Glossary of heating equipment: Terminology explained clearly and simply

Three-pass boiler, standard seasonal efficiency [to DIN] or surface losses: Have you come across one of these terms before, but not known exactly what they mean? Our glossary of heating equipment provides you with information and explanations regarding this and other terms from the field of heating equipment, along with technical terms that are specific to Viessmann.

Flue gas losses

The energy released by the combustion of oil or gas in a boiler cannot be supplied to the heating system without an element of loss. The hot flue gases that escape into the atmosphere via the chimney contain a relatively large amount of heat known as 'flue gas loss'.

During the annual emissions test, flue gas inspectors determine whether the combustion quality and flue gas loss arising during operation of the burner meet statutory regulations. They check that the burner operates correctly and the system is safe. Even if they award it a perfect score, this says little about the actual energy consumption of the boiler (its standard seasonal efficiency), since this is also significantly affected by the level of surface losses.

Sectional view of a solar collector


Absorbers are an integral part of every solar collector. They sit beneath the transparent, low-reflectivity glass cover of the collector, so that the sun's radiation reaches them directly.

The absorber absorbs the insolation almost entirely and the solar energy is converted into heat. As far as high efficiency is concerned, absorbers that have a highly selective coating – which includes all solar collectors made by Viessmann – are particularly notable.

Combined heat and power unit (CHP unit)

A combined heat and power (CHP) unit essentially consists of an engine, a synchronous generator and a heat exchanger. The synchronous generator, driven by the internal combustion engine (drive unit), generates a 3-phase alternating current with a frequency of 50 Hz and a voltage of 400 V, which is generally used on site.

The low voltage grid (0.4 kV level) is used for the electrical connection. As a rule, CHP units are operated in parallel to the mains. In principle, however, they can also be used in mains substitution mode by deploying synchronous generators.

Surplus power can be exported to the mains. The engine generates heat that can be absorbed in the 'internal cooling circuit' successively from the lubricating oil, the engine coolant and the exhaust gas, and transferred to the heating system via a plate heat exchanger.

This system of energy generation and utilisation is referred to as combined heat and power (CHP) generation because the mechanical energy (power) generated by the engine and the thermal energy (heat) given off by the engine as it drives the generator are both used simultaneously.

Function diagram

A gas combustion engine drives a generator to produce power. The heat this creates is extracted from the coolant and exhaust gas via the heat exchanger and can then be utilised.

Combined heat and power unit (CHP unit) functional diagram
Dual mode DHW heating

Dual mode DHW heating

In dual mode DHW heating, domestic hot water is heated by two different heat generators – a boiler and solar collectors for example. The heat from the solar collectors is transferred to the DHW via an indirect coil in the DHW cylinder. If necessary, the water can be reheated by the boiler via a second indirect coil.

Fuel cell technology

Fuel cell technology

Hydrogen and oxygen are all that is needed to generate heat and power. The chemical reaction between the two substances forms the basis of what is sometimes referred to as "cold combustion". It occurs between two electrodes: Hydrogen is conveyed to the anode, where a catalyst splits it into positive ions and negative electrons. The electrons travel to the cathode via an electrical conductor, causing electrical current to flow. At the same time the positively charged hydrogen ions reach the cathode through the electrolyte (an ion exchange membrane), where they ultimately react with oxygen to form water. Heat is released. The entire process is completely free of pollutants and is environmentally responsible.

Gross calorific value (Hs)

The gross calorific value (Hs) defines the amount of heat released by complete combustion including the evaporation heat latent in the water vapour of the hot gases.

Up until recently, the evaporation heat could not be utilised, as the technical capabilities did not exist for this. The net calorific value (Hi) was therefore chosen as the basis for all efficiency calculations. Referring to Hi and utilising the additional evaporation heat can thus lead to efficiencies above 100 %.

