Central Heating: The Wet System

For far too long central heating has been known by the fuel which supplies the heat energy. We have for instance gas central heating, or oil central heating. This is entirely due to the efforts of advertisers, of whom the biggest and most persistent spenders have been the fuel interests.

The truth of the matter is that the important aspect of a system is the heat emission side. Suppose you have radiators installed, and a wet system, with a gas boiler. It is a couple of hours work to remove the gas boiler and fit instead a coke or oil boiler. But to take out the radiators and fit instead ducting or some other type of system is a major operation. Equally, substituting a boiler makes no difference at all to the heating. whereas any departure from the radiator system will cause some noticeable change, probably introduce a new method of control for comfort. That is why we are going to consider wet systems before going on to boilers.

In spite of a few half hearted attempts to make changes, the wetness of wet systems is water. Partly with car practice in mind it is possible in a closed circulation system to introduce an antifreeze solution. Another additive is aimed at reducing internal corrosion, and this is vehemently defended by its suppliers and supporters but by no means universally accepted.

The operation of a wet system is entirely dependent upon circulation. Water is heated in the boiler, is piped to the heat emitters and returns, partly cooled, to be reheated. In earlier days, and to a very limited extent nowadays, heating circuits operated by gravity. A warmed column of water is lighter than a cold column, and so the one rises and the other falls. A system based upon this natural movement has to have a substantial vertical component. The rise and the fall must be fully established in order that any sideways movement, into radiators etc., can take place with continuing circulation and so with renewal of heat introduction.

The principle of gravity circulation is still the one most used on the domestic hot water side of combined systems. The pipes, known as the primaries, flow to and return from the cylinder, which must be at a higher vertical level than the boiler.

But heating circuits cannot rely upon finding a high vertical to horizontal ratio, except perhaps in a lighthouse. Most gravity inspired heating circuits were either sluggish or failed to circulate. Those that worked were characterised by a wide difference between flow and return temperatures so that radiators became progressively cooler along the line. The gravity circuit has passed, therefore, without regret. Its successor is the pump assisted, or pumped, circuit, which began seriously with what became known very quickly as Small Bore. Small bore employs pipes mainly in the range 15 or 22 mm, occasionally 28 mm. Gravity systems could never fall below 1 inch pipe.

The reduced diameter, the extra mobility contributed by the use of copper instead of iron, the design freedom due to pumping, have all led to small bore being widely and readily applicable. It is no more difficult to make work in a single storey barrack style building than in a conventional semi. It can of course be made to misbehave, for example by having too much pump power, or by using pipes inadequate for the designed load at any point in the system. But those are matters to be sorted out during design, as we shall be showing in a preceding post2.


After twenty years of adherence to small bore we have more recently come to microbore, in which the pipework is almost entirely smaller than 15 mm or ½ in, being more of the order 10 mm. This material can be installed in the same fashion as medium weight electric cable, which makes it a good deal easier, quicker and therefore cheaper to handle than small bore tube. The material used is copper, though nylon is on offer. The latter must be viewed with some reservation, since its only real advantage is in first cost.

The pump to be used with microbore develops more pressure than a small bore pump, since the system still has to carry the same weight of water in unit time. Microbore circuits have another notable difference, compared with what has gone before. In other sizes of circuit the pipe goes from one appliance to the next, giving up as much as each requires. In microbore circuits there is no reserve for such tributary treatment. The total amount of heat for the system, as hot water, is led to a central distributing point called a manifold, by a flow pipe of standard size. The manifold should have as many connecting points as there are heat emission appliances. From this central distribution point a pair of microbore pipes is led to each appliance, by the shortest route, and connected to the inlet and outlet. The microbore pipe connected to the appliance outlet is then connected, back at the centre, to another manifold, which in turn is connected to the return pipe to the boiler.

In spite of the ease of pipe running and other advantages of microbore, it is not destined to take over the market, though it will achieve a share. To give only one reason: a gravity circulation system could be brought to a standstill by almost anything — a dip in a supposedly horizontal pipe, or an air bubble; a clean small bore system will fight on through most disturbances, short of having scale or similar foreign matter choke up the pump; the cross section of microbore pipe is so small that it can be blocked by quite small pieces of foreign matter. We mention this because there is always a tendency to imagine that the latest of anything is about to become the only one. In the end anything of value assumes a reasonably constant share of the market after a settling in period.

