This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The working of the apparatus is quite simple. The boiler g, and the syphon air-vessel r', are filled with water to the required level, and as soon as steam is generated in the boiler, the air, occupying the steam-space of the boiler and distribution-pipes v v, is displaced by the steam and driven through the radiators into the vessel r; displacing in turn the corresponding volume of water from this vessel, and driving it into the vessel R through the syphon-pipe w. Each radiator has a specially-constructed steam-valve, as shown in Fig. 552. The valve has an indicator, and by reference to this the degree to which the valve is opened may be ascertained , and accordingly as the valve is more or less opened, more or less air will be forced out of the radiator into the vessel r'. If the regulating-valve be quite closed, the steam in the radiator will quickly condense, and the radiator will again fill with air from the vessel r'.
The advantages of this system are considerable. The air in the heating-system cannot escape , as it is trapped on the one side by the water in the boiler, and on the other by the water in the vessel R, and therefore no fresh air from outside is taken into the system, This fact is of the greatest importance, as the air, hermetically inclosed in the system, loses its oxygen in a very short time, and ceases to have the slightest corrcsive action upon the inside of the pipes and radiators. Air-valves are not required, and the regulation of each radiator can be effected perfectly and easily by one steam-valve The heating may be carried on either continuously or with breaks, as may best suit the character of the building, the season of the year, or the preference of the owner. As all the connections which contain water when the heating is out of use are without exception in the basement of the building, the risk of freezing is very Blight indeed, and damage to the radiators and pipe-connections above the basement from this cause is quite impossible. The water-level in the boiler is not subject to any variation during working, as all the condensed water is returned direct by gravitation, and perfect noiselessess of working is secured, if the steam and water never come into direct contact with one another in the pip
The maintenance of a constant pressure is of special importance in low-pressure steam - heating, and the inventors of this system have designed an automatic draught-regulator (shown in Fig. 520, page 122), by which this is secured, q q, are vessels containing mercury, D is the steam connection to the boiler, w the water-connection to the stand-pipe, s a float, h the lever arm. f a movable weight, and v v1 valves for regulating the admission of the air. The action of the regulator depends upon the change of level of the surface of the mercury in the vessel Q. The upper part of this vessel is connected by the pipe D to the boiler, and the mercury rises and falls as the variation of the steam-pressure in the boiler causes the movement of the float s, which works the le\ h, on the ends of which the double valves v and V1 are suspended. When the pressure in the boiler rises to a certain height, the valve v1, which regulates the air-admission to the furnace, commences to close, and the valve v, which allows air to pass by a second canal in the bolier -setting direct into the flue, begins to open. When the maximum pressure desired is reached, the valve v1 is completely closed and the valve v fully opened, so that the fire is deadened and does not again burn briskly, until the reduction in steam-pressure has resulted in the sinking of the float s, and the consequent alteration of the position of the valves v and v1
Fig- 661-View of Steam- valve.
An improvement has been made in the regulator by the introduction of the movable weight F. By altering the poisition of this weight, the valve v and the float can be nearly balanced, and it is possible to obtain a constant pressure of from 4½ lbs . down to 0.15 lbs. per square inch. It is thus possible during the night, or at intervals when the heating is not required, to lower the pressure thus causing the coila in the various rooms to be partly filled with air, and thereby reducing the fuel-consumption during this time to a minimum. When heating is again required, the weight is moved, and the steam-pressure rises; the air is forced out of the coils, and the rooms are warmed without anyone in the rooms troubling in the least about it. The heating of all the rooms is in this way under full control from the boiler-house.
Although, in ordinary working, an excessive rise in the steam-pressure is prevented by the draught-regulator, it is conceivable that, by some accident. such an increase may occur. The steam-pressure would then force the water out of the syphon in the stand pipe; the steam would escape, and the pressure would altogether disappear. In such a a.case the pressure on the mercury in the regulator would also cease, and owing to the consequent sinking of the float, the air. which had been kept from the furnace by the closed valve v1, would be admitted again, and there would be risk of burning out the boiler. The regulator is, however, so designed that all such risk is prevented. There is a second vessel Q, containing mercury, connected by a small pipe w to the upper part of the stand pipe Should the water blow out of the syphon, part of it flows out of the stand pipe into q,, which is thus under water-pressure equal to the height of the stand-pipe. As this pressure is at least as high as the maximum steam-pressure needed to work the float, this will maintain the float in the position in which air is cut off from the furnace, until the water in the pipe w is allowed to run out through the valve E, Fig. .519, page 122.