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1198 CHAPTER 49 1957 Guide been used in summer homes as far north as Chicago. When properly installed and proportioned, solar water heaters render satisfactory service, especially in climates where the outside temperatures are high and ex tremely hot water is not necessarily desirable. Such installations consist essentially of a storage tank, heating coil, and hot box. The coil is installed in the hot box, and is arranged to circulate water to and from the storage tank. The advantage in the use of this type of heater is the fact that it -requires no fuel. The same materials should be used for the coil, circula tion lines, and tank. A copper coil is more efficient in absorbing heat in the box, but galvanized iron or steel may be substituted, depending on the local water conditions, cost, and other considerations. The storage tank must be able to store sufficient heated water for the night period of about 16 hr when the coil is not functioning, or is operating under such poor sun conditions as to make its heating effect negligible. Due to the fact that the no sun period includes the night period when little or no hot water is used, an available storage of 50 percent of the average daily usage is' considered adequate. Since about 25 percent of stored hot water cannot be drawn out of a storage tank before the incoming cold water reduces the temperature of all of the water in the tank to an unsatisfactory point for usage, the equation for calculating the storage capacity of the tank becomes : ,, Qi X 0J50 S " 0.75 0.666Qa (5) where S = storage capacity of tank, gallons. Qd = average daily usage, gallons. Thus, for a family of four persons using an average of 40 gal of hot water per (person) (day), the size of the tank would be 4 persons x 40 gal x 0.666 or 106 gal, and the nearest standard size of tank would be used. The tank should be well insulated to prevent undue loss of heat during the 16-hour period when the coil is inoperative, and it should be located as high as possible in the building (under the peak of the roof if such exists) so as to secure a maximum circulation head from the coil. The hot water supply line .to.the house, as shown in Fig. 12, is.connected to the top of the tank and serves to vent the air from the tank through the hot water faucets as fast as it accumulates.. The coil should be of the return-bend type (square or slightly rectangu lar in form), and should have the pipes running east and west, with the coil on the south side of the building where it can receive the full sun effect all day long without shadows from the building itself, or from the adjacent ob structions such as trees or other structures. The coil should be placed as low as possible in relation to the storage tank level, such as on a porch roof, the roof of a one-story extension or, if necessary, even on the ground. Both the coil and the circulation lines should be designed to facilitate the circula tion flow as much as possible, using long radius copper fittings or recessed galvanized iron fittings to match the materials of the coil, circulation lines, and tank. The coil should be inclined, as shown in Fig. 13, so that the north end is raised above the south end to secure an angle with the hori zontal of about 53 deg. This will result in the inlet end of the coil being on the south side (or bottom), and the outlet end being on the north side (or top). This will satisfy conditions along the 30-deg N latitude, which Water Services 1199 includes the portions of Florida and Southern California where these heaters are most frequently used. The hot box is usually constructed of wood on the four sides and bottom, and is insulated. Glass sash are placed over the top of the box, and the box should be constructed as air-tight as possible. The interior surfaces should be painted white to reflect the heat, while the coil should be painted black to absorb the heat. The box need not be deeper than necessary to house the coil and to protect it from the weather. Table 12. Suggested Solar Heater Design Data* Design Item Based on Rate op 30 Gal peb Day peb Pebson Based on Rate op 40 Gal peb Day peb Pebson No. of Occupants in Residence.. I 2 3 4 5 6 7 8 2345678 Hot Water Used at Night, gal per person................................ 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 Hot Water Used at Night, gal total........................................... 15 30 45 60 75 90 105 120 20 40 GO 80 100 120 140 160 Retained in Tank, 25 per cent, gal 4 8 11 15 19 23 27 30 5 10 15 20 25 30 35 40 Tank Capacity Required, gal... 20 40 59 75 94 113 130 150 25 50 75 100 125 150 175 200 Hot Water Used During Day, gal 15 30 45 60 75 90 105 120 20 40 GO 80 100 120 140 160 Total Water to be Heated: Gal per 8 hr period.................... 35 70 104 135 169 203 235 270 45 90 135 180 .225 270 315 360 Gal per hour............................... 4.6 9 13 17 21 26 29 34 6 12 17 23 28 34 39 45 Copper Coil Required: Surface area, sq ft..................... 25 50 75 100 121 145 168 192 32 64 96 128 160 192 224 256 Equivalent length 1 in. coil, ft. 100 200 300 400 484 580 664 768 128 256 384 512 640 768 896 1024 Box Size: Area, sq ft.................................... Width, ft...................................... 25 4 50 6 75 100 121 145 168 192 7 8 9 10 10 11 32 4 64 6 96 128 160 192 224 256 8 9 10 11 12 12 Length, ft.................................... 6 8 11 12.5 13.5 14.5 16.S 17.5 8 10 12 14 16 18 19 21 131*). Sun Effect and the Design of Solar Heaters, by H. L. Alt (A.S.H.V.E. Transactions. Vol. 41. 1935. Kd. The addition, on the bottom of the box, of a light gage copper plate to which the pipe of the coil is soldered for good metallic contact, will add to the amount of heat received by the coil, due to the fact that this plate will receive all of the sun's rays which fail to directly strike the Coil. The heat from this source is transmitted to the coil through the plate rather than from the heated air surrounding the coil. Otherwise, only part of the heat enters the coil, the balance being transmitted through the glass. Design data given in Table 12 may be used with judgment in selecting the size of solar heater coil and box for a particular application. These data are based on consumptions of 30 and 40 gal of hot water per day per person. DOMESTIC HOT WATER BY HEAT PUMP Hot water may suitably be obtained by using a heat pump installation. The hot water heater may be either a heat exchanger installed just ahead of the compressor of a heat pump installation, or may be a self-contained domestic-water heat pump. Various designs of self-contained domestic water heat pumps are avail able, and one particular arrangement is shown in Fig. 14. Hot water heating by means of a heat pump is not yet advisable where