This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
A hot water system is similar in construction and operation to one designed for steam, except the hot water flows through the pipes, giving up its heat by conduction to the coils and radiators, which in turn transfer it to the air of the room by conduction and radiation.
The flow through the system is produced solely by the difference in weight of the water in the supply and return pipes, due to the difference in temperature. When water is heated it expands, and thus a given volume becomes lighter and tends to rise, and the cooler water flows in to take its place; if the application of heat is kept up the circulation thus produced is continuous. The velocity of flow depends upon the difference in temperature between the supply and return, and the height of radiator above the boiler. The horizontal distance of the radiator from the boiler is also an important factor. Types of Radiating Surface. Cast iron radiators and circulation coils are used for hot water as well as for steam. Hot water radiators differ from steam radiators principally in having a horizontal passage at the top as well as at the bottom. This construction is necessary in order to draw off the air which gathers at the top of each loop or section. Otherwise they are the same as steam radiators, and are well adapted for the circulation of steam, and in some respects are superior to the ordinary pattern.
The form shown in Fig. 29 is made with an opening at the top for the entrance of water and at the bottom for its discharge, thus insuring a supply of hot water at the top and of colder water at the bottom.
Some hot water radiators are made with a cross-partition so arranged that all water entering passes at once to the top, from which it may take any passage toward the outlet. Fig. 30 is the more common form of radiator, and is made with continuous passages at top and bottom; the hot water is supplied at one side and drawn off at the other. The action of gravity is depended upon for making the hot and lighter water pass to the top, and the colder water sink to the bottom and flow off through the return. Hot water radiators are usually tapped and plugged so that the pipe connections can be made either at the top or at the bottom. This is shown in Fig. 81.
The efficiency of a hot water radiator depends entirely upon the temperature at which the water is circulated. The best practical results are obtained with the water leaving the boiler at a maximum temperature of about 180 degrees in zero weather and returning at about 160 degrees; this gives an average temperature of 170 in the radiators. Variations may be made however to suit the existing conditions of outside temperature. We have seen that an average cast iron radiator gives off about 1.5 B. T. U. per hour per square foot of surface per degree difference in temperature between the surrounding air and the radiator, when working under ordinary conditions, and this holds true whether filled with steam or water.
If we assume an average temperature of 170 degrees for the radiators then the difference will be 170 - 70 = 100 degrees, and this multiplied by 1.5 = 150 which may be taken as the efficiency of a hot water radiator under the above conditions, which represent good average practice.
This calls for a water radiator about 1.5 times as large as a steam radiator to heat a given room under the same conditions. This is common practice although some engineers multiply by the factor 1.6 which allows for a lower temperature of the water. Water leaving the boiler at 170 degrees should return at about 150; the drop in temperature should not ordinarily exceed 20 degrees.