What Is a Heat Meter? How It Works, Types and Selection
A heat meter in a building calculates the heat consumed by an apartment in kWh from the flow rate and the supply–return temperature difference. In this guide we explain the working principle, types, size selection, remote meter reading and re-verification rules.
The word calorimeter is used for two different devices. The calorimeter vessel or bomb calorimeter known from chemistry and physics classes is a laboratory setup that measures the heat released in a reaction. The subject of this article is the device used in buildings, the heat meter: a measuring instrument that records the heat energy consumed by each apartment in a building with central heating and forms the basis for heat cost allocation. Below we explain how it works, the types, how to choose the right size and what regulations typically require, using the technical data of the meters we at Novus Energy sell, read and re-verify.
What Is a Heat Meter and What Does It Measure in Buildings?
Turkish cost-allocation regulations define a heat meter as a device that measures the heat energy passing through the heating line on which it is installed, based on the flow rate and the temperature difference. In installations with manifolds (one entry per apartment), each apartment has a single supply and return line; the heat meter is installed on these lines, usually on the return line, and totals the energy used by the apartment. The main value on the meter display is the energy accumulated since the meter was first commissioned (the meter reading). Depending on the manufacturer, the unit can be MWh, kWh or GJ. The same display also shows the total water volume, the current flow rate, the supply and return temperatures and the values of past periods.
You can find all heat meter models on our Heat Meters page.
How Does a Heat Meter Work? Flow × Temperature Difference = Energy
The logic of a heat meter is simple: the water that cools down while passing through the radiators has carried as much energy as the heat it released into the apartment. The meter measures two things:
- The amount of water passing through the circuit (flow rate and volume),
- The temperature difference between the supply and return water.
The calculator unit measures these two values at set intervals (from a few seconds to half a minute, depending on the model), multiplies them and accumulates the result. Since the heat capacity of water is about 4.19 kJ/(kg·°C), 1 m³ of water releases about 1.16 kWh of energy when it cools by 1 °C.
Example: Suppose 0.4 m³ of water passes through the apartment in one hour; the supply water is 70 °C and the return water is 55 °C. The temperature difference is 15 °C. Consumption is approximately 0.4 × 15 × 1.16 ≈ 7 kWh. If the same conditions last 6 hours a day, the daily consumption is about 42 kWh. Because a real meter accounts for the change in water density and heat capacity with temperature according to EN 1434, its result may differ slightly from this manual calculation.
Parts of a Heat Meter
- Flow sensor: Measures the volume of water. It can be ultrasonic or mechanical (impeller type).
- Temperature sensor pair: Two matched PT1000 sensors. One measures the supply temperature, the other the return temperature. The return sensor is usually fitted in the flow sensor body, while the supply sensor is installed directly in the pipe or in a sensor pocket. Installation parts are in the heat meter spare parts category.
- Calculator unit: Houses the display, memory and battery. It calculates the energy and stores monthly values and annual due-date values.
- Communication: M-Bus, wireless wM-Bus, LoRa or pulse output. Enables remote meter reading.
In apartment-type meters these parts are combined in a single body (compact meter). In larger sizes, the flow sensor, calculator unit and sensors can be installed as separate components; these models are in the large-diameter heat meter category.
Ultrasonic vs. Mechanical Heat Meters
The way the flow is measured divides heat meters into two groups. The values below are taken from the data sheets of two compact meters from the same manufacturer (ZENNER zelsius C5-IUF ultrasonic, C5-ISF mechanical).
| Ultrasonic | Mechanical (single-jet) | |
|---|---|---|
| Flow measurement | Difference in transit time of sound waves with and against the flow | Single-jet impeller turned by the water |
| Moving parts | No | Yes |
| Accuracy class | Class 2 (optional 3) | Class 3 |
| Minimum flow, 2.5 m³/h model | 25 or 50 l/h | 50 or 100 l/h |
| Installation position | Any position | Horizontal, tilted 90° or vertical |
Ultrasonic meters are preferred in new projects because they have no wearing parts and capture low flow rates better. Mechanical meters are more economical and remain a sensible option for replacing an existing mechanical meter of the same dimensions. Models: ultrasonic heat meter and mechanical heat meter.
