Low-Pressure Hot Water Heating Systems

By the time, Thomas Messenger began his business the use of hotbeds, hot air stoves, flues, and steam, as methods of providing heat were in decline and being replaced by more efficient and controllable hot water heating systems using boilers and low-pressure hot water pipes. The initial capital outlay of installing a hot water system was generally higher than that of other forms, but proved cost effective due to its durability, relatively low running costs and heating capabilities. At the time, the advantages of using low-pressure systems over other forms of heating were that it was simple, safe, and reliable. An even temperature was relatively easy to obtain and maintain something that was difficult with previous heating systems. It was also comparatively economical being able to utilise most kinds of fuel, available at the time, be it coal, coke[1] or slack[2]. The risk of fire was relatively low with pipe temperatures rarely exceeding 88ºC (190ºF), resulting in a mild and humid heat. Finally, a relatively unskilled workforce could fix it.

The basic principle of low-pressure hot water heating systems is that a heat source, i.e. the boiler is placed close to, if not actually within, the structure, to be heated. Attached to the boiler are a series of pipes which form a closed circuit, running around the periphery of the greenhouse or other building and it is the radiation from the pipes that heats the structure. To ensure that the hot water circulated around the system properly, as no pump was used, the boiler had to be set at a lower level than the structure being heated, with the pipes gently sloping up to a point then descending back to the boiler. The boiler had one or more outlets and one or more returns to which the heating pipes were connected. The outlet connections were at the top of the boiler and the return pipes entered close to or at the base of the boiler. One or more cisterns were used to ensure that there was always sufficient supply of water for the heating system. The circulation is caused by the water in the return-pipe, which, owing to the difference in temperature, is heavier than that in the boiler, consequently forcing the hot water, which is lighter, upwards, and thus sets the whole circulation in motion. This arrangement allowed hot water to flow out of the top of the boiler move around the heating system, cooling along the way to return much cooler to the base of the boiler to be reheated. At the time, it was normal to use cast iron pipes closest to the boiler to help prevent cracking.

The water circulation speed was controlled by three factors. Firstly, the difference in height between the outgoing and returning columns, the greater the distance the faster the flow Secondly, the length of pipes, the longer the pipes the greater the temperature difference. Thirdly, by increasing or decreasing the pipe diameters, the smaller the diameter the larger the cooling surface in relation to the volume of water.

The size of the boiler and the amount of pipework required were dependent upon a number of factors, such as the size and number of greenhouses or structures to be heated, the temperatures required, etc. These heating systems could become quite complex involving numerous individual circuits controlled by a number of valves, especially where several structures were being heated by a single boiler. On small systems, the boiler was usually installed at one end of the greenhouse or structure, such that all the connecting pipes are inside of the house and only the fire and ash pit doors project through to the outside; whilst for larger heating systems a separate, purpose built stove house was the norm. At the time, it was standard for pipework to be constructed of four-inch internal diameter circular pipes, which were found to be ideal, giving sufficient radiating surface to provide a good ‘top heat’, whilst small three-inch pipes were adequate for smaller structures and beds. The advantage of using hot-water pipes is that it provided a much larger radiating surface than other methods and as the heat is radiated at relatively low temperatures, the risk of condensation on the glass was reduced. Another advantage is that the lower the surface temperature, the lower the moisture loss due to evaporation. However, to compensate for any moisture loss water troughs were often sunk into the floor of the structure, strategically placed to be “out of the way”. Ideally pipes were placed as near to the front of the structure and as low as possible, allowing about 6-inch clearance off the ground and were usually supported on floor or wall mounted cast-iron brackets.

Simple low-pressure hot water heating system

The design of heating systems suitable for various and varied solutions was the subject of a much effort by a number of individuals and organisations. One such person was Charles Hood who wrote a book entitled “A Treatise on Warming and Ventilating Buildings…” first published in 1844, which went into a number of editions, the sixth of which was published as late as 1885. In the fourth edition published in 1869, which no doubt Thomas Messenger was familiar with, Charles Hood gave details of the amount of hot water pipers required in various situations. For example, he reckoned that to heat churches and large public buildings with an average number of doors and windows and with moderate ventilation required about 5 feet of four-inch pipe to every 1,000 cubic feet of space to give a temperature of 55◦F in very cold weather. However, if there were more than moderate ventilation then 50 to 70 per cent more pipe would be required. Dwelling rooms required about 12 feet of four-inch pipe to every 1,000 cubic feet of space to give a temperature of 65◦F and 14 feet. to raise the temperature to 70◦F. Halls, shops, etc., required about 10 feet of four-inch pipe every 1,000 cubic feet of space to raise the temperature to about 55◦F. Workrooms where a temperature of 50◦-55◦F was required needed 6 feet. of four-inch pipes for every 1,000 cubic feet of space.

Low-pressure hot water heating system table

The principles for heating horticultural buildings using hot water systems are much the same as those involved in heating other buildings, except that the loss of heat is greater from glass than from wood or brick structures, and a higher and more constant night temperature is normally required than is necessary in dwellings. For this reason, relatively more radiating surface is required and boilers of larger capacity needed. In general, the quantity of air to be warmed is 1¼ cubic feet per square foot of glass per minute[3]. Greenhouse and conservatories requiring a temperature of about 55◦F, in the coldest weather, should, according to Charles Hood, have 35 feet of four-inch pipe for each 1,000 cubic feet of space. Vineries and stove houses requiring a temperature of 65◦-70◦F, in the coldest weather, required 45 feet of four-inch pipe for each 1,000 cubic feet of space. Whilst pineries, hothouses and cucumber pits requiring a temperature of 80◦F, must have 55 feet of four-inch pipe for each 1,000 cubic feet of space.



  1. The solid residue of impure carbon obtained from bituminous coal and other carbonaceous materials after removal of volatile material by destructive distillation.

  2. A mixture of coal fragments and coal dust.

  3. Iron frames and sashes are measured with the glass; for wood frames deduct one-eighth from the gross area of surface.