Water is cheap and plentiful, and although used by all as the heat transfer medium in wet central heating systems, beyond the basic engineering requirements, many equipment manufacturer and installers give little consideration to its fundamental characteristics. In many cases, this leads directly to the creation of waterside problems and the inability to resolve them.

The water molecule

Amongst all molecules, naturally occurring or man-made, water is unique. It is present in the environment in all three states of matter, solid as ice, liquid as water, and gas as vapour (steam).

Because of its unique structure, the amount of energy required to vaporise water and the amount of energy released when water freezes, is greater than for other simple chemical compounds. Linked to this, the amount of heat absorbed or released with change of temperature is greater than many substances, i.e., it has a high heat capacity. This makes water a highly effective heat transfer medium. Despite its high surface tension, the viscosity of water which can be visualised as its 'thickness' is low, making it easy to pump.

Water is non-hazardous and non-flammable, but it is sometimes referred to as the universal solvent, as it will tend to dissolve anything with which it comes into contact. Water containing a low concentration of solids is corrosive and often behaves highly aggressively, as it seeks to increase its solids content. Hence, the purer, low solids content waters, such as occur in Ireland and Scotland are far more corrosive than the chalk rich waters of Southern England.

Three quarters of the Earth's surface is covered with water, in places to a depth of 11,000 meters. This tremendous volume of water is never still. Besides the physical movement caused by tides and marine currents, there is a continuous process of evaporation and condensation, which we call the Water Cycle.

Most people have heard of acid rain produced by absorption of sulphur and nitrogen oxides by rainfall, but the presence of pollutants like these gases is not the only source of acidity. Carbon dioxide makes up 0.3% of the Earth's atmosphere and is readily absorbed by water to form carbonic acid - the acid found in carbonated beverages. Hence, all rain is acidic to some extent and therefore has a strong tendency to dissolve minerals and rocks with which it comes into contact.

By knowing the recent history of your water and the length of time it has been in contact with soil and rock, it is possible to predict its likely quality. In the Lake District and most of Scotland, the water is soft because the extractions are mainly of surface water and the contact time between water and earth is short. Also there's not much chalk in the soil to be dissolved. In the South East of England, there's a lot of chalk and much of the water is pumped from underground where there has been a longer contact time, so the water is hard. The water used in a domestic central heating system is normally drawn from the towns mains supply.

The quality of water in the British Isles is generally good, but it must be remembered that the only statutory obligation on the Water Authorities is to supply water fit for potable use. Consequently, the water distributed via the mains system is unlikely to be ideally suited to the central heating system. British Standard 7593: 1992 categorises towns mains supplies as follows.

Hardness Concentration
mg/i (as CaCo3)

We will discuss the consequences of the different qualities of waters for the central heating system.

Most people are familiar with the build up of scale on the elements of kettles and immersion heaters in hard water areas in the country, however, scale formation in closed circuit central beating systems is rarely considered.

In central heating systems, scaling is caused by the deposition of either lime or black iron oxide corrosion debris. Lime scale consists of calcium carbonate (calcite) with lesser amounts of other calcium salts such as the sulphate. Sometimes lime scale deposits contain corrosion debris which has become trapped.

Unlike most substances, most calcium salts become less soluble as the temperature rises (inverse solubility). Such salts are naturally present to some degree in all mains water supplies, therefore, any system where mains water is heated is liable to suffer from lime scale formation. Consider a mains water containing 300 milligrams per litre (mg/l) of calcium carbonate hardness. The potential weight of scale produced in a 100 litre central heating system is 30 grams, and this is from only the initial fill of water. Once formed, calcium scale tends not to redissolve, so water lost from the system for whatever reason will leave it behind.

The fresh make-up water will then bring further calcium salts into the system to begin the process again producing an ever increasing thickness of scale. Being a direct result of increased temperature, lime scale will form in the hottest part of the system, usually the heat transfer surface in the boiler. In contrast, iron oxide first forms as a sludge at the point of corrosion and will only become a scale if carried by the water to the heat exchanger where it can become hardened by baking.

