Hotwater

Hot-water heating is now effected by one of two systems, high-pressure and low-pressure. In the high-pressure system small wrought-iron pipes of great strength pass from a boiler-coil in the furnace upwards through the set of rooms, etc., in the building to be warmed. The boiler-coil is simply that part of the arrangement which acts as a boiler and which is composed of lengths of pipes in series instead of a single boiler-shell, for the sake of extra strength and greater heating surface. The pipes rise to the highest part of the building, and then return by vertical stretches to the boiler coil again. The system forms a complete circuit, through which water flows to the exclusion of all air except such as is dissolved in the water. But to allow for expansion of the water on heating a special cylindrical expansion-pipe is attached to the topmost portion of the circuit. It contains air, and is so placed that expansion of the water is permitted therein by compression of the air. When the furnace is fired the water in the coil becomes hot, and rises by reason of its lightness as compared with that of the colder column of water in the return pipe. The speed of flow is kept up continually by this difference of temperature between the supply and the return, the latter always being at the lower temperature on account of the heat losses during the flow through the building. The whole circuit being of great strength, it is possible to raise the temperature safely up to 200° C. The water being completely enclosed, it does not boil, but its pressure rises to perhaps ten atmospheres. The speed of flow is very great, but the working seems safe.

In the low-pressure system the same theoretical conditions hold for the flow of water, but the pipes are larger, the boiler is of the more usual form, and the speed of flow is much lower. It has the advantage over the other systems of supplying a more uniform temperature; the pipes are not so inconvenient to touch, and do less harm by contact with woodwork, etc. But the high-pressure system employs smaller pipes, more convenient to fix, and proportionately stronger. There is less liability of clogging in the tubes, and the boiler efficiency is greater.