Hot-water heating.

Although direct hot-water heating, owing, mainly, to its better regulation, but also, perhaps, in some degree, to the lower temperature of the heating surfaces, is less oppressive than direct steam heating, neither system affords any ventilation; and, to secure a supply of fresh air in connection with the special advantages of a hot-water system, indirect radiators, similar to those used for steam, must be placed in the cellar, and the fresh-air current passed through them. As the temperature of the hot-water radiators, under the open-tank system generally used in this country, is considerably lower than that of a steam radiator, their size must be proportionately increased, in order to obtain sufficient power for the coldest weather; and an indirect hot-water heating system is on this account both bulky and expensive, although, where the necessary space and money can be afforded, it gives admirable results, with a comparatively small expenditure of fuel. The intermediate system, known as direct-indirect, where the radiators standing in the rooms have special air-supply, although common with steam, is very rarely applied to hot-water heating, probably on account of the danger of freezing a hot-water radiator in which the circulation happened to be cut off or obstructed.

Indirect hot-water heating.

The difficulties encountered in the management of steam or hot-water heating apparatus, either direct or indirect, are usually due either to insufficient heating surface, insufficient boiler power, or imperfect circulation. Where heating work is done by contract, the contractors, who have usually secured the work by putting in the lowest bid, in competition with other contractors, are naturally tempted to furnish the smallest radiators and pipes, and the cheapest and smallest boiler, that they think the owner can be prevailed upon to accept. It seldom happens, under such circumstances, that the house is comfortably warmed in the coldest weather; and, if the owner obstinately declines to accept argument in place of heat, the contractor, if, as should always be the case, he has given a written guarantee to heat a certain list of rooms simultaneously to 70° when the thermometer outside stands at zero, without overtaxing the boiler, will generally begin by taking out the radiators in the coldest rooms, and substituting larger ones. If the boiler is really large enough, this process, if carried sufficiently far, and if the pipes are of the proper size, and well arranged, will remove the difficulty; but if the boiler is too small, so that, with steam, it is necessary to maintain more than the usual standard, for house boilers, of ten pounds' pressure, to furnish the required heat, this also should be rejected, and a larger one put in.

Difficulties in management.

Where a large house is to be heated, it is advisable for the owner, or some expert for him, to calculate, by means of the simple rules given in the text-books, the sizes of the radiators which will be needed for rooms of the given dimensions, considering also the window surface and the exposure, and deduce from these the necessary size of the pipes, and the required heating and grate surface of the boiler, and to make the contract accordingly. As all these conditions must be fulfilled before the building can be properly heated, it is a saving of time and money to see that they are complied with at the outset.

Even where the boiler, pipes, and radiators are large enough, some of the rooms may fail to heat, owing to imperfect circulation in the pipes. This may occur either with steam or hot water heating, but it is more easily remedied with the latter. In general, the ideal hot-water heating system may be said to consist of a vertical loop, with the boiler at the bottom, the hot water flowing out of the top of the boiler into the ascending portion of the loop, and, as it cools, returning by the descending portion of the loop into the bottom of the boiler, where it is again heated, and again rises, to go through the same course. In practice, however, this simple plan must be modified in various ways. In order to heat several rooms on the same story, it is necessary to use branch pipes, running nearly horizontally, to supply some of the radiators, at a distance from the main ascending and descending pipes. If these branch pipes, both for flow and return, can be kept either perfectly horizontal, or inclined in parallel lines, the flow-pipe continuously ascending to the radiator which it supplies, while the return-pipe continuously descends, the circulation will still be good; but a dip in either pipe below the horizontal, or a curve above it, will, unless special precautions are taken, stop the flow, for the reason that, in a downward dip, cold water, being heavier than hot water, accumulates, interposing a barrier to the current, while an upward bend is soon filled with air from the bubbles carried along in the water, and, as the current has not force enough to push the air out of its path, the circulation is stopped in this way as effectually as in the other. In fact, an "air-bound " pipe is more effectually stopped than one with a dip in it; for, by carrying up a loop of pipe, in such a way that the water, before it reaches the dip, ascends as high above the general line of the pipe as the dip is below it, or, if possible, a little higher, the resistance of the cold water in the dip will be overcome, and circulation will go on without interruption, while an air-bubble confined at the top of an upward bend can only be got rid of, and the flow restored, by letting the air escape through a valve or otherwise, which is not always an easy matter.

Imperfect circulation.

In practice, hot-water systems are usually so arranged that the water rises from the boiler almost immediately to an "expansion-tank" at the top of the house, the pipes descending thence, and supplying the radiators on the way, so that the force gained by the elevation of the expansion-tank above the general heating system is available to lift the water out of any dips that may be necessary to carry the pipes past doors or under floors; but this is not always the case, the radiators being sometimes fed from the upward current. In either case, the whole system, including any upward bends, if these are used, should be so arranged that air-bubbles in any pipe will rise to the expansion-tank and escape.