# Heat meter

A heat meter (heat meter , WMZ, measuring device for heat quantity) is a measuring device for determining the heat energy that is supplied to consumers via a heating circuit or taken from heat exchangers via a cooling circuit. The heat meter determines the thermal energy from the volume flow of the circulating medium and its temperature difference between the flow and return. The measured thermal energy is generally given in gigajoules (GJ) or megawatt hours (MWh) , previously also in gigacalories (Gcal) .

Distribution of heating costs with heat meters
Heat meter in a
heat transfer station for district heating from EVN Wärme with an output of 300 kW in a residential complex. The fitting piece, which was installed before the meter was installed, lies on the floor. Right above in blue and white: the arithmetic unit; in the middle slightly right in bronze: the ultrasonic flow meter
Mechanical heat meter (no longer common today, but easy to recognize water meter as a volume measuring part)
Electronic heat meter

## commitment

Heat meters are mainly used at

Further areas of application are:

• Measurement of the extracted heat ("released cold" or, better, cold work) of a refrigeration system. It is important to ensure that most of the anti-freeze additives in the cooling circuit are correctly taken into account when calculating enthalpy and density.
• Review of the profitability of solar thermal systems.

## Functional principle and measurement

The functional principle of the heat meter is based on the physical determination of the amount of heat that arises when the determined heat output is integrated over time: The heat output is calculated using the formula: ${\ displaystyle Q}$${\ displaystyle {\ dot {Q}}}$

(1)  ${\ displaystyle {\ dot {Q}} = {\ dot {m}} \ cdot c_ {W} \ cdot (\ vartheta _ {HV} - \ vartheta _ {HR})}$
(2)  ${\ displaystyle {\ dot {Q}} = {\ dot {V}} \ cdot \ rho \ cdot c_ {W} \ cdot (\ vartheta _ {HV} - \ vartheta _ {HR})}$

The following also applies to volume flow measurement using ultrasound :

(3)  ${\ displaystyle {\ dot {Q}} = {A} \ cdot v \ cdot \ rho \ cdot c_ {W} \ cdot (\ vartheta _ {HV} - \ vartheta _ {HR})}$

The subsequent integration gives the amount of heat:

(4)  ${\ displaystyle Q = \ int {\ dot {Q}} _ {t} \, \ mathrm {d} t}$

With:

${\ displaystyle {\ dot {Q}}}$ = Heat output (in MW or kW)
${\ displaystyle {\ dot {m}}}$ = Mass flow (kg / s)
c w = specific heat capacity of water (kJ / (kg K); Wh / (kg K))
${\ displaystyle \ vartheta}$HV = heating flow (° C)
${\ displaystyle \ vartheta}$HR = heating return (° C)
${\ displaystyle {\ dot {V}}}$ = Volume flow (l / s)
${\ displaystyle \ rho}$ = Density (kg / m³)
${\ displaystyle A}$= Cross-sectional area (dm²)
${\ displaystyle v}$ = Flow velocity (m / s; dm / s)
${\ displaystyle Q}$ = Amount of heat (MJ; kJ; kWh)
t = time (s; h)

As can be seen from formula (2), the volume flow and the temperature difference between the heating flow and heating return must be measured in order to record the heat , and the density and the specific heat capacity of the water must be known. Heat meters therefore consist of a measuring device for the volume flow ( flow meter , water meter ), a temperature sensor pair and an electronic calculator. With the ultrasonic measurement, instead of the volume flow, the mean flow velocity is used together with the pipe cross-section   . ${\ displaystyle {\ dot {Q}}}$${\ displaystyle {\ dot {V}}}$${\ displaystyle \ Delta \ vartheta}$${\ displaystyle \ rho}$${\ displaystyle c_ {W}}$${\ displaystyle {v}}$${\ displaystyle {A}}$

## Measuring device

• The arithmetic unit combines the incoming signals from the flow sensor and the temperature sensors and takes into account the temperature-dependent physical characteristics of the density and heat capacity c W . The integration in the calculator provides the amount of heat that is shown on the display in a unit to be selected. Additional information can be called up on request, such as the current flow rate, the heat output or an accumulated amount of heat within key dates to be entered.${\ displaystyle \ rho}$
• The temperature sensors for recording the heating flow and return are mostly platinum resistance thermometers, which are firmly integrated in the small heat meter area or, in the case of larger units, can be attached from the outside using special connectors.
• The volume measuring part is either mechanically rotating or statically based on the ultrasonic principle. Impeller meters in single-jet or multi-jet design, as well as Woltman meters , are used as mechanical measuring parts . Single-beam meters are suitable as compact meters for small measuring units, as even low volume flows of around 1.5 l / h can be reliably recorded. With multi-jet meters, such as those used in larger residential units, the volume flow flowing onto the wheel is divided up by orifices and the impeller is thus evenly loaded; the increased pressure loss is therefore offset by less wear on the bearing axles. Multi-jet meters are used in the larger nominal width range. In the same area of ​​application, Woltman meters are used which have lower pressure losses because the axis of rotation is parallel to the flow. They are available for horizontal installation (WS) and for vertical installation (WP) in risers.
• Ultrasonic counters based on the entrainment principle make use of the fact that sound waves running in opposite directions between two reflectors have different transit times, so that a transit time difference can be determined. This is proportional to the mean speed ofthe pipe flow, which, when multipliedby the pipe cross-section, results in the volume flow.${\ displaystyle \ triangle t}$${\ displaystyle {v}}$${\ displaystyle {A}}$