The problem of scale

The formation of scale in a central heating system has three important effects:

The first of these is the problem of boiler noise or kettling. Correct operation of the boiler relies on the removal of heat from the transfer surface such that the temperature of the metal skin and the re circulating water, which are the same, remain below boiling point. Boiler noise is usually caused by localised overheating of the re circulating water leading to steam generation. This can happen in two ways:

b) Scale deposition is rarely uniform. Thicker areas will always exist where the metal temperatures are higher. This will concentrate the bulk of the heat transfer over small areas at above design heat transfer rates, so causing simmering and boiling. Cooling and reheating of scale as the system cycles will cause cracking and flaking of the surface giving rise to boiling in crevices in the fractured deposit, either between deposit and metal, or in the body of the deposit. This problem is dependent on the amount of scale present, as once the quantity becomes very great, the fractures will tend to 'heal' and the water become completely insulated from the metal surface. At this time boiler efficiency will drastically reduce and failure become a definite possibility.

• Reduced efficiency
  The loss of boiler efficiency due to reduced heat transfer is becoming increasingly significant in the central heating system. The insulation afforded by an egg-shell thickness of scale can lead to as much as a 10% rise in fuel consumption, and further build-up can rapidly lead to overheating of the metal and potential failure. The extent to which scale will affect efficiency depends on thermal conductivity, e.g., a porous lime scale conducts heat 100 times less effectively than boiler metal and has a very marked effect.

• Increases thermal stress in metal
  In the extreme case, scale can build up to the point where the restriction of heat flow can cause the insulated metal surface to overheat, leading to stress failure.

• Corrosion in the system
  Corrosion will take place to a greater or lesser extent in all domestic central heating systems. Just how severe it is will depend on many factors including the types of metal in the system; the degree to which air can be drawn into the system; the nature of the supply water. Even the quality of the installation work can have an effect. The strength of the forces involved in the corrosion process can readily be appreciated by considering the energy necessary to oppose them, e.g., to produce metal in a steel works, or in a smelter, from naturally occurring ores. The purer the metals produced, the more unstable they are and the greater their tendency is to revert to their natural state. the more unstable they are and the greater their tendency is to revert to their natural state.

Corrosion will take place to a greater or lesser extent in all domestic central heating systems. Just how severe it is will depend on many factors including the types of metal in the system; the degree to which air can be drawn into the system; the nature of the supply water.

Even the quality of the installation work can have an effect. The strength of the forces involved in the corrosion process can readily be appreciated by considering the energy necessary to oppose them, e.g., to produce metal in a steel works, or in a smelter, from naturally occurring ores. The purer the metals produced, the more unstable they are and the greater their tendency is to revert to their natural state. the more unstable they are and the greater their tendency is to revert to their natural state.

The presence of oxygen in the system is possibly the most important factor. It is often wrongly assumed that the oxygen present in the initial fill will soon be consumed by reaction with the metal surfaces and that once this has occurred no further corrosion will take place. This is not the case, as in practice systems are found never to be oxygen free.

At room temperature, water contains 8 milligrams of dissolved oxygen in every litre, so an open vented system using make-up, or pumping over, will steadily admit oxygen. Even where no make-up is used, the water at the surface of the feed and expansion cistern will be saturated with oxygen and an oxygen gradient will extend down through the tank and the cold feed into the circulating water.

Oxygen ingress will also occur at pumps, joints and valves. At points under negative pressure, i.e., those between the neutral point (the point where the cold feed is connected to the re circulating system) and the suction side of the circulator, depending on the soundness of the connections, ingress can be rapid. It should be noted that negative pressure points can also be generated in sealed systems.

Even where there is a positive pressure, yet no leakage of water, there may still be contact with the atmosphere the surface tension of the water may be preventing any flow through capillaries. Under such conditions oxygen will enter the system.

In its simplest form, the reaction between iron (steel) and oxygen in the presence of water produces iron oxides (initially rust) and hydrogen gas. The effect of hydrogen production is to depress the water level in the radiators, so producing a cold area at the top. Sufficient pressure can be produced to split aluminium radiators. The gas will often end up in one radiator at the point of lowest pressure, furthest from the circulator. The gas is slightly soluble in water and, although produced elsewhere, will transfer to the point of least resistance.

