucket and mixed with sufficient water to form a fairly stiff paste. The mixing must be done thoroughly, as any material left dry may give trouble.
The sides of the cistern are now spattered all over with small pellets of the paste, flung with the hands or a square-ended paint-scraper. These are left to dry on. In the process of drying the asbestes becomes much whiter. The manhole, by the by, should not be covered, as breaking away any part of the insulation may cause the rest to crack and separate from the metal.
When the rough key coat is dry, a layer half an inch thick may be added with a round-nosed trowel. The surface should be left rather rough, to give a better hold to the succeeding layer. The final coating is smoothed down with the trowel, small quantities of paste being added to fill up any hollows.
If any asbestes remains over, use it on the top of the cistern, where it will be of most service. Round the asbestes off at all corners, and taper it towards the manhole. Sharp edges are easily damaged and liable to crack.
The asbestes may be painted over to match the surroundings. A more secure job will be made if, before painting, coarse canvas is stuck to the asbestes with a liberal amount of flour-and-watcr paste. The asbestes will then be held permanently in place, whether it cracks or not. The canvas, when dry, may be given a coating or two of paint.