## Measurement error

Measurement error of a volume counter

The measurement errors of a heat meter result from the errors of its subcomponents. As can be seen from the diagram opposite, the measurement errors of the volume measuring section depend on the flow rate. In the start-up area  , the flow energy has to overcome the bearing friction until the wheel begins to turn and the flow rate can be determined. This is where the greatest measurement errors occur, and to avoid them, the meters must be designed so that this range does not occur during operation. In the range there  are measurement errors of up to ± 5%, which should be avoided by short-term operation. Volume flows with  represent the ideal flow range with errors of less than ± 2%. With larger volume flows, the dynamic forces increase quadratically with the speed of the flow, so that the bearings are exposed to extreme mechanical loads and continuous operation can lead to premature failure of the component. There are also increasingly strong flow noises that are transferred to the heating system and can disturb the residents. Another disadvantage is the increased pressure loss, which has to be compensated for by the pump. ${\ displaystyle {\ dot {V}} <{\ dot {V}} _ {\ mathrm {min}}}$${\ displaystyle {\ dot {V}} _ {\ mathrm {min}} <{\ dot {V}} <{\ dot {V}} _ {T}}$${\ displaystyle {\ dot {V}} _ {T} <{\ dot {V}} _ {max}}$

The measurement of the temperature difference is also subject to an error that can easily become relatively large with small temperature differences. For example, with a measurement inaccuracy of ± 0.3 K of the individual sensor, the measured temperature difference can in extreme cases be 0.6 K too high or too low. In this case, with a temperature difference of 10 K, the error is already 6%.

If the current heat output is to be determined, it must be ensured that the temperature profile in the flow and return is stable over time, i.e. does not increase or decrease significantly. The volume element of the cooler water in the return needs a certain amount of time to get from the return thermometer to the flow thermometer. If the temperature level rises and the temperatures are measured at the same time, a temperature difference that is too low is displayed, because only the temperature measured later gives the true temperature difference of the heated volume element and thus the current heat output. The usual heat meters (see above) mathematically integrate the heat output over time and again average out the errors in the instantaneous measurement that occurs when the temperature rises and falls.

## Types

Heat meters come in various designs, in particular as

• Compact heat meter: Calculator and volume measuring section are built into one housing ( compact body heat meter ). The volume measuring part can be designed as an ultrasonic flow meter. The temperature sensor pair is connected from the outside.
• Combination of a volume measuring unit (usually a hot water meter), an arithmetic unit and a pair of temperature sensors
• In addition, heat meters are equipped with key date modules that do not require the tenant to be present for reading, or with radio modules using plug-and-play technology and for remote transmission of consumption values.

## calibration

If heat meters are used for billing heating costs, they must be calibrated in Germany on the basis of the calibration law . During calibration , the temperature sensors are first checked individually and then pairs with matching error characteristics are formed, which may no longer be separated. The volume measuring parts are checked independently. All components must first be connected for assembly. The calibration validity for heat meters is five years. Afterwards, a re-calibration is required, which requires a complete repair. In the field of building services, single-use devices are predominantly used, which is questionable for reasons of ecology.

The currently valid calibration ordinance (EO-AV) still contains the exception rule (§8 Appendix A - 28 a + g) that heat meters with a heat output of 10 MW or more and water meters with a flow rate of more than 2000 m³ / h do not have to be (re) calibrated. These meters can be used unchecked for years (also for billing purposes). Such heat meters can be found in large industrial plants, waste incineration plants, thermal power stations, airports, clinics, large real estate complexes, trade fairs, etc.

## Interfaces

Heat meters are usually equipped with electrical interfaces. With these interfaces, measured values ​​are passed on to downstream processing units. The interfaces of the current heat meters are implemented as pluggable and can be programmed for one or more measured values.

• potential-free contact , for the transfer of impulses
• S0 interface according to DIN 43 864 for the transfer of impulses
• Analog interfaces 0… 5 V or 4… 20 mA, for transferring analog measured variables
• M-Bus (EN 13757-2 (physical and link layer) / EN 13757-3 (application layer))
• Other proprietary or open interfaces (wired or wireless)

In the case of the potential-free contacts and the S0 interface , the instantaneous power, cumulative power or the amount of water are usually transmitted as a weighted pulse, i.e. H. One pulse is transmitted per kWh or m³. The following units accumulate the pulses and then generate a value that can be displayed.

Analog interfaces are used for the transmission of instantaneous values. The current power and the current flow rate of the water can be used as measured values.

The M-Bus interface is a serial computer interface that works in the master-slave process. All values ​​recorded and generated in the meter are transmitted in one telegram.