This system is similar in construction to one for direct steam, except that hot water flows through the pipes and radiators instead of steam. It is largely used for the warming of dwelling houses to which it is especially adapted owing to the ease with which the temperature of the water can be regulated.

Direct Hot Water 100013

Fig. 9.

Where steam is used the radiators are always at practically the same temperature, and regulation must be secured by shutting off steam and turning it on at intervals depending on the outside temperature; while with hot water, the radiators can be kept turned on all the time, and regulation secured by varying the temperature of the water flowing through them.

There are two distinct systems of circulation employed; one depending on the difference in temperature of the water in the supply and return pipes, called "gravity circulation;" and another where a pump is used to force the water through the mains, called "forced circulation." The former is used for dwellings and other buildings of ordinary size, and the latter for large

Direct Hot Water 100014

Fig. 11 buildings, and especially where there are long horizontal runs of pipe.

For gravity circulation some form of sectional cast iron boiler is commonly used although wrought iron tubular boilers may be employed if desired. In the case of forced circulation a heater designed to warm the water by means of live or exhaust steam is often used. A centrifugal or rotary pump of the type shown in

Direct Hot Water 100015

Fig. 10.

Fig. 12 is best adapted to this purpose; this pump may be driven by an electric motor, or a steam engine, as most convenient. Fig. 13 shows the general form of a hot-water radiator, which is similar to those used for steam, except the sections are connected at the top as well as at the bottom; this is shown by the cap over the opening at the top of the end section, which does not appear on the steam radiator shown in Fig. 2. A system for hot-water heating costs more to install than one for steam as the radiators have to be larger and the piping of larger size and more carefully graded.