In this system it makes no difference whether the floor of your boiler-house is on the same level as the greenhouses or two feet below or two feet above. It will make no difference in the working of the system. In my case the ground at the north-east end of the range happened to be two feet lower than the average level of the greenhouse site. I mention this because I think the north-east is where a building such as a boiler-house necessitates will cast the least shade and do the least harm. Being almost on a level with the surrounding ground the fuel can be wheeled to the furnace doors, but better than that, the ashes can be wheeled out. There is no iron ladder to climb down and break your neck on Christmas eve or other occasions when a faltering step is excusable, no water or stopped up sewer to fret and annoy. Now all there is about this system besides saving labor and expense in installation is that you carry from forty to sixty pounds of steam on the boiler all the time. One practical friend said he carried sixty-five pounds and believed it was more economical of fuel than forty pounds, and by the use of a controlling valve the pressure is reduced one, two, or five pounds on your system of heating. The controlling valve recommended to us was the Mason, made in Boston, Mass., and not once in three years' use has there been the slightest hitch or trouble with it. The slightest turn of the brass key on top of the valve will increase or diminish the quantity of steam passing through the valve. A small steam clock connected with the main steam pipe a few feet beyond the controlling valve will tell you how much steam you have on your heating system. In our case the main steam pipe leaving the boiler is only a 3-inch, carrying of course the fifty pounds of steam till it reaches the controller, which in moderately cold weather is set at, three, in severe weather at five or six, and this is enough to carry the steam in some of the houses for full 500 feet. It is just worth mentioning here that while you have fifty pounds of steam in your boiler, if a sudden storm comes up or a sudden fall of temperature, all of which often happens, there is no great commotion at the fire door but simply a turn of that little brass key on the controller and the opening of a few valves. You have the reserve steam at all times and it costs no more to keep it there than it does to keep up two pounds of steam on your gravity system.
Now, whatever arrangement of heating pipes you have there will be one pipe you call your main return, which in the gravity system empties the condensed steam into the boiler. In the pressure system it empties into an automatic steam pump or a trap. It is my duty to be honest, so I must confess I have no experience with a steam trap of any make, but I have watched them working and have consulted practical men who have used both the trap and automatic pump and in every case they gave a strong preference for the pump. I understand that a trap does not work unless you have five or six pounds pressure on your heating system. In the case of the pump you can have but one on your radiating pipes. That will make no difference because your pump is supplied by a small pipe (1-inch will do) from top of your boiler - where it gets whatever pressure is in the boiler. The return, pipe will most likely leave the greenhouse about the level of the floor and run as directly as convenient to the pump, where it falls into a drum or cylinder which in a small pump is about two feet long and fifteen inches in diameter. In this drum there is a float and as soon as there is a gallon of water returned the float rises and lets in a jet of steam, which starts the little pump working, and by a 1 1/4-inch pipe throws the water into the boiler smoothly and almost silently. This pump keeps your system clear of any condensation. Our pump is in a small brick pit eighteen inches below the floor of the boiler-house. This can always be arranged, but it appears that is not necessary, and if the pump were set on the floor it would work perfectly, because the pressure behind the condensed steam would raise the water two feet into the drum. Wherever convenient, however, I would prefer to have the drum a few inches below the return pipe so that there would be decided fall into the drum.
In the article on gravity I said about all I could on the distribution of the radiating pipes, so what more I can say on the high pressure system will be devoted to remarks on its advantages. Although reduced in pressure by the controller the steam is still hotter and drier than steam only raised slightly above the boiling point. When occasion arrives, it will travel to the farthest point of your system much quicker than by increasing the supply of steam with a stronger fire. In many places the draining of a deep pit is almost impossible without great expense. There is no need of it in the least. It is money wasted. You will find that after this steam under the pressure system has been through your heating pipes it will return to the pump in the shape of steam, showing that you have got the utmost benefit from it, but the chief claim of the advocates of the high pressure system is that it is a decided saving of fuel, and I quite agree with them. Why this should be I am not able scientifically to argue or demonstrate, yet it seems that after you have once got the forty to sixty pounds on your boiler, very little additional fuel will hold it there. It is certain that any condensation returning to the boiler is much hotter and nearer steam than under the gravity system, hence less fuel is required to convert into steam.
In conclusion, without an elaborate drawing it would be difficult to demonstrate the arrangements of pipes, position of the controller, pump, by-path and other points, but any one thinking of installing this up-to-date system would surely visit one of the many establishments where this system is in use. He could take quite a journey for the price of digging ten feet into the earth.
There is one point more. Whatever steam system you use, don't depend on one boiler. If you need eighty horsepower, buy two forties, if a hundred get two fifties, and it is a saving in fuel to have plenty of boiler power. As it is in hot water heaters, a little recent real experience may be worth quoting. For two years we heated about 22,000 feet of glass with one boiler rated at fifty horsepower. Then we built on another 12,000 feet of glass situated 250 feet from the boiler-house, and put in another boiler rated at seventy-five horse-power. Now on occasions either one of these boilers will heat and has heated the whole place, but both night and day firemen are positive that one-third less coal is burned when they are using both boilers to do the work.
What wonderful stuff this asbestos covering is and how it enables you to carry steam long distances in the open an! You have heard of the rival safe makers, one of whom, to demonstrate the imperviousness of his safe to heat, locked a mouse in the safe and then subjected it to great heat. On unlocking the safe the poor mouse was found frozen to death. A more truthful experience, and one nearer home, has astonished us; in running a 2-inch steam pipe from the boiler house to some carnation houses 250 feet distant, we have to cross an orchard ten feet above the ground. The 2-ineh pipe is covered with asbestos an inch thick and then a thin covering of tar paper. We have noticed frequently that while five pounds of steam was coursing through the 2-inch pipe, icicles were hanging from the tar paper covering.