Condensing technology

Condensing technology

Condensing technology not only utilises the heat generated by combustion as a measurable temperature of the hot gases (net calorific value), but also the water vapour content (gross calorific value). Condensing boilers are able to extract almost all of the heat contained in the flue gases and to convert it into heating energy.

Condensing boilers use high performance heat exchangers. These cool the flue gases before they escape through the chimney, to the extent that the water vapour contained in these gases is deliberately condensed. This releases additional heat which is transferred into the heating system.

With this technology, a condensing boiler achieves a standard seasonal efficiency [to DIN] of up to 98 % (relative to Hs). Condensing boilers are therefore particularly energy efficient, looking after both your wallet and the environment.


Three-pass boiler

The design principle of the three-pass boiler contributes to the reduction of harmful emissions. The hot gases flow first through the combustion chamber, then return to the front via a reversing zone and enter a third pass. This cuts the time that the combustion gases spend in the hottest part of the boiler, reducing the formation of nitrogen oxide (NOx).


In every combustion process involving fossil fuels, the harmful gases carbon monoxide (CO) and nitrogen oxide (NOx) are formed, alongside the unavoidable carbon dioxide (CO2). Nitrogen oxides are particularly relevant here. An increase in these gases not only leads to higher levels of poisonous ozone, but is also one of the factors responsible for acid rain.

Heat pipe principle

Heat pipe principle

In heat pipe systems, the solar medium does not flow directly through the tubes. Instead, a process medium evaporates in the heat pipe below the absorber and transfers the heat to the heat transfer medium. The dry connection of the heat pipes inside the header, the small amount of fluid inside the collector and the automatic temperature-dependent shutdown in the case of the Vitosol 300‑T, ensure particularly high operational reliability.

Graphic: The medium heated by the sun evaporates and shifts to the colder part of the tube. There, the steam condenses, transfers the heat to the header and the water is then reheated in a new cycle.


A system boiler is a wall mounted appliance intended solely to provide heating. Such appliances can also be combined with a DHW cylinder to provide DHW heating.

Setting the heating curves

Heating curves

A weather-compensated heating controller ensures that the flow temperature is matched to the actual heat demand (the flow temperature is the temperature of the water fed to the radiator or underfloor heating system).

To this end, the outside temperature is measured and the flow temperature calculated in relation to the required room temperature and the conditions at the periphery of the building.

The relationship between outside and flow temperature is described by the heating curves. To put it simply, the lower the outside temperature, the higher the boiler water temperature or flow temperature.

Net calorific value (Hi)

Net calorific value (Hi) refers to the amount of heat released by complete combustion if the resulting water is discharged as steam. The evaporation heat latent in the water vapour of the hot gases is not used.

Gas compact hybrid appliance Vitocaldens 222-F

Hybrid appliance

A hybrid appliance is an appliance supplied by a number of energy sources. Such systems include, for example, dual mode heat pump systems. These are heating systems with an electrically operated heat pump in combination with at least one fossil fuel boiler and a higher ranking control unit.

During operation, the heat pump covers the base load with a high proportion of free ambient heat. For this, the external unit extracts the heat from the outdoor air and, via the compressor, heats it to a flow temperature of up to 55 °C.

The gas condensing boiler is only activated when this is beneficial in terms of the pre-set operating mode, i.e. when it results in reduced operating costs for the system user, lower CO2 emissions, or increased DHW convenience.


Inox-Radial heat exchanger

All Viessmann wall mounted and compact condensing appliances are now equipped with the stainless steel Inox-Radial heat exchanger. This technology brings with it an extremely high efficiency rate of up to 98 percent [to DIN] and exceptionally reliable and efficient operation during its long service life.

The Inox-Radial heat exchanger cools the flue gases before they are routed into the chimney, to the extent that the water vapour contained in these gases is deliberately condensed. The additional heat released is transferred into the heating system. This function not only saves valuable energy, but also protects the environment through significantly reduced CO2 emissions.

