Heat Meter Selection: DN20 Ultrasonic or Mechanical?
Short answer: For a new apartment installation, or for a meter that should run untouched for many years, choose a DN20 ultrasonic heat meter: it has no moving parts, every DN20 ultrasonic model on our site is MID accuracy class 2, and it measures low flows better. A DN20 mechanical (single-jet) heat meter makes sense when you replace an existing mechanical meter with the same 130 mm body length on a tighter budget; the mechanical model on our site is accuracy class 3.
We explain what a heat meter is and how it works in What Is a Heat Meter?. This guide compares ultrasonic and mechanical meters in DN20, the most common size for apartment installations, in terms of accuracy, low-flow performance, installation, communication and battery life. Values come from the manufacturer data on our product pages and from the ZENNER data sheet.
Why DN20?
DN20 corresponds to 3/4″ pipe and is the most common heat meter size in apartment installations with a manifold (distribution) station. Every DN20 model on our site with a technical table has a nominal flow (qp) of 2.5 m³/h, a body length of 130 mm and a 1″ body thread; the meter is connected with unions. Because these dimensions are the same across brands, a DN20 meter of one brand can easily replace another's.
To confirm the size, calculate the design flow of the apartment: flow (m³/h) ≈ heat load (kW) ÷ (1.163 × ΔT). With an 8 kW heat load and a 20 K supply-return difference, for example, the flow is about 0.34 m³/h, well below the 2.5 m³/h nominal flow of a DN20 meter. In spring and autumn the flow drops much further, so what really matters is how the meter behaves at its minimum flow (qi).
| Apartment heat load | Flow at ΔT = 20 K | Flow at ΔT = 10 K |
|---|---|---|
| 6 kW | ≈0.26 m³/h | ≈0.52 m³/h |
| 10 kW | ≈0.43 m³/h | ≈0.86 m³/h |
| 15 kW | ≈0.64 m³/h | ≈1.29 m³/h |
The values are calculated with the formula above and all fall below the DN20 nominal flow. Where a maximum flow (qs) is stated for our DN20 ultrasonic models, it is 5 m³/h.
Ultrasonic vs mechanical DN20 heat meters
| Criterion | DN20 ultrasonic (models on our site) | DN20 mechanical (Minol Zenner C5 ISF) |
|---|---|---|
| Flow measurement | Transit-time difference of sound waves; no moving parts | Single-jet impeller turned by the water |
| MID accuracy class | Class 2 | Class 3 |
| Minimum flow qi (qp 2.5) | 25 l/h (SonoSafe 10, Calmet PM02, Digitermo DTL) or 50 l/h (EW700, Integral UK-PLUS) | 50 or 100 l/h (ZENNER data sheet) |
| Mounting position | Any position on most models | Horizontal, tilted 90° or vertical |
| Heating / cooling | Heating only or heating + cooling versions | Heating |
| Communication | Wired M-Bus as standard; wM-Bus, LoRaWAN, NB-IoT depending on model | M-Bus or wM-Bus, 3 pulse inputs |
| Battery (manufacturer statement) | 10 years; 16+1 years for SonoSafe 10 | At least 7 years (10 years with long-life option) |
| Best for | New projects, long service life, accurate low-flow metering | Like-for-like replacement of an existing mechanical meter |
DN20 ultrasonic models: which one for which job?
| Model | Use | Pipe | qi | Communication | Battery |
|---|---|---|---|---|---|
| Danfoss SonoSafe 10 | Heating | Return | 25 l/h | Built-in M-Bus; retrofit wM-Bus (OMS) module | 16+1 years |
| Minol Zenner C5 IUF | Heating + cooling | Return | — | M-Bus or wM-Bus, 3 pulse inputs | At least 7 years (10-year option) |
| Honeywell EW700 | Heating (cooling in a separate register) | Return | 50 l/h | M-Bus + IrDA | 10 years |
| Calmet PM02 | Heating + cooling | Supply or return | 25 l/h | M-Bus; wM-Bus, LoRaWAN, pulse versions | 10 years |
| Integral UK-PLUS | Heating + cooling | Return | 50 l/h | M-Bus | — |
| Novus Digitermo DTL | Heating + cooling | Return | 25 l/h | M-Bus | — |
| CEM CM-HR | Heating, cooling or both | Return | — | M-Bus + optical; optional wM-Bus/LoRaWAN, NB-IoT | 10 years |
Installation details differ by model: the SonoSafe 10 can be mounted in any position and needs no straight pipe before or after it; the Minol Zenner C5 IUF suits any position, including upside down; the Honeywell EW700 installation instructions require at least 10×DN of straight pipe before and 5×DN after the meter. All DN20 models are listed side by side in the DN20 ultrasonic heat meters category.
