A much better tank for the purpose is that indicated in Fig. 118. This is a tank made particularly for such a location. The cold water enters at the bottom of the tank and also leaves the bottom on its way to the water-back. Circulation takes place through the water-back as before but when hot water is drawn from the top of the tank, the entering cold water at the bottom mixes with only that at the lower part of the tank and so cools but a small amount of the hot water in storage. Hot-water tanks of this kind are tapped for pipe connections in two places on both the top and bottom sides and also at the ends as shown in the drawing. =Tank Heaters.=--When the demand for hot water is sufficient to warrant a separate hot-water heater the apparatus similar to Fig. 119 is used. With such a heater, the conditions of overheated water--to be described later--may be almost entirely avoided. In this case the connections are arranged similarly to those of the range boiler but a separate furnace takes the place of the water-back. The heater is simply a small furnace made expressly for heating water. Connected with the discharge pipe _p_ is a draft-regulating valve which controls the drafts of the heater. The draft-regulator is set to so control the furnace that water at the desired temperature will always be in the tank. The mechanism of this regulator is the same as the draft-regulator described under hot-water heating plants. =Overheated Water.=--Under ordinary conditions the water contained in the range boiler is below the atmospheric boiling point (212°F.) but at times when a hot fire is kept up in the range for a considerable period, the temperature will rise to a degree much above that amount. The temperature to which the water will rise will depend on the pressure of the water supply. As an example--suppose the gage pressure of the water supply is 25 pounds. The temperature corresponding to that pressure is 258°F. The temperature of the water in the tank will rise to that amount but not further because any additional temperature will produce a higher pressure, but a higher pressure would be greater than the pressure of the water supply and hence will back the water into the supply pipe. This condition of things, then, acts as a safety valve to the tank to prevent excessive pressures. [Illustration: FIG. 119.--Independent hot-water heater with temperature regulator.]