Heating equipment jargon buster: Terminology explained clearly and simply

CHP unit, standard seasonal efficiency [to DIN], surface losses: Have you come across one of these terms before but not known exactly what they mean? If so, we are here to help. Our heating equipment jargon buster provides you with the definitions to many heating equipment terms, including many technical terms that are specific to Viessmann.


Absorbers are an integral part of a 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.


A unit of pressure relating to water, gas or air.


Biomass refers to any fuel which comes from biological matter. Viessmann offer a range of biomass products including log boilers, pellet boilers, woodchip boilers, wood heating systems and wood combustion systems.

Biomass is often referred to as a carbon neutral fuel, this is because the energy used to burn it is equivalent to the energy absorbed when the plant was growing.


Bleeding is the process applied to a radiator in order to release any air trapped within a central heating system. The need for this is usually signalled by the tops of a radiator feeling cooler than the rest. This is easily done using a radiator key, which opens the small valve at the top of the radiator to let the air out.

Combi boiler

A combi boiler combines a high-efficiency water heater and a central heating boiler within the same compact unit. With a combi boiler, no separate hot water cylinder is required and hot water is delivered to the taps and shower(s) at mains pressure.

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.


In addition to the heat which is generated, when gas or oil is burned within a boiler, condensation also forms. This is discharged from the boiler as a gas and is often noticeable in colder weather. In addition to being discharged in the air, condensate also collects inside the boiler. This is completely safe and nothing to worry about, and is simply drained away either inside or outside your home.

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.


DHW stands for domestic hot water. This refers to the water that comes from a hot tap or is stored in a hot water cylinder. 

Dual mode DHW heating

In dual mode DHW heating, domestic hot water is heated by two different heat generators – for example, a boiler and solar collectors. 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.


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), these are known as emissions. 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.

ErP rating

ErP Rating is a requirement set by the European Union in September 2015 to drive improvements in the efficiency and performance of heating and hot water products. ErP ratings are measured by efficiency classes from A++ to G.

Flow rate

Also known as LPM/litres per minute, Flow rate refers to the number of litres of hot water which comes out of the tap from a combi boiler in a minute. It is determined by the volume and pressure of water entering your property from the water main and the ability of the boiler to heat the water to the desired temperature.

The more powerful the boiler, the faster it will heat the water coming into your property.


The flue is the section of the boiler which allows gases to exit. This is usually located directly behind the boiler to allow waste gases to exit straight out of the property’s external wall.

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. 

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.

Gas Safe Register

Replacing CORGI from 1st April 2009, the Gas Safe Register is the official gas registration body for the United Kingdom, Isle of Man and Guernsey.

By law, all gas engineers and installers must be on the Gas Safe Register to legally work on a boiler or any other gas appliances. The purpose of the Gas Safe Register is to protect the public from unsafe gas work.

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%.

Ground source heat pump

Using pipes buried under the ground to extract stored thermal energy, ground source heat pumps use the ground as their primary energy source and convert this energy into clean, efficient and renewable heating and hot water for the home.

Heat only boiler

Commonly also known as conventional or regular boilers, this type of boiler can often be found in older homes. In order to provide water, heat only boilers need to be used in conjunction with a feed and expansion tank in the loft for the storage of cold water, and a hot water storage cylinder and pump for providing hot water.

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.

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.

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.

Lambda Pro Control

The Lambda Pro Control combustion controller in the Vitodens 200 boiler range 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.

LPM/litres per minute

Also known as flow rate (FR), this the rate at which water comes out of your tap per minute. It is determined by the volume and pressure of water entering the property from the water main and the ability of the boiler to heat the water to the desired temperature.

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.

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.

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.

Open-vented system

An open-vented system is a central heating system which features a tank in the loft known as a feed and expansion tank, or a header tank. It is the role of this tank to fill the boiler and radiator system, and keep it topped up with water.


The output of Viessmann boilers is measured in kilowatts (kW). The output of the boiler you choose for your home depends on your requirements. If you intend to be using a lot of hot water then a combi boiler with a high output is recommended. Whilst if you are intending for a system or heat only boiler to be installed, you will require a boiler with a lower heat output to heat your radiators, as hot water will be stored in the cylinder.

PCB/printed circuit board

The PCB is the central control unit of the boiler and where all of the other components are controlled from electronically. If another boiler component is not working properly, the PCB will shut down the boiler.


PV/photovoltaic is the name given to the process of generating electric power by using solar collectors to convert energy from the sun. 

Pilot light

Usually found in older boilers, a pilot light is a small flame which burns constantly to ignite the burner for heating and hot water. Modern Viessmann boilers do not use a pilot light as they are lit by electronic ignition.

Power flushing

Power flushing refers to the cleaning process which removes all of the sludge and debris which has built up inside a heating system. Power flushing is integral to the health of a central heating system and it can provide the solution to cold radiators and noisy boilers.

Power flushing should also be performed when installing a new boiler, as if your installer is installing a new boiler on to a dirty system it could damage or block the heat exchange. 

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.


SEDBUK stands for Seasonal Efficiency of Domestic Boilers in the UK. This is the rating given to a boiler’s efficiency. The rating system was developed in conjunction with boiler manufacturers and the government and provides a fair comparison of average boiler efficiency.

The SEDBUK rating scheme rates boilers from A-G, with A being the most efficient. 

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 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 thermal systems

A solar thermal system uses solar collectors to convert energy from the sun into the energy needed for hot water. With a solar thermal system, you are demonstrating your commitment to protecting the environment, by sustainably lowering CO2 emissions.

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 therefore lost as usable heating energy.

Surface losses occur as radiation losses whilst the burner is running or as standby losses when the burner is idle, especially in spring/autumn. However, they can also occur 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.

System boiler

A system boiler is a boiler which works by pumping hot water to radiators and a sealed hot water storage cylinder but does not need a tank in the loft to fill the system. For these reason, all components of the heating and hot water system are contained within one arrangement.

System boilers are ideal for properties with more than one bathroom, or those those who require higher flow rates or large capacities of hot water.


A solar collector generates heat whenever sunlight falls on the absorber – even at times when no heat is required. For example, when you 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 Viessman systems with ThermProtect temperature-dependent shutdown, the formation of steam is reliably prevented.

ThermProtect technology prevents overheating by switching the solar collectors off when they reach a specific temperature.

TRVs/thermostatic radiator valves

TRVs/Thermostatic radiator valves are control valves which are fitted to individual radiators so that you can control the temperature of a room by changing the flow of hot water to the radiator.

Weather compensation

Weather compensation is technology available on the Vitodens range of boilers, this uses an outdoor mounted weather compensation to work with the boiler and match its temperature to that needed based on the current weather conditions. Providing a pleasant indoor temperature, whatever the weather, weather compensation allows for fuel savings and a cut down on carbon emissions.