Exhaust steam from an engine is often used for heating. The water of condensation from an exhaust steam heating plant is frequently allowed to run to waste, but as its temperature is near boiling, coal is saved if the water is collected in a receiver and pumped back into the boiler. Mill engines that are run with condensers cannot furnish exhaust steam for heating. In such a case, live steam must be taken from a branch opening in the main steam drum in the boiler-room. This requires the use of a reducing valve to let the pressure down between 5 and 15 lbs. for the heating coils. A receiving tank is necessary for the return water, and a pump must be installed to force it back to the boilers. Some mills have spare boilers that are used only for heating purposes. These may be run at a low pressure, 10 lbs., and the steam may be passed directly into the heating system without the use of a reducing valve. When the return water is piped directly to the feed pipe of the boilers we have what is known as a gravity return system. Since there is the same pressure in the heating system that there is in the boiler, the water of condensation runs back into the boiler simply by its own weight. This requires that all heating pipes be on a higher level than the water line in the boiler. If any radiators or coils were lower than the boiler, they would, of course, fill with water, and a pump would be required to return the water from the low coils to the boiler. The gravity return system is used in many dwellings, office buildings, churches, and stores

Fig. 174.   Indirect Heating System.

Fig. 174. - Indirect Heating System.