When energy costs were low, hot water heating in residential units was considered to be of minor importance because adequate quantities of inexpensive energy were available to heat service water in a random manner to hot water temperature. In many cases, the water was heated to unnecessarily high scalding water temperatures which had to be lowered with large amounts of cold water to usable temperatures. For smaller dwelling units, e.g., single-family houses, less importance was attached to the economic aspects of heating water.
In large residential units, such as large hotels and the like, even with relatively favourable basic energy costs, the costs of heating water became a significant expense so that measures to economize were taken at an early stage. However, these measures were mainly limited to reducing line loses by adequate insulation. Measures involving the dynamics of hot water consumption were rarely used.
In 1973, DOS No. 2,231,231 disclosed an automatic mixing water installation for minimizing the temperature differences of the mixed water used which indirectly led to considerable energy savings. Considerable interest can be attached to the described measures, e.g., in community or municipal showers, to ensure that the user received the water at physiologically usable temperatures.
The described arrangement keeps the temperature differences in the mixed water drawn-off very low by an automatic mixing water installation with lines for supplying hot and cold water and with means for regulating the mixing of the cold and hot water. The temperature is maintained whether large or small mixed water quantities are removed as compared with the maximum possible water delivery. This function can be achieved by a closed mixed water circuit, possibly with feeding in mixed water and draw-off points, which circuit is also connected to the secondary side of a heat exchanger. The mixed water quantities circulating in this circuit having the desired discharge temperature cause a temperature compensation to take place so that thermal losses are compensated by the heat exchangers. An important advantage is that the installation is independent of a boiler or hot water tank. Thus, it requires no connecting lines between the mixed water and a boiler because the mixed water circulation forms its own operationally reliable, closed circuit.
Since 1973, the raw energy prices have increased considerably, making the earlier, simple solutions unacceptable. subtle and better thought-out solutions have been proposed and are now used. Ever-increasing importance is attached to different energy sources for the purpose of heating water, and in particular, energy from low temperature-generating units is being coupled into the known high temperature-generating systems.
Apart from solar cells, the heat pump is the most widely used means. However, the difficulties encountered in inexpensively heating water by means of a heat pump are described, e.g., in the article entitled "Warmepumpe, Argumente fur Beratung und Verkauf in Einfamilienhausern", Haustechnik, Vol. 12, 1981, No. 10, p.36ff. The decisive problem in using heat pumps is that without hot water accumulation or storage, heat pumps are unable to provide easily the required hot water with the presently conventional calorific power levels of 3 to 10 kW. With an adequate storage volume, equipment with a low calorific power can cover a high daily demand if sufficiently long intervals are permitted between the draw-off periods. Additional heating only improves the hot water capacity if there is sufficient heating time between two large water removal periods. Even with acceptable efficiency figures, the storage problem continues to exist because sufficiently large water tanks are relatively expensive and the amortization period is too long. Therefore, heat pumps cannot be used in large residential units, e.g., apartment houses or hotels, due to the storage problem, i.e., the cost/use ratio is disadvantageous.
The possibilities of using solar cells for heating water is described, inter alia, in chapter 17, "Solar Energy--Water Heating, Possibilities of Use", p.456ff. This discusses the additional problem of limitation to small residential units at peak loads.