If adopted it may be either on the hot-air or radiating plan, as in heating by means of furnaces. In the first method the fresh air is introduced into a chamber containing a coil of steam-pipes, and passes thence into the laconicum by a shaft or conduit, as in the case of air heated by a stove. In the second method, steam radiators—compact batteries of pipes—must be placed in recesses in the hot rooms, fresh air being introduced over them. The steam-pipes employed should be of the "small bore" type, about 5/8 inch internal diameter, and of wrought iron or copper. In order to ensure as far as possible against the danger of explosion, the system of pipes should be tested, when fixed, by severe hydraulic pressure.
It is certainly a great advantage, in point of ease and economy, to be able to warm a building, drive machinery, and heat Turkish and Russian baths from one boiler, which can readily be done, very ordinary pressures of steam giving sufficient heat to keep the radiators of the requisite temperature. But the nature of the heating accomplished by means of steam-pipes is very inferior to that from large radiating surfaces of firebrick.
The average temperatures of a public bath should range from about 110° in the shampooing rooms to 250°-260° in the hottest part of the laconicum, taking the readings of the thermometer at a level of 6 ft. 6 in. above floor-line. Between the entrance of the heated air and its point of furthest travel in the shampooing rooms, the bather should be able to select any temperature that may be most agreeable to him, and as many find by experience that a certain degree of heat is best suited to themselves, it shows attention to the habitués of the bath, if the hot rooms are carefully maintained at the same uniform temperatures throughout the year. This may be 110°-120° in the shampooing rooms, 140° in the tepidarium, 180° in the calidarium, and 250° in the laconicum. These must be the maxima of the average temperatures of each room at 6 ft. 6 in. above the floor. In a pure atmosphere the highest temperatures are comfortable, but in a foul one they become insupportable.
In a good bath, where there is a rapid and continuous flow of air, there will be comparatively little difference between the temperature at say 4 ft., 6 ft., and 8 ft. above the floor. In badly-ventilated rooms, where the air stagnates, there will be a considerable difference. And here we may note a serious objection to the heating of a bath by convection; for while the head may be in a high degree of heat the feet are in comparatively cool air, whereas, if possible, it should be just the reverse. In convected heat, this of course applies in its entirety, as where so-called radiant heat is employed the evil is not quite so marked. And here, too, we may note the admirable nature of the Roman system of heating, where the floors radiated the majority of the heat, and the walls a slightly less amount. The fresh air under the ancient system must have entered through the cooler rooms, and being drawn towards the calidarium found its exit through the ceilings, at times by way of the regulating device mentioned by Vitruvius. Thus the ancient bather would not suffer the inconvenience that accrues to the bather in the modern hot-air bath, whose head, when he is standing upright, is in a considerably higher temperature than any other portion of his body.
The temperature of a bath should not be regulated by the firing of the furnace. This should be regularly stoked, and kept at one uniform heat-giving condition. Bad firing and forced firing may crack the stove should it be of iron, and the air may be overheated. The temperature should be regulated by means of the hit-and-miss ventilators at the floor level. Fanlights between the various hot rooms, with screw-rod adjustment, serve as a means for regulating their relative temperatures.
The heating power of furnaces must be studied. Having calculated the cubical contents of the rooms to be heated, and given the heating power of the stove or apparatus to be employed per cwt. of metal or superficial foot of radiating surface, we arrive at the necessary size.
Messrs. Constantine give the following tables to show the heating power of the "Convoluted" stove. The figures give the requisite size of stove to raise the air to about the relative temperatures I have mentioned before, and with ordinary firing.
| Weight of metal. | Sq. ft. of heating surface. | Area capable of heating. |
| cwt. | sq. ft. | cub. ft. |
| 14 | 35 | 500 |
| 20 | 55 | 1,200 |
| 22 | 69 | 2,000 |
| 34 | 119 | 3,500 |
| 36 | 139 | 5,000 |
| 45 | 180 | 8,000 |
| 50 | 231 | 12,000 |
| 56 | 296 | 16,000 |
When different kinds of heating apparatus are employed, their heating power must be carefully ascertained and calculations entered into, or it may be found necessary to resort to the costly and humiliating process of dragging out the stove or pulling down the furnace and refitting a larger one. This point is worth attention. Such mistakes are not unfrequently made.
As regards the amount of air that should flow through the hot rooms, an allowance of 40 cubic feet per head per minute should be the minimum, if purity of atmosphere is to be maintained. In a bath, the importance of perfect ventilation cannot possibly be over estimated, as not only has the respired air from the lungs to be removed, but also the deleterious exhalations from the skin which are produced by perspiration.