279. Mixing valves are indispensable to the success of any system of ventilation. They form the only practicable method of quickly regulating the temperature, and at the same time securing an unvarying amount of fresh air.

Fig. 89.

The registers which are commonly used for controlling the admission of air to rooms, having louvres or valves that can be opened or closed, are all wrong in principle and should be discarded. Notwithstanding the fact that they are almost universally used with indirect-heating apparatus of all kinds -steam, hot-water, and hot-air furnaces-yet they are radically defective in the most important particular. They do not permit the heat to be shut off without shutting off the supply of fresh air at the same time.

In large rooms which are supplied with fresh air at several points, it is desirable that each flue should be provided with a separate mixing valve, so that the temperature of the several air-currents may be varied independently when required. Fig. 89 shows an ordinary indirect radiator box, having a cold-air inlet at a. A swinging valve b is hung near the end of the radiator, in such a manner that it can be made to deflect any proportion of the air-current and compel it to pass over the heating surfaces. The valve is operated by means of a chain c and a weighted handle d.

The importance of the improvement in ventilation to be made by using these devices is not understood or appreciated, either by the public, who are vitally interested in the matter, or by the architects and others who supply heating apparatus.

280. In designing a system of flues for warming and ventilating purposes, it is necessary to carefully consider not only the proper dimensions, but also the efficiency, durability, convenience, and cost of construction.

The velocity of the air-current in wall flues and in branch conduits should always be made lower than in the main trunk pipes, not only for the purpose of reducing the frictional resistance, but to avoid delivering the air into the rooms at a velocity too great to be easily controlled and diffused, and also to prevent perceptible drafts.

When the air is driven by a fan, the velocity may be 20 feet per second in the main flue, and somewhat less in the large branches, but should not exceed 10 feet per second in the wall flues. With the aspiration system, only half of these velocities can be obtained.

Brick flues should be plastered on the inside in order to present a smooth surface to the air, and especial care should be taken in this particular when the aspiration or natural draft system is to be employed.

281. Metal flues possess the advantage of being easily made to any reasonable shape, thus permitting round corners and easy curves; a further advantage is that of occupying less space than any other kind. Where they are exposed to moisture, or to the corrosive action of plaster or mortar, their durability may be insured by coating them thoroughly with asphaltum.

In the plenum system of ventilation, arrangements are often made to carry hot and cold air separately in duplicate flues. Usually the flues are made of equal size, but in most cases this is not necessary, because the volume of cold air required for tempering or mitigating the temperature of the hot air-current is usually considerably less than the volume of hot air.