When does a mechanical heat meter make sense?
The Minol Zenner C5 ISF DN20 mechanical heat meter has the same body length and connection as the ultrasonic models of the same size. That makes it suitable for renewing an existing mechanical meter without changing the pipework while moving to remote reading over M-Bus or wM-Bus. Know its limits too: it is accuracy class 3, its minimum measurable flow is higher than that of ultrasonic models, and its moving impeller is more affected by scale and sediment than an ultrasonic meter, so a strainer should be fitted upstream. It measures heating only. Options are in the DN20 mechanical heat meters category.
Communication, battery and verification
- Communication: wired M-Bus needs a reading cable in the building; where cabling is not possible, wM-Bus (RF) models are used. In a building that is already read remotely, the new meter's communication type must match the existing system. Wireless options are in the wireless (RF) heat meters category.
- Battery: manufacturer statements range from at least 7 years to 16+1 years, depending on the model.
- Re-verification: in Turkey, heat meters are subject to periodic re-verification every 5 years.
- Pipe: most models on our site are for the return pipe; the Calmet PM02 can go in the supply or the return. See our heat meter installation guide for the steps.
Selection checklist
- Does the design flow fit the 2.5 m³/h nominal flow of a DN20 meter? If it is higher, look at DN25–DN40.
- Do you need heating only, or heating and cooling?
- Does the existing meter position accept a 130 mm body and a 1″ connection?
- Is the building's reading system M-Bus or wM-Bus?
- Do the installation pipe (supply/return) and straight-pipe requirements match the model's instructions?
All sizes and brands are in our heat meters category. The current price is shown on the product card, with the VAT-inclusive price first and the VAT-exclusive price below it.
Frequently Asked Questions
Does an ultrasonic heat meter measure more accurately than a mechanical one?
The DN20 ultrasonic models on our site are MID accuracy class 2, while the mechanical C5 ISF is class 3. Ultrasonic models also start measuring at a lower flow: qi is 25–50 l/h at the same nominal flow, against 50–100 l/h for the mechanical meter.
Can an ultrasonic meter replace a mechanical one?
Yes, if the body length (130 mm) and connection thread (1″) are the same. In remotely read buildings, check that the new meter's communication type is compatible with the existing system.
Should I choose DN20 or DN25?
Size is set by design flow, not pipe size. For most apartments the 2.5 m³/h nominal flow of a DN20 meter is enough; for higher-flow apartments, shops and villas, look at DN25–DN40 models.
How long does a heat meter battery last?
It depends on the model: 10 years is stated for the Honeywell EW700, Calmet PM02 and CEM CM-HR, 16+1 years for the Danfoss SonoSafe 10, and at least 7 years (10 years with the long-life option) for the Minol Zenner C5 range.
Which model if cooling must be metered too?
Choose a model that keeps heating and cooling in separate registers, such as the Minol Zenner C5 IUF, Calmet PM02, Integral UK-PLUS or CEM CM-HR. For the C5 IUF, a separate national type approval is stated for cooling measurement.
Is a heat meter installed in the supply or the return pipe?
The model's instructions decide. Most DN20 models on our site are for the return pipe; the Calmet PM02 can be installed in either. A meter installed in the wrong pipe will measure incorrectly.
Prepared by the Novus Mühendislik ve Enerji Sistemleri technical team · Updated: November 2026