Which Size (DN) for an Apartment?
The heat meter size is selected according to the design flow rate of the apartment, not the pipe size. The values below are taken from the technical tables of the products on our site; the body length may vary by brand and model.
| Size | Inch equivalent | Nominal flow (Qn) | Body length (L) | Connection thread |
|---|---|---|---|---|
| DN15 | 1/2” | 1.5 m³/h | 110 mm | 3/4” |
| DN20 | 3/4” | 2.5 m³/h | 130 mm | 1” |
| DN25 | 1” | 3.5 m³/h | 160 mm | 1 1/4” |
| DN32 | 1 1/4” | 6 m³/h | 180 mm | 1 1/2” |
| DN40 | 1 1/2” | 10 m³/h | 200 mm | 2” |
The most common size in apartments is DN20. If you are replacing an existing meter, choosing a model with the same size, body length and connection thread makes installation easier. Keep in mind that the same size may come in different lengths: for example, the Minol Zenner C5 IUF DN25 on our site has a body length of 260 mm and a nominal flow of 6 m³/h. The connection size in the table refers to the thread on the meter body; the meter is connected to the installation with couplings. For flanged models DN50 and above, see the large-diameter pages starting with DN50.
Heating, Cooling and Combined Metering
The same principle applies to cooling; the difference is that the supply water is colder than the return. Heating–cooling (combined) meters store the two energies in separate registers. Before buying, check the application type in the product table: some models are listed for heating only, others for heating and cooling. Manufacturer documents may show an MID certificate for heat metering and a separate national type approval for cooling metering (for example ZENNER zelsius C5-IUF). Cooling circuits with glycol require a special version calibrated for the glycol mixture instead of a standard meter (for example the glycol version of the ZENNER C5-ISF).
Remote Meter Reading: M-Bus, wM-Bus (RF) and LoRa
- M-Bus: Wired reading. Meters are connected to a two-wire bus; the same line carries data and powers the meter's communication circuit. The bus is connected to an M-Bus converter, and the data can be transferred via GSM or Wi-Fi. Example: Novus DFN-M500 GSM/Wi-Fi M-Bus converter.
- wM-Bus (RF): Wireless reading. The meter transmits data telegrams in the 868 MHz band; reading is done by walk-by or with fixed receivers installed in the building. OMS-compliant devices can be read with receivers from different manufacturers, and the data can be encrypted.
- LoRa (LoRaWAN): Long-range reading over a wide area network; daily or monthly values are transmitted.
For a comparison of the methods, see our article on remote reading methods; for reading equipment, see our meter reading category; for the service, see our Remote Meter Reading and Billing page.
Are Heat Meters Mandatory? Regulatory Example: Türkiye
Metering obligations differ from country to country, so check your local regulations. As an example, this is how the rules work in Türkiye: the Regulation on Energy Performance in Buildings, based on Energy Efficiency Law No. 5627, requires buildings with central heating or connected to district heating to use systems that allocate heating costs according to the amount of heat used. According to the same regulation, new buildings with a construction area of 2,000 m² or more are built with central heating, and the installation is designed so that a heat meter can be used in each dwelling unit, with the meters located outside the unit; it is stated that this rule may not apply to the 1st degree-day zone defined in the Turkish standard TS 825.
In Türkiye, how the cost is divided is set by the Regulation on the Allocation of Heating and Domestic Hot Water Costs in Central Heating and Domestic Hot Water Systems: in central heating, 70% of the total heating cost is allocated according to measured consumption and 30% according to floor area. Measurement can be done with a heat meter or with a heat cost allocator fitted to each radiator. The regulation makes the building owner, manager or management board responsible for ensuring that the metering equipment is regularly re-verified and calibrated. For a step-by-step explanation of the calculation, see our central heating cost allocation guide.
