Glossary of heating technology terms

Annual efficiency
Key figure to assess the energy efficiency of a heating boiler.The annual efficiency shows the percentage relation between the utilisable thermal energy and the energy consumed for a full year.
> Standard efficiency

Combined air and flue gas systems
Combined air and flue gas systems allow heating boilers to operate independently of the room air.
The air required for combustion is drawn in directly from the outside atmosphere. Such combined systems operate on the basis of a pipe-in-pipe concept: the inner pipe serves as flue pipe, while the combustion air is delivered to the heating boiler via the shaft between inner and outer pipe.

Condensing boiler
A condensing boiler operates with infinitely variable temperature; unlike conventional boilers, it additionally utilizes the condensation heat contained in the flue gas. The condensing boiler cools the flue gases so that the water vapor contained in the flue gas condenses. The heat liberated in the process is used for heating purposes. Gas-fired condensing boilers achieve standard efficiency values of up to 110 %.

Control
The purpose of the heating control system is to ensure that each room is heated in accordance with individual requirements. Depending on the configuration, it can control one or more heating circuits with different feed and return flow temperatures. It can also coordinate the production of hot water or the incorporation of additional heating systems, such as a solar thermal system, into the existing heating system.

Room-temperature-based control:
The feed temperature of the heating medium is controlled in accordance with the existing and desired room temperature of a selected reference room All factors acting on the room, such as additional heat due to sunlight, are included when measuring the temperature of the room and taken into account accordingly when determining the feed temperature.

Control based on outside temperature:
The feed temperature of the heating medium is controlled in accordance with the outside temperature. The relationship between momentarily prevailing outside temperature and the feed temperature to be selected is reflected in the heating characteristic set on the controller.

Cooling loss
The cooling loss is that portion of the thermal energy that the heating boiler loses into the ambient air in an uncontrolled manner.A warm
boiler releases heat through convection and radiation into the ambient air, irrespective of whether the burner is running or not.

Efficiency
The efficiency of a heating boiler indicates the relation between the heat output and the heat input in a defined operating point.
> Standard efficiency, Annual efficiency

Electric hot water boiler
Electrical appliances which produce hot water. They are available in the form of instantaneous water heaters or storage tanks.

Emission
The emissions of a heating system are products of combustion that are discharged into the atmosphere through the chimney together with the flue gas.

Energy intensity
The energy intensity is a key ratio defining a national economy ’s energy consumption in relation to the generated gross national product.

Energy performance certificate
The energy performance certificate shows the energy efficiency of a building. It contains all relevant energy data of a building.

EU Directive on the "Energy performance of buildings"
The EU Directive on the "energy per formance of buildings" or EU Buildings Directive, as it is known for short, came into force in January 2003 with the aim of reducing energy consumption in buildings As a matter of principle, the Directive was to be transposed into national law in all EU Member States by the beginning of 2006.

Feed temperature
The feed temperature is the temperature of the heating water when it leaves the boiler.

Flue gas loss
The flue gas loss is that portion of the thermal energy that escapes unused together with the flue gases.

German Energy Savings Directive (EnEV)
The German Energy Savings Directive (EnEV) came into force in February 2002 and defines minimum standards for the insulation and the quality of the heating system of new and existing residential buildings. The aim of the Directive is to reduce the primary energy demand for space heating and the hot water preparation.

Heat pumps
Heat pumps utilize the energy contained in the environment to heat rooms and the domestic water. Their principle of operation corresponds to that of a refrigerator, but the other way around.
The refrigerant in a heat pump circuit extracts heat from the soil, air or water and evaporates. It is then pressurized and heated. This "heated" vapor releases energy which is transferred to the heating water, causing the vapor to condense. The refrigerant resumes its original form when it is subsequently depressurized.

Gross calorific value
The gross calorific value indicates the amount of heat that is released when the fuel is burned, including the heat contained in the flue gases (so-called condensation heat).

Low-temperature boiler
Low-temperature boilers operate with infinitely variable temperature control. As a result, they operate with distinctly lower heating water temperatures than used to be the case with constant-temperature boilers.
Since the flue gas loss and the heat lost through the boiler sur face are distinctly lower, these low-temperature boilers achieve standard efficiency values of 92 % to 96 %.

Net calorific value
The net calorific value indicates the amount of heat that is released when the fuel is burned without taking into account the heat that is released as water vapour together with the flue gases.

Pellets
Pellets are pressed pieces of firewood, wood chips and untreated offal timber. They measure approx. 6 millimetres in diameter and 10 to 30 millimetres in length.

Primary energy
Primary energy is energy obtained from natural sources such as coal, oil, natural gas, solar radiation or natural uranium.

Primary energy demand
The annual primary energy demand is the key figure in the German Energy Savings Directive (EnEV) and the energy performance certificate. It indicates how many kWh of energy need to be obtained from an energy source e.g. coal or natural gas to cover a building ’s heating and hot water requirements.

Regenerative energy
Energy that is not obtained from fossil energy carriers but from renewable resources such as wood or from water power or solar radiation.

Return flow temperature
The return flow temperature is the temperature of the heating water when it returns to the boiler.

Solar storage tanks
Bivalent hot water storage tanks, combination storage tanks and buffer storage tanks are used, depending on the purpose to which the generated heat is to be put.
Bivalent hot water storage tanks are normally used for solar heating of the domestic water, while buffer storage tanks are used as solar support for the heating system. The combination storage tanks are used for combined heating of the domestic water and support of the heating system.

Solar collectors
Solar collectors convert the short-wave radiation from the sun into long-wave thermal radiation.
A special heat transfer liquid in the collector absorber is heated by the captured sunlight and transported to the solar storage tank, where the transfer liquid releases its heat and is returned to the collector.

Solar thermal systems
Solar themal systems convert solar energy into useful thermal energy for heating the domestic water and to support the heating system.
> Solar collectors > Solar accumulators

Standard efficiency
The standard efficiency indicates the percentage relation between the utilisable thermal energy and the energy consumed according to a standardised measuring system. The standard efficiency enables a comparison between different boiler types and between the products of different manufacturers.
> Annual efficiency

Thermosiphon systems
Thermosiphon systems are simple solar thermal systems which are only used to heat the domestic water. They comprise a solar collector and an storage tank above the collector Radiation from the sun heats the solar liquid in the collector; the liquid becomes lighter and rises through the piping to the storage tank above.

Thermotechnology
The term thermotechnology refers to all technologies for the generation of space heat and hot water.