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.
I do not intend to encroach upon your space to discuss further what has been termed the side issues of this question, as they are comparatively unimportant. Mr Hammond differs from me when I say that a house situated above the level of others will be the hottest. He does not think that that proves anything in favour of elevation increasing circulation. It only proves that the hotter water gets there. He also says that " it is not a well-known fact that a house situated above the level of the others in a range is the hottest;" and probably it may not be so in all cases - that may depend a little on the arrangement - but I could point to many practical examples in support of my statement if it were necessary. I may remark that I merely stated that as a practical illustration of the fact that elevation increases circulation, thinking that it was self-evident to all gardeners. But though the practical workings of different apparatus may be disputed as not being uniform, the principle upon which hot water circulates remains unaltered.
I will now deal with what Mr Hammond considers the points of practical value at issue on this subject; but first, I may remark that if what Mr Makenzie has already written on the subject, with his numerous quotations from the best authorities, does not convince Mr Hammond that he is in error, I despair of being able to do so. He asks, " Is a continuous rise in the flow from the top of the boiler to the furthest points of extension in the compartments to be heated essential to rapid circulation?" In answer to this question, I agree with Mr Hammond that it is not essential to circulation, but in my opinion is most convenient in general practice. What I contend is, that elevation is essential to rapid circulation, but I differ with him when he says that in place of it being essential the continuous rise is a hindrance to circulation. I hold that it makes not the least difference whether the rise be vertical or gradual, if the height and difference between the mean temperatures or densities of the ascending and descending volumes of water be the same.
If Mr Hammond can prove that the circulation will be less rapid under these circumstances, I will submit that a gradual rise hinders circulation; but if he fails to prove that, as I predict he will, then of what avail is all Mr Hammond's talk about an imaginary return current hindering circulation in a gradient rise? [Is it imaginary? - Ed.] Lastly, he asks, "Is it necessary or essential to rapid circulation that the top of the boiler should be below the main body of flow and return pipes whether there is only one house to heat or a range of houses?" In answer to that question, I may say that that would depend on the height of the boiler and what Mr Hammond may consider a "rapid circulation." If I understand Mr Hammond to mean what he stated in the February number, that the circulation will be as rapid with the bottom of the boiler one foot below the level of the return as if it were several feet deeper, then I must again differ with him. I maintain that the circulation in hot-water pipes increases in proportion to the difference of weight or pressure between the ascending and descending volumes of water on the base.
Now this difference of pressure is increased in two ways: by raising the elevation, and also, to a limited extent, by increasing the difference between the mean temperatures of the two volumes of water. Mr Hammond thinks that the circulation will be as rapid with the bottom of the boiler one foot below the level of the return as if it were several feet deeper. Now it is plain that the water before entering the boiler is at its lowest temperature, and consequently at its greatest density. And as it has been already stated that the pressure of a liquid on a given base is measured by the vertical height or depth and density, independent of the shape of the vessel or the quantity of water which it may contain, hence it is evident that the greater the fall before entering at the bottom of the boiler, the greater is the difference of pressure and gain of power to counterbalance the lighter volume in the flow. This will appear more plain when we consider that it is only the ascending and descending volumes, equal to the vertical height, that press on the base, and therefore act on circulation.
Now, as an illustration of the fact that elevation increases circulation, we will take for example an apparatus with an elevation of 4 feet: we will suppose the mean temperature of the ascending column of the flow is at the temperature of 98° Fahr., and that of the descending column of the return at 62°; the pressure on the base of the water in the flow at the temperature of 98° in a 4-inch pipe 4 feet high is approximately equal to 344.72063918 ounces avoirdupois, and that of the return is equal to 348.17828489 oz. The difference between the pressures is 3.45764571 oz. Now suppose we raise the elevation to 8 feet, the pressure of the flow is increased to 689.44127836 oz., and that of the return to 696.35656978 oz. The difference of pressure is 6.91529142. Hence we see that the difference of pressure at 8 feet is double that at 4 feet. Bat with the same source of heat, circulation would not be doubled, as it is plain that the difference between the mean temperatures of the two volumes becomes less as the rapidity of circulation increases. Thus we see that the rapidity of circulation increases in proportion to the difference of pressure on the base, and that the difference of pressure is increased by elevation.
I will not trespass further on your valuable space; but I may say, in conclusion, that though what has been said may not convince Mr Hammond that he is in error, it may help to convince some of your intelligent readers who may have any doubts on the subject. C. M.
I Received a few days ago a copy of the August number of ' The Gardener,' and was so interested in the articles it contained on heating by hot water, that I at once procured all the earlier numbers having reference to the subject. Allow me at once to say that I agree with most of the views so clearly expressed by Mr Makenzie, and most especially with that as to the obligation we hot-water engineers should be under to Mr Hammond, if he is able to show us a method by which three or more houses can be heated from one boiler without a deep stoke-hole. I presume I am right in confining the discussion within these limits, as it is, I believe, admitted by all engineers that where only one, and occasionally two houses, have to be heated, it is easy to obtain a satisfactory result with the boiler level with, or even above, the pipes. I shall be glad to supplement Mr Makenzie's list of heating apparatus so erected during the last fifteen years.