Periodic Re-verification (Every 5 Years in Türkiye)
Re-verification periods and procedures depend on national rules; check your local regulations for periodic re-verification requirements. In Türkiye, heat meters are subject to periodic re-verification under the Heat Meter Inspection Regulation based on Law No. 3516 on Measures and Calibration. The period is 5 years and is tracked by year, not by date; the year in which the meter was stamped counts as the first year, regardless of the month. For example, the period of a meter stamped in 2021 expires at the end of 2025, and the re-verification application is made between the beginning of January and the end of February 2026. Re-verification is carried out at services authorized by the Ministry; a meter within the error limits is stamped, while a meter outside them is repaired and adjusted or replaced. For the process and our service, see our Heat Meter Re-verification and Calibration page.
How Long Does a Heat Meter Battery Last?
The calculator unit usually runs on a lithium battery; battery life varies by model and battery capacity. For example, the data sheets of the ZENNER zelsius C5 series state a battery life of at least 7 years, and at least 10 years with the long-life battery option. The manufacturer also notes that battery replacement is subject to different rules depending on the country. Do not open the meter seal yourself to replace the battery: a meter with a broken seal or stamp cannot be re-verified.
What Determines the Price of a Heat Meter?
- Measurement technology: ultrasonic or mechanical.
- Size and connection: DN15–DN40 threaded or DN50 and above flanged.
- Communication: wired M-Bus, wireless wM-Bus, LoRa or display reading only.
- Application type: heating only or heating–cooling.
- Brand, MID approval and sensor type.
- Installation material and reading service: couplings, sensor pocket, reading infrastructure.
Our range includes Novus Digitermo, Minol Zenner, Calmet, Cem, İntegral, Danfoss and Honeywell heat meters, as well as Qundis heat cost allocators. You can see current prices on the product cards and filter the list by brand, size and communication type.
Read more: How to read a heat meter · Heat meter battery · Heat meter error codes
Frequently Asked Questions
What does a heat meter measure?
A heat meter used in buildings measures the heat energy that an apartment draws from the heating circuit. It records consumption in kWh or MWh by measuring the amount of water passing through the circuit and the supply–return temperature difference.
What is the difference between a heat meter and a heat cost allocator?
A heat meter is installed on the apartment's entry line and measures the energy consumed in kWh. A heat cost allocator is mounted on each radiator and records a unitless consumption value; it is used in buildings where the radiators are fed from different risers.
Which heat meter is better, ultrasonic or mechanical?
An ultrasonic heat meter has no moving parts, measures low flow rates over a wider range and has a higher accuracy class in manufacturer data. A mechanical heat meter is more economical and suitable for replacing an existing mechanical meter of the same dimensions.
How often must a heat meter be re-verified?
This depends on national rules. In Türkiye, the Heat Meter Inspection Regulation sets a periodic re-verification period of 5 years. The year in which the meter was stamped counts as the first year; for a meter whose period has expired, the application is made between the beginning of January and the end of February of the following year.
How long does a heat meter battery last?
Battery life varies by model and battery capacity. For example, the data sheets of the ZENNER zelsius C5 series state a battery life of at least 7 years, and at least 10 years with the long-life battery option.
How is kWh calculated in a heat meter?
Approximately, the volume of water passed (m³) is multiplied by the supply–return temperature difference (°C) and by 1.16. For example, if 0.4 m³ of water passes through and cools by 15 °C, consumption is about 7 kWh. The meter calculates more precisely by taking into account the change in water properties with temperature.
How to Read a Heat Meter: Display Values and Brand Guide
Heat Meter Battery: Lifetime, Replacement and What Happens When It Runs Out
Heat Meter Error Codes: Meanings by Brand and SolutionsGet free product advice on size, communication and reading system compatibility.