Hydrogen is a colourless, odourless gas . It is not corrosive, but it is lighter than air and flammable (take care when venting). When gas build up is detected, the first step is to test for the presence of hydrogen - the gas may just be air. To do this, the radiator should be vented into an upturned glass container, e.g., a jam jar. Being the lighter gas, hydrogen will displace air from the jar. The jar should then be removed from the radiator and a lighted taper inserted into it. If hydrogen is present, the gas will ignite with a pop (without risk of explosion), and it can be concluded that corrosion is occurring.

Metals are affected differently by pH conditions, e.g., mild steel is most stable from pH 10.5 to 11.5, yet aluminium, always coated with a layer of aluminium oxide, is readily attacked above a pH of 8.7. Copper may be adversely affected by a pH greater than 9.5. The ideal pH for the domestic central heating system is between 6.5 and 8.0, when the corrosion rates for all metals are acceptable.

When dissimilar metals are in contact a galvanic (electrochemical) cell is created. One metal will go into solution, determined by the metals' relative positions in the Electrochemical Series. This series is a law of nature and a property of the metal in the form in which it is used. Copper is the most noble metal used in the domestic system, so it never corrodes, but all other metals will sacrifice in its presence. Aluminium is particularly susceptible being far away from copper in the table, i.e., a very high corrosion current is generated. (N.B. pH plays an important role in the case of aluminium, due to the oxide coating). The sacrificial metal wastes away at the closest contact point. Insulation minimises the damage, but even fibre washers are not foolproof as they absorb salts from the water and can then conduct electricity.

The metal most commonly corroded in a domestic system is mild steel in radiators. Here, depending on the amount of copper present, the corrosion can take two forms:

(a) Where a large area of a radiator becomes plated with copper (as can happen in new systems where flux has been abused), inevitably there will be points within the area of copper not covered. Galvanic corrosion then occurs with the current being concentrated at the point where the sacrificial metal is in contact with the water. The current is intense, and as all the corroding metal emanates from the same point, perforation is rapid. The hole on the outside of the radiator is the same size as that on the inside, as though the metal has been drilled. Such corrosion is termed 'pinhole' corrosion.

Generally, chemical reactions proceed more rapidly as the temperature increases, hence, a central heating system will corrode faster than a similarly constructed chilled water system. The effect of temperature is not, however, restricted to reaction rate. Most installers are aware that galvanised metal should not be used in central heating systems, but the reason is not widely appreciated. If the steel pipe, or surface, is well coated with a tightly adherent layer of zinc, the degree of protection afforded will be good. However, the surface is often scratched or pitted, allowing the water to contact the steel below. The Electrochemical Series indicates that steel is more noble than zinc, so corrosion of the steel will not occur. At temperatures below 60-65°C this is indeed the case, however, at higher temperatures, the relative potentials of mild steel and zinc reverse, making steel the sacrificial metal, so inducing pinhole corrosion. Galvanised feed and expansion cisterns are particularly susceptible.

Stress

Points of stress, e.g., welds, bends, etc., are always more liable to corrosion. Certain salts such as chloride will attack stressed areas along the grain boundaries in the metal. Sources of high chloride include inefficient water softeners, washing up liquids (misguidedly used to cut the noise from noisy boilers) and excessive flux application.

Fouling

Fouling in the central heating system can be biological, organic or silt based, or more often a combination. Foulants include bacteria, slimes, scale and corrosion debris. Once on the metal surface the insulating layer creates a local dissolved oxygen gradient which promotes under-deposit corrosion - usually in the form of pits. The very low level or complete absence of oxygen under the foulant also encourages the growth of anaerobic (oxygen hating) bacteria such as sulphate reducing bacteria (SRB's). Such bacteria produce sulphuric acid as part of their metabolism, with obvious consequences for metal in the vicinity.

John R Lane, EurChem C.Chem M.W.M.Soc MRSC B.Sc ARCS