=Hot-water Pipes= are probably the best means of carrying heat to various parts of a large house, and hot water is more thoroughly under control and less dangerous than either hot air or steam. There are _two systems_ of heating by hot water.
In the first, which we may call the =low pressure system=, there is a boiler from which water circulates through pipes to every part of the building, and as it cools down returns again to the boiler. At the highest points of the pipes, outlets are provided for air. In this system the water is not heated above 200° Fahr., and there is consequently no great pressure on the pipes.
In the =high pressure system= (Perkin’s patent), the pipes have an internal diameter of about 1∕2 an inch, and have thick walls made of two pieces of welded iron. There is no boiler, but one portion of the tube passes through the fire and the water is heated to 300-350° Fahr., thus subjecting the pipes to great pressure. In dwelling-houses with the low pressure system, for every 1,000 cubic feet of space to be warmed to 50°, 12 feet of 4-inch pipe should be given; with Perkin’s pipes, probably about two-thirds of this will suffice.
=Steam Distributed by Pipes= may be employed instead of hot water. This method has been used in factories in which there is a surplus supply of steam.
=Warming by Hot Air= is only applicable on a large scale, and should only be used in association with a system of ventilation by propulsion (page 153), in which the temperature, humidity, and freedom from dust of the entering air are carefully regulated.
=Warming by Electricity= both for cooking food and for warming rooms has a large future, but in most districts the supply of electricity is not hitherto sufficiently cheap to be used for these purposes. By its means the atmosphere will be prevented from becoming impure, labour will be reduced, and life rendered more pleasant.
=Hot Water Supplies.=—Nearly every modern house is supplied with a bathroom, and this may be supplied with hot water either from a geyser or from the kitchen boiler. In a _geyser_ the water is made to flow over a large heating surface furnished by burning coal-gas, and with the best varieties a bath of 98° F. can be supplied in from five to ten minutes. As the bathroom is usually small and unprovided with an open fire-place, persons have occasionally been suffocated by remaining in such a room while the gas continues burning. This is due to the production and in-breathing of carbonic oxide. No geyser ought to be allowed to be used which is unprovided with a flue passing into the chimney flue or in its absence through an external wall of the house. Short of fatal poisoning, violent headaches often occur when a warm bath obtained by means of a geyser is taken, unless such a flue is provided. In Ewart’s lightning geyser, additional protection is furnished by the fact that a dual valve is so arranged, that immediately the water is turned off or the supply fails from any cause, the supply of gas is also cut off.