There are several reasons why a building loses warmth: the greater the difference between inside and outside temperatures, and the larger the area of the border constructions (walls, windows, roof, etc.), the greater the heat loss. Heat loss also depends on the materials of which the border construction elements are made. For example, heat is lost through a thin wall more than through a thick one of the same material. Walls made of wood or of brick, of the same thickness, retain heat differently: a building with wooden walls will get cold more slowly than one with walls made of brick. The reason is that some materials (e.g. brick and metal) transmit warmth better than others (e.g. wood).
The heating system must replace the warmth which has left the building:
- through the border constructions of the building (walls, windows, doors, ceilings of the upper storeys, and the floors of the bottom ones);
- through any windows or doors which may be open at times;
- because of any vehicle, equipment or material which has entered the building from the outside, which brings cold air with it.
The warmth needed to heat a building is obtained from the burning of fuel in boilers or heating stoves. Some fuels which are used in boilers are: different types of coal, different types of fuel oil, wood or wood products, peat, and gas.
The amount of heat obtained by burning fuel depends on the kind of fuel. For the complete burning of one kilogram of solid fuel, the fuel efficiency is measured in kCal/kg (or in BTU/lb; note that 1 kCal/kg = 1.8 BTU/lb); for gas, for the burning one cubic metre, it is measured in kCal/m³ (or in BTU/ft³; note that 1 kCal/m³ = 0.112445529 BTU/ft³). For example, the fuel efficiency of coal (boghead) is 5600–7000 kCal/kg; for lignite, it is 2200–3200 kCal/kg; for wood, it is 2700–3200 kCal/kg; for natural gas, it is 8400 kCal/m³.
To burn one kilogram of different types of fuel requires different amounts of air to be supplied to the boiler. On average, for every 1000 kCal, it is necessary to deliver 1.5 cubic metres of air. During burning, the oxygen contained in the air combines chemically with the fuel because of the high temperature. Warmth, which can be used for heating, is a result of this chemical reaction.
A building’s heating system must provide: even heating of air in the rooms; ability to regulate the system; working with the ventilation system; convenience of usage and repairs.
The means of heat delivery in heating systems is water of not more than 105 degrees C; water steam of not more than 130 degrees C; or air warmed to 60 degrees C. Respectively, these systems are called hydronic, steam or air. The heating devices and pipes of the heating system should be placed in such a way that not more than ten percent of the heat is lost through the border constructions.
For easier access to the pipes, they should be exposed. However, there may be hygienic, structural, architectural or technological reasons why this may not be possible.
In heating systems where the local heating devices are placed not more than one metre above the floor, the heat carrying medium should not be hotter than 95 degrees C (in one-pipe systems, however, the temperature may be up to 105 degrees C); in floor-heating systems, it should be not more than 55 degrees C.
Heating systems can be either individual (one house or apartment) or multi-unit. Individual systems receive warmth directly from the unit’s heating room (e.g. boiler room). The types of heating devices used in individual systems can be: stoves; boilers which use solid, liquid or gas fuel; electric heaters; etc. In multi-unit systems, the warmth is produced in a centralized station, and is transported to customers through pipes.
There are two types of hydronic heating systems: middle-temperature and low-temperature. The temperature of the warmth carrier can be up to 95 degrees C in middle- temperature systems. In low-temperature systems, not more than 70 degrees C (with an average of 50–55 degrees C), and there should not be more than 14 degrees C difference between the temperature of the water as it leaves and returns to the boiler. Mid-temperature systems use small but high-heat-output radiators, while low-temperature systems use radiators which are barely warm but are of large surface area. Low-temperature systems are slightly more popular than high-temperature ones because the room temperature is more even and comfortable with the low-temperature systems.
Hydronic heating systems are categorized using several criteria:
- Using means of water circulation, a hydronic heating system is categorized as a “gravity system” or a “pumped system”
- Using design of the system, it can be either single-contour or multi-contour
- Using direction of the water in the supply and return paths, it can be either “dead end” or “continuing flow”
- Using method of connecting pipes to the heating units, it can be: one-piped, with units in sequence; two-piped, with units in parallel (direct return); or two-piped, with inputs connected in sequence in supply and outputs connected in sequence in return (reverse return)
- Using the criterion of whether the radiators are connected horizontally or vertically by the pipes
- Using the criterion of pipe assembly: t-joint, collectors, or mixed.
- Using path location: whether the supply pipe is above the heating units (upper setting), both supply and return pipes are below the heating units (lower setting); or whether the return pipe is above the heating units (upside-down setting). (Upper settings are used in buildings with an attic but no basement and lower settings are used in buildings with a flat room or a technical basement.)
Now we will examine all of the above variants.