To transport the heat energy from the heat source (boiler, for example) to the user, it is necessary to have a warmth carrier. There are three types of warmth carrier: water (or anti-freeze), steam, and air. The most common of these is water, and such a system is called «hydronic heating». Water is an incompressible fluid, which is capable of absorbing heat as it is heated and giving it out again as it cools. Water, like all physical objects, expands as it is heated, and it has good fluidity. Because of such features, it is not difficult to force it go through the heating system, delivering warmth. Water is easily available and you need only to pour it into the system. It is the source of life on our planet and if there are any leaks, it will not be a health problem. Water in its natural state has pressure under gravity: a ten-metre column of water = 0.981 atmospheres = 0.1 megapascal = 1 bar (1 pascal = 1 Newton/m²).
Water molecules get extra kinetic energy when heated, and start to move chaotically, and the volume of water increases. Since water is an incompressible liquid, increasing its volume in an open heating system leads to the lifting of a column of water and, as a result, to increasing pressure which will be equal to the new height of the water column. In closed heating systems, the pressure of the inter-molecular bonds is added to the gravitational pressure, and that will prevent compressing of the water. Water has not only water molecules but also oxygen and salt molecules. Upon heating, salt and oxygen molecules are released, and this leads to sedimentation on the internal walls of the pipes, and too much air, which causes bubbles in the system, impeding the flow of the water. Each addition of water to the system brings more salt and oxygen. Below freezing, molecules of water stop moving and create crystal lattices, and water becomes hard and increases in volume. In closed contours, expansion of water as a result of freezing leads to an increased pressure which is capable of breaking pipes, joints and fittings. To carry the warmth, it is possible to use anti-freeze instead of water. Anti-freeze is a liquid which does not freeze, made with ethylene glycol as basis. Ethylene glycol is an aggressive substance and can corrode the internal surface of both pipes and radiators, and therefore elements (e.g. water) are added to neutralize the chemical activity. In a heating system, one should not use anti-freeze designed for cars, but rather that designed for heating systems.
While using anti-freeze, one should pay attention to the fact that the warmth capacity of anti-freeze is 15–20% lower than water. Therefore it absorbs and releases warmth less efficiently than water. Thus, in designing a heating system using anti-freeze, the radiators should be more powerful; the viscosity of anti-freeze is higher than that of water, so it is more difficult to force it to move in the system, and because of this increased friction, diameters of pipes and fittings must be larger (by one level). In pumped systems, a more powerful circulator will be needed. Anti-freeze has more fluidity than water and therefore greater demands are made on detachable fittings: specialized gaskets for connections of pipes and fittings must be used. Zinc-covered pipes should not be used because they cause chemical changes in the anti-freeze. Usually anti-freeze is sold in two varieties: concentrated, which freezes only at -65 degrees C; and diluted, which freezes at -30 degrees C. Concentrated anti-freeze can be diluted with water as needed. For example, to obtain anti-freeze with a freezing temperature of -30 degrees C, add one part water to two parts concentrated anti-freeze. For -20 degrees C, mix water with the anti-freeze half and half. Later, in the description of heating systems, we will use the term «warmth carrier» to mean either water or anti-freeze. While using anti-freeze in a heating system, some changes must be made; in particular, when more liquid is needed, anti-freeze must be added by hand.