In most commercial buildings, air supply is among the most costly utility services. Until recently, however, one essential capability was missing – a capability that has existed for decades for all other relevant utilities: consumption measurement by means of meters and legally compliant billing based on these measurements.
With the invention of the air energy meter and the official establishment of this new metrological quantity in 2020, Luftmeister was the first worldwide to create the basis for consumption-based and legally compliant billing of conditioned air.
The basis for legally compliant billing using the air energy meter is the metrological quantity “air energy”, which has been officially established for several years. This quantity is specified by the product category standard DIN 94701, which defines the minimum requirements for air energy meters.
On this basis, Luftmeister was granted a type examination certificate by the Physikalisch-Technische Bundesanstalt (PTB) in 2020, authorising the manufacture, placing on the market and commissioning (verification commissioning) of this innovative metering system.
In addition, the VDI guideline 2077-4, which has been in force since 2019, provides the applicable billing methodology and confirms that air energy meters represent the state of the art.
Legally compliant, consumption-based ventilation cost billing
Operating principle of the certified air energy meter
The air energy meter records three meter values for its respective consumption zone: air consumption in m³, as well as heat consumption and cooling consumption in kWh. To achieve this, the air volume flow rate, air mass flow rate and the enthalpy of supply air and outdoor air must be measured with high accuracy, as illustrated in the following figure.
The accumulated air volume flow results in the air consumption in m³. At the same time, the mass flow rate is multiplied by the enthalpy difference (supply air minus outdoor air) to determine the thermal power (in kW). During periods of positive thermal power (when the enthalpy of the supply air exceeds that of the outdoor air), the air-side heat consumption counter is incremented. During periods of negative thermal power, the air-side cooling consumption counter is incremented.
Operating principle of the certified air energy meter
Components and installation procedure
Luftmeister develops and manufactures the patented air energy meters. These devices feature a cascade of differential pressure sensors (i.e. several overlapping measurement ranges) in order to achieve high accuracy across both high and low flow ranges. In addition, the air energy meters are equipped with a precise absolute pressure sensor and digital inputs for external enthalpy sensors, which measure temperature and humidity with high accuracy.
On this basis, air density and enthalpy are known to the air energy meter, enabling it to calculate not only the volume flow rate but also the mass flow rate, thermal power and the meter values described above (air consumption, heat and cooling meters). The air energy meters are equipped with a user-friendly display showing both instantaneous values and the meter readings of the last 24 months. A wide range of output signals (M-Bus, Modbus RTU, pulse, switching and analogue outputs), as well as an integrated 9-channel data logger, complete the system.
In the certified version (Luftmeister EZ 55), the air energy meters are connected to Luftmeister measuring sections featuring the patented double-blade sensor design. A detailed description of this precision sensor technology can be found in the section Air flow measurement. The dimensions of the measuring sections are individually adapted to the respective supply air duct dimensions.
Each measuring section and each air energy meter has its own serial number and is calibrated and adjusted together in the high-precision Luftmeister air flow calibration laboratory in accordance with the requirements of the Physikalisch-Technische Bundesanstalt (PTB).
While the installation of the measuring sections and the electrical wiring are carried out on site by the customer, the components themselves (air energy meter and enthalpy sensors) are installed by Luftmeister technicians. They also perform the final verification commissioning on site.
Components and installation procedure
Planning considerations
It must be taken into account already during the planning phase that the air energy meter measurement system must fulfil certain mandatory verification requirements. The measuring sections must be installed in a supply air duct section that provides a straight, unobstructed length of 4 × D, including the measuring section itself. For rectangular ducts, D refers to the hydraulic diameter.
The power supply (230 VAC) of the air energy meters must be protected together with the supply air fan of the respective HVAC system. The HVAC system itself must be equipped with certified primary meters (electricity, heating water and, if applicable, chilled water energy meters).
The Luftmeister measurement system must remain accessible for maintenance and re-verification (e.g. sufficiently large inspection openings). Finally, all wiring must be carried out in accordance with Luftmeister specifications.
The following graphic summarises some of these requirements.
Planning considerations
Air delivery monitoring
The air energy meters are installed at the utility transfer point of the conditioned air supply. This is where the delivery (supply) of air from the HVAC system operator to the zone tenant or zone owner takes place.
In addition to consumption-based billing, the air energy meter also enables monitoring of this air delivery. Specifically, air flow rate, temperature and humidity are measured with high precision at all times. If these measurement data are also read out and made transparent by the operator, many of the typical disputes regarding “incorrect” air volumes and temperatures can be avoided.
Furthermore, the time profile of the air flow rate and temperature can be analysed jointly by the operator and the air recipient to define and implement savings measures on a long-term basis. For example, if it becomes apparent that a lower air flow rate and a later start of air supply in the morning are sufficient, these parameters can be specifically reduced – resulting in lower ancillary costs and improved energy performance indicators for the building.
Air delivery monitoring
Sustainability and savings
Building projects equipped with air energy meters regularly achieve higher ratings in sustainability certification schemes, as consumption measurement and air delivery monitoring represent direct savings levers for the costly medium of conditioned air. For the same reason, the German Building Energy Act (GEG) stipulates that, with the next revision of the Heating Costs Ordinance, HVAC systems with multiple consumption zones must be equipped with meters that account for the individual consumption of each zone – these are the air energy meters.
From a building owner’s perspective, maintaining control over costs is naturally a key concern. In this context, it is beneficial that the recurring re-verification costs can be passed on to tenants. In addition, as illustrated in the graphic below, the use of air energy meters allows many projects to supply a building with a reduced number of central HVAC units.
Whereas conventional buildings were often equipped with a separate central HVAC unit for each major tenant (such as a supermarket or a law firm), neighbouring rental areas can now be supplied by the same HVAC system. The result is substantial savings in planning effort and, in particular, in investment costs and annual operating expenses.
Sustainability and savings