Viessmann heat pump

Seasonal performance factor (SPF)

In heat pumps, the coefficient of performance (COP) is the ratio of heat transfer to power consumption. The seasonal performance factor is the average of all COPs occurring in a year. The COP is used to compare heat pumps with regard to efficiency, yet it is derived from a particular operating point under defined temperature conditions.

When planning a system, it is necessary to consider its operation over the whole year. For this, the amount of heat transferred over the year is given in relation to the overall electrical power drawn by the heat pump system (including power for pumps, control units, etc.) over the same period. The result is given as the seasonal performance factor. Example: A SPF of 4.5 means that, averaged out over the whole year, the heat pump has required one kilowatt hour of electrical energy to generate 4.5 kilowatt hours of heat.

Combi boiler

A combi boiler is a wall mounted appliance that is used both for central heating and for DHW heating. The DHW is heated using the instantaneous water heater principle.

Lambda Pro Control

Lambda Pro Control

The Lambda Pro Control combustion controller in Vitodens wall mounted gas condensing boilers ensures constantly stable and environmentally responsible combustion, a consistently high level of efficiency and high operational reliability, even if gas quality varies.

The Lambda Pro Control combustion controller automatically recognises every gas type used. This makes manual adjustments and measuring during the commissioning superfluous. In addition, the Lambda Pro Control continuously manages the gas-air mixture to ensure constant clean and efficient combustion, even when the gas quality varies. The ionisation electrode supplies the raw data required for this purpose directly from the flame.

Micro CHP unit

Decentralised heat and power provision is proving increasingly relevant. Viessmann offers solutions that can contribute towards levelling out the volatility of electricity supply from renewables. Wind farms and photovoltaic plants have been built in large numbers to replace nuclear power stations and conventional large scale power stations.

However, as the availability of these renewables fluctuates and, consequently, cannot be scheduled, controlled combined heat and power plants (CHP) have become important components in the push towards a successful energy transition. This development is led by the political objective of increasing the share of power generated by CHP plants to 25 percent by 2020.

Decentralised power generation

Where there are shortages in volatile power generation, micro CHP units can make an important contribution to covering demand. Because this happens locally and the power is generated on site, this also reduces pressure on power grids. Generating your own electricity by means of CHP units is now a viable replacement for drawing power from the grid. In combination with a power storage unit, a standalone power supply can be achieved, particularly with micro CHP systems.

Micro CHP unit system diagram

Micro CHP unit system diagram

[1] Micro CHP unit
[2] Vitocell 340-M heating water buffer cylinder
[3] Vitovolt power storage unit
[4] Inverter (power storage unit)
[5] Heating circuit
[6] Consumers
[7] Meter
[8] Domestic power circuit
[9] Home Manager
[10] Public grid
[11] Wireless remote control

Natural cooling

The primary purpose of heat pumps is to provide comfortable and convenient central heating and reliable DHW heating. However, they can also be used to cool a building. While the ground or groundwater is used in winter to provide energy for heating, in summer it can be used for natural cooling.

With the natural cooling function, the heat pump's control unit starts only the primary pump and heating circuit pump. This means the relatively hot water from the underfloor heating system can transfer its heat via the heat exchanger to the brine in the primary circuit. This extracts heat from all rooms that are connected, which makes natural cooling a particularly energy efficient and affordable way to cool interiors.

Standard seasonal efficiency [to DIN] 

Standard seasonal efficiency [to DIN] was introduced to enable the energy consumption of different types of heat generator to be compared. As a measure of the energy utilisation of a boiler it shows, across the whole year, up to what percentage of the energy utilised is converted into usable heating energy.

The level of the standard seasonal efficiency [to DIN] is significantly affected by the level of flue gas losses and surface losses arising during operation.

Surface losses

Surface losses are the proportion of the combustion output released to the surrounding air by the heat generator surface, and thus lost as usable heating energy.

