111. The method which may be used for controlling the temperature of the air within a building depends upon the system of heating which is employed, and also upon the heating agent, whether steam, hot water, or hot air. The emission of heat from a steam radiator may be graduated in several ways:

1. By dividing the radiator into several independent sections and admitting steam to more or fewer of them, as desired.

2. By admitting steam at full pressure and shutting it off again at moderate intervals, in regular alternation; the average temperature of the radiator thus obtained depends upon the relative length and frequency of the intervals.

3. By varying the pressure of the steam.

The first method is a good one and is used to some extent, but its general use is prevented by the lack of suitable apparatus. The expense and bother of adapting the varieties of radiators now on the market, and making the necessary connections, with valves of the ordinary kind,is almost prohibitory.

The second method is usually operated by means of automatic valves, which are similar in general principle to pressure-reducing valves.

The third method is commonly applied by varying the intensity of the fire under the boiler, or by the use of an automatic pressure-reducing valve, the adjustment being varied to give the desired steam pressure.

The emission of heat from a hot-water radiator may be graduated by adjusting the inlet valve, thus controlling the amount of hot water which flows through it.

When the indirect-heating system is employed, the temperature of the air which is delivered by the apparatus may be controlled, not only by modifying the emission of heat from the radiators, but by mixing the hot air with a sufficient quantity of cold air to obtain the temperature desired. The former method is so slow in operation that it is not generally satisfactory; but the latter method produces the desired result very promptly and is easy to manage.

When hot-air furnaces are used for heating, the methods of controlling the temperature by mixing, or by operating the registers, are the only ones which will give satisfaction. Regulation by varying the fire, even although it is most commonly done, is altogether too slow and uncertain.

112. Mixing Valves

Mixing Valves. Fig. 40 shows the mode of applying a mixing valve to a duct leading from the mains of an ordinary forced-blast system. Hot air flows through the duct a, and cold air is supplied by the duct b. The opening of b into a is covered by a valve v, which is hinged as shown. By moving the valve up or down, the proportions of hot and cold air which pass into the flue f may be varied as desired. As the opening for cold air is increased, the flow of hot air is restricted, and vice versa, but the valve does not operate to increase or diminish the volume of the current passing up the flue f.

Fig. 41 shows the application of a mixing valve to an indirect radiator.

The radiator is enclosed in the case a, and is supplied with fresh cold air by the duct b. Warm air is delivered through d into the flue.

The duct b is connected to d by a by-pass pipe c, and the opening is controlled by a valve v, which is hinged as shown. This valve permits more or less cold air to pass directly into the flue without passing over the radiator. The flow of air through the radiator box a is checked to the extent that air is allowed to pass up the pipe c. The area of the opening of the valve v usually does not require to be more than one-third to one-half that of the vertical duet

Fig. 40.

Fig. 41.

The mixing valve should be located at the foot of the vertical duct, and it may be operated from any floor by means of suitable connections of wire and chain. If desired, the connections may pass down through the flue.

All mixing valves should operate to change the proportions of the hot and cold air, without affecting the volume of the currents passing into the rooms.