This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
In 'The Gardener' of last month we have an interesting paper from Mr Hammond on heating by hot water. All who have to do with deep, and in too many cases imperfectly drained, stokeholes will agree with the writer as to the desirability of doing away with those evils. A small boiler may easily be fitted up to heat a single house without taking out a deep stokehole; but when pipes have to be extended to various houses there are very few instances indeed where the heating could be accomplished without some obstacle in the shape of footpaths, etc, coming in the way. After referring to the almost universal custom of giving flow-pipes a continuous ascent from the boiler to their extremity, and the practice of setting boilers below the level of the return-pipe, Mr Hammond says: "Notwithstanding, we do not hesitate to say that the circulation of the water in the pipes will be as rapid with the bottom of the boiler one foot below the level of the return-pipes as it would be supposing the boiler was sunk several feet deeper." With all due respect for Mr Hammond's opinion, I am not quite prepared to accept this statement.
Some time ago I had to see to the fitting of a boiler for the heating of a house at some considerable distance off; and from the nature of the ground, and to avoid excavating deeper than was absolutely required, the boiler was fixed at a level which made it necessary to give the flow-pipe rather less than the usual rise. Otherwise there was nothing different in the fitting from what is the usual mode. The circulation in this boiler, when set to work, was very unsatisfactory, and when hard fired kept blowing out at the air-pipes at their highest point. To keep the boiler from wasting its energy in this direction I cut the air-pipe and stopped it with a small cork, and when this was done there was a marked improvement in the circulation. My opinion at the time was, that the water in the return-pipes did not move back to the boiler as fast as the heating power of the boiler required; and the consequence was that the water in the boiler boils, or becomes so disturbed as to cause the water to fly out at the air-pipe. Some will be ready to say that this blowing at the air-pipes takes place just the same in many instances where the pipes are high above the boiler.
This I admit, but think it is waste of coals to have this go on, and further, that when a boiler is fed by a ball-cock in the supply-cistern, or carefully attended to, air-cocks are far better than the usual air-pipes.
Mr Hammond and most gardeners know that when two or more houses on different levels have to be heated from the same boiler, the most heat always goes to the highest house, if there is no check put on the circulation by means of valves. If two houses are fitted with the same quantity of pipes, but the one higher than the other, and if the one that is highest maintains the highest temperature, is it not fair evidence of a quicker circulation of water in those pipes than in those on the lower level?. The chief reason of this, I consider, is because of the additional weight of water coming from the return-pipe in the higher house over that coming from the lower house, and, in consequence, its power to draw the hot water from the boiler in its wake. The heating of the water and its consequent expansion and lightness are the first motive power; but I am disposed to attribute the force of circulation more to the weight of cold water returning to the boiler, and acting on the warm water in the boiler as a syphon would do on a cistern of water. If you wish to empty a cistern of water by means of a syphon, the length of time required to do so will be much shortened by having the end of your syphon a good way below the level of the cistern.
And so, I think, it is with the circulation in our boilers, - the greater the difference between the highest point of the pipe and the bottom of your boiler, the quicker will the circulation be. The water in a boiler at work being much lighter than that in the return-pipes, and assisted by the agitation and expansion conveyed to it by the heat of the fire, circulation will take place if the return-pipe is simply fixed immediately below the flow; but it would be slow. Mr Hammond, I think, makes too much of the inclination of the water to form a return-current in the flow-pipes, and too little of the fact that the water in the return-pipes is very much heavier - that this heavy column of water is pulling, as it were, a lighter one, and at the same time is pushing it from below. Whether the rise in the pipes is made by a slow gradient or by a vertical rise, I think, will not make material difference to the circulation; but every foot that they are thus elevated above the boiler gives, in my humble opinion, greater force to the circulation of the water.
I, like Mr Hammond, would not discuss the merits of the various boilers, each of which is supposed to be the best by its supporters. I have had experience of a few; and I must say that when properly fixed, kept regularly cleaned, and otherwise judiciously managed, all have worked well.
I however venture to say, that a boiler that is fitted inside for the regular flow of the water in divisions from the lower to the higher parts, so that the water has to traverse the whole of the boiler before getting out by the flow, will have an advantage over those that are not thus divided. R. Inglis.
In your last month's impression I noticed an article on the above subject, condemning the present system of fitting up hot-water apparatuses for heating plant-houses, which contains various statements the accuracy of which I question. Your correspondent, after alluding to the difference of opinion regarding the best pattern of boiler for economising fuel, goes on to say "that the circulation of the water in pipes will be as rapid with the bottom of the boiler one foot below the level of the return, as it would be were it several feet deeper: and instead of a continuous ascent of the flow-pipes being necessary it retards circulation." He then gives his reason why water circulates under any condition, and asserts as the basis of his theory that the particles of water are incapable of transmitting heat to each other. I would ask your correspondent, if the particles of water are unable to transmit heat, how does hot water impart its heat to cold water when, if there is an equal weight of each put into a vessel, the temperature will become the mean between the two ? Perhaps your correspondent may answer that it is caused by the hot getting so minutely mixed with the cold particles that it equalises the whole, which is true to a certain extent.
But if hot water is incapable of conducting heat, how does it come to pass that hot water poured upon ice imparts its heat to it, the ice being then a solid? It cannot be caused by circulation or the mixing of the particles with that of the ice, and the ice has come in contact with no heated body but the heated particles of water. But the fact is, water transmits heat to water in the same degree as it does to any other substance or body. Again, he says that if particles of water were capable of transmitting heat in the same way as that of solids, it would be impossible through the application of heat to cause circulation in the pipes. I venture to say that it would make no difference whatever; the expansive properties of the water would remain the same. All bodies expand by heat, and if it were possible to apply sufficient heat to liquefy bodies, circulation would take place in the same manner. It is by motion in the ultimate particles of matter in either case, whether solids or liquids, by which heat is transmitted. Heat creates motion, and motion creates heat.
The difference is, that in solids heat is transmitted by conduction only, but in liquids by both convection and conduction.
Then your correspondent endeavours to show why the rise in the flow hinders circulation. He says the reason for this is plain: as s >on as the fire acts upon the boiler, the water in contact with its inner surface bounds upward till it comes in contact with the inner surface of the upper side of the pipe, where it parts with a portion of its heat, and would now return, were it not still lighter than the body of water at the bottom of the pipe which it has just passed unaffected; therefore it has to continue its course on the top of the cold water until it is reduced to the same temperature, when it returns from whence it came by an opposite cold current at the bottom of the pipe to the boiler, and descends through the hot water and takes its seat at the bottom to be heated over again. He therefore asserts that we have two currents of different degrees of temperature moving in opposite directions in the same pipe. If your correspondent's theory is correct, we would have no use for a return-pipe at all - a flow would be sufficient. Bat such is not the case: the water in the boiler on being heated expands, consequently it then becomes lighter than the water in the pipes, hence its tendency to rise.
And as the water from the flow which proceeds from the top of the boiler cannot descend without mixing and equalising the temperature of both, therefore the water from the return which is situated at the bottom of the boiler rushes in and gets heated likewise, and continues to expand and ascend the flow-pipe, equalising and forcing the cold water before it. And that part of the structure which is situated farthest from the boiler will be the hottest, the pipes being at the highest elevation; which goes far to prove that a continuous rise in the pipes does not hinder circulation, but the reverse.
[It is quite evident that this is a subject that requires to be discussed. - Ed].