They occur as radiation losses while the burner is running or as standby losses when the burner is idle, especially in spring/autumn, but also in the summer months when the boiler is required solely for DHW heating.

As a rule, the surface losses of an old boiler will be substantially higher than the flue gas losses checked by the flue gas inspector. The level of surface losses is thus a critical factor in the cost effectiveness (the standard seasonal efficiency) of the heat generator.

Open flue operation

Open flue operation

The terms "open flue" and "room sealed" describe how a boiler is supplied with the air that it needs for combustion.

In open flue operation, it takes its combustion air from the room where it is installed. The room must, of course, therefore have adequate vents. There are a number of possibilities here. Frequently, the combustion air supply is ensured via openings or gaps (vents) in the exterior wall. If the appliance is sited inside the living space, another option is the 'interconnected room air supply', in which adequate ventilation is ensured by means of air connections (slits in the doors) to a number of other rooms.

Room sealed operation

In room sealed operation, the combustion air required is supplied from outside via ventilation pipes. In essence, three solutions can be identified:

1 Air supply via a vertical roof outlet
2 Air supply via an external wall connection
3 Air supply via a balanced flue stack

The benefits of room sealed operation are that it provides even greater flexibility than open flue operation when it comes to siting wall mounted gas boilers. The appliance can be installed anywhere – whether in living rooms or in recesses, cupboards or roof spaces.

Independence from indoor air also reduces losses, since the heated air in the room is not being used for combustion. Room sealed appliances can therefore be sited within the thermal building envelope.

Room sealed operation
Solar DHW heating

Solar DHW heating

The dual mode DHW cylinder is key to this type of system. When there is sufficient insolation, the heat transfer medium in the solar thermal system heats up the water in the DHW cylinder via the lower indirect coil. The boiler starts when the temperature drops through water being drawn off, such as for a bath or shower – if necessary – in order to provide additional heating via the second circuit.

Solar central heating backup

The heat transfer medium heated up in the solar collectors can also be used to heat the heating water, as well as for DHW heating. For this, the heating circuit, via a heat exchanger, uses the water in the solar cylinder that is continuously heated by the solar collectors. The control unit checks whether the required room temperature can be achieved. The boiler will also start if the temperature is below the set value.

Solar central heating backup


A solar collector generates heat whenever sunlight falls on the absorber – even at times when no heat is required. This may, for example, be the case in summer when residents are on holiday. If heat transfer, through the DHW cylinder or heating water buffer cylinder, is no longer possible because either is already fully heated, the circulation pump switches off and the solar thermal system goes into stagnation.

If further insolation falls on the collector, its temperature will rise until the heat transfer medium evaporates, causing high thermal stresses on system components such as seals, pumps, valves and the heat transfer medium itself. In systems with ThermProtect temperature-dependent shutdown, the formation of steam is reliably prevented.

ThermProtect prevents overheating

An intelligent absorber layer protects the collectors from overheating. Viessmann’s patented ThermProtect technology switches the solar collectors off when they reach a specific temperature. At temperatures of 75 °C and above, the crystalline structure of the absorber layer changes, increasing the rate of heat radiation many times over and reducing collector output. This significantly lowers the maximum temperature inside the collector and prevents vapour formation in the solar circuit.

By lowering the collector temperature, the crystalline structure returns to its original state. More than 95 percent of the incoming solar energy can now be absorbed and converted into heat. Only the remaining five percent is reflected. There is no limit to the number of times the change in crystalline structure can be activated, making this function available at all times.

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ThermProtect prevents overheating

An intelligent absorber layer protects the collectors from overheating. Viessmann’s patented ThermProtect technology switches the solar collectors off when they reach a specific temperature. At temperatures of 75 °C and above, the crystalline structure of the absorber layer changes, increasing the rate of heat radiation many times over and reducing collector output. This significantly lowers the maximum temperature inside the collector and prevents vapour formation in the solar circuit.