Small bore

Small bore is likely to dominate the wet system market for quite a while. It is split into two categories, single pipe and two pipe systems, which we will examine. At present the balance lies in favour of the single pipe system, but only on account of first cost. The two-pipe system, which is technically a much better job, uses almost twice as much pipe. But first, to see what single and two-pipe systems are. Each radiator is connected so as to take water from, and return water to, this pipe. The process is repeated on the ground floor, until eventually the single pipe, now known as the return, enters the boiler.

At any one radiator there are two important things to note. First, any water which passes through the radiator gives up heat and so drops in temperature. Consequently the water in the flow pipe, just after the point at which the radiator returns to it, will be cooler than before due to the cooling effect of the water from the radiator. But that same water is of course the water which enters the next radiator, and so there is a progressive cooling of the heating medium, as the flowing water is called, along the circuit. This same thing occurred, of course, with gravity systems, and the only benefit of the new system is that thanks to pumping the temperature drop is roughly halved.

The second thing which should be noted is that, while the system is pump circulated, the circulation within each radiator depends upon natural forces. The force causing flow to start within a radiator is the very small difference in pressure in the flow pipe due to the frictional resistance of the length of pipe between radiator inlet and outlet. That is why, in some cases of reluctant circulation, a cure has been made by transferring the radiator outlet connection to a point further downstream on the circulation pipe. Reluctance, it may be added, is rarely so fundamental. No radiator should be bought without an air valve key, unless an automatic air vent is fitted. That key should be used whenever sluggish behaviour is noticed, and at regular intervals if there seems to be a tendency to accumulate air.

It can be seen that the flow pipe goes only as far as the inlet to the last radiator in the circuit, and that the return pipe begins at the outlet of the first radiator. But in the two-pipe system the flow and return functions are kept apart, the return coping only with that water which is waste, or spent, from the heating process.

It will be obvious from this that, subject only to a small practical loss, each radiator receives its water at the same temperature, which is the predetermined boiler temperature. This in turn means that the heating system has more chance of being fully effective, and it can actually save money on radiators, since to get a given heat output from a radiator operating at a lower temperature calls for a larger radiator.

There is thus a wide differential pressure causing flow though the radiator, and the manner of flow is much nearer to being positively pumped. This means that the velocity of water flow through the radiator, hence the rate of warming and the response to change, are all more rapid.

A further advantage to come from the two-pipe system and its near-positive water flow is that we need no longer rely upon gravity circulation within the radiator, as was the case with a single-pipe system. Indirect Systems

All systems, whether single or two-pipe or microbore, are capable of being installed in two ways – direct and indirect. They should certainly not be installed as direct systems, but we must at least consider the possibility in order to see why not.

A direct system is the basic one, in which water passes from boiler to pipework and back to boiler. It goes, too, usually through a cylinder, to hot taps. So every time hot water is drawn off, more water enters the system and passes through the boiler. Raw water is a mixture of many things, depending upon its source, but most of the ingredients can be harmful to a heating system, in or after going through a boiler. Hard waters will deposit scale in and beyond the boiler;reducing waterways, coating heating surfaces so that they must be forced to higher temperatures and in the end break from heat fatigue, or burn out. Many soft waters act in quite another way. Their reaction being acid, they attack metal surfaces, and bring about corrosion. This usually destroys a system more quickly than the scale build-up in hard water systems.

Direct systems give a choice, therefore, between the devil and the deep blue sea, and the safest course is to have no dealings with them. The way out is by an indirect system, in which water drawn from taps does not pass through the boiler, but gets heated indirectly. The key apparatus is an indirect cylinder. In appearance like any other cylinder, it contains a heat exchanger, in the form of a coil or other suitable type. Boiler water passes through the heat exchanger, and the domestic hot water for taps passes around the outside of the heat exchanger.

Thus it is that in a leak-free indirect system the primary circulation, which is the water passing through the boiler, heating system and heating coil in the cylinder, never changes. It is the same water going round all the time. If we suppose a system to contain 45 litres of water, and the water in raw condition to have in it 100 parts per million of scale forming or acid ingredient, the total weight of that ingredient to enter the system is 4.5 grams, which is negligible. Raw water must of course enter the cylinder on the secondary side, I.e. outside the heat exchanger, with its scaling or corrosive potentiality unchecked. But this is nowhere near as detrimental because under the less drastic heating conditions it does not behave so badly; (2) it is easier and cheaper to clean or renew any apparatus on the secondary side, if this should ever become necessary; and (3) it may be avoided entirely, since water treatment, most often softening, can be applied to the supply to the cylinder secondary side.