Water Meters: Cold, Hot and Woltman Models

A water meter (water metre in British English) is a measuring instrument that measures and totalizes the volume of water flowing through it, in cubic meters (m³). This category features Klepsan, CEM, Minol Zenner and Polimeter meters in three groups: threaded DN15–DN50 cold water meters and hot water meters for apartments and businesses, and DN50–DN250 Woltman (flanged) water meters for mains, industry and irrigation.

Reading apartment meters and allocating costs in buildings with central systems is part of our everyday work, so you can source meters with remote reading modules and the matching meter reading equipment from a single supplier.

How Does a Water Meter Measure?

The meters in this category are mechanical: the flowing water turns an impeller or turbine, and the rotation is converted into volume and passed on to the number rollers of the register. In a single-jet meter, the water strikes the impeller through one channel; in a multi-jet meter, through several. In a dry-dial meter, the register does not come into contact with the water, as the movement is transmitted to it magnetically. In a Woltman meter, the water drives a turbine aligned with the direction of flow. Ultrasonic meters, by contrast, have no moving parts: they calculate the flow rate from the difference between the transit times of sound waves traveling with and against the flow.

Types of Water Meters

What distinguishes cold from hot water meters is the water temperature they can withstand (for Klepsan, up to 50 °C for cold and up to 90 °C for hot water meters), so a cold water meter must not be installed on a hot water line.

Choosing the Size: DN, Inches and Body Length

Meter size is stated as DN (nominal diameter) and corresponds to the pipe size; the external thread on the meter body, however, is one size larger than the pipe, and the meter is connected to the pipe with unions. For example, a DN20 meter has a G1 thread on its body, and its unions connect to a 3/4” pipe. The second dimension to check when replacing a meter is the body length (L): the length of the meter without its unions must match the existing gap.

The size is determined by the flow rate through the meter, not by the pipe size. The permanent flow rate (Q3) is the highest flow rate at which the meter operates within its maximum permissible errors in normal use; short peaks must not exceed the overload flow rate (Q4 = 1.25 × Q3). For meters on a utility account, the flow, temperature and pressure ratings are specified by the water utility that installs the meter (MID, MI-001).

Meter Markings: Q3, R, T30/T90 and M

• Q3: permanent flow rate (m³/h). A meter marked “Q3 2.5” measures within its maximum permissible errors at up to 2.5 m³ per hour in normal use.
• R (e.g., R160): the ratio of Q3 to the minimum flow rate (Q1); the higher the R value, the smaller the flows the meter can measure. For a Q3 2.5 R160 meter, Q1 = 2.5 / 160 ≈ 15.6 L/h and the transitional flow rate Q2 = 1.6 × Q1 = 25 L/h. R can depend on the installation position: Zenner ETKD/ETWD meters are rated R80 when installed horizontally and R40 when installed vertically.
• T30, T50, T90, T30/90: temperature class. T30 is for water at 0.1–30 °C, T90 for 0.1–90 °C and T30/90 for 30–90 °C (OIML R 49).
• CE and M marking: shows conformity with the MID (2014/32/EU). “M24” inside a rectangle means the marking was affixed in 2024; the four-digit number next to it identifies the notified body.

The maximum permissible errors also depend on these flow rates: ±2% between Q2 and Q4 (±3% for water above 30 °C under the MID) and ±5% between Q1 and Q2.

Remote Reading: Pulse, M-Bus and wM-Bus

A mechanical meter is read remotely by means of a module fitted to its register. A pulse output generates a signal each time a set volume passes (e.g., every liter); M-Bus transmits the reading over a wired bus (EN 13757-2/-3) and wM-Bus by radio (EN 13757-4). The options depend on the brand: on our site, Zenner is offered with a 1 L pulse, M-Bus or wM-Bus module and Polimeter PHW with an M-Bus module; for CEM KT13, the manufacturer supplies pulse, M-Bus, wM-Bus/LoRaWAN and NB-IoT modules, and some Klepsan models have an optional pulse output. Readings can be collected with meter reading equipment.

Re-verification: How Often Are Water Meters Replaced?

Periodic re-verification of water meters is regulated nationally, so intervals and responsibilities differ from country to country; check your local regulations for periodic re-verification requirements. As an example, in Türkiye the Regulation on the Inspection of Electricity, Water and Gas Meters sets a re-verification period of 10 years for water meters, counting the year of the verification stamp as the first year.

Hot water meters may be treated differently: the Turkish regulation, for instance, does not cover meters that measure water in hot water lines. Requirements vary by country, so check the rules that apply to your installation before choosing hot water meters.

Separate Meters for Each Unit

In buildings supplied through a single meter, each dwelling unit can have its own meter and its own utility account; our Water Meter Separation and Individual Accounts page describes this process for buildings in Istanbul. Because the local water utility specifies the meters that may be used on utility accounts, check its current requirements before you buy.

Frequently Asked Questions

How often should a water meter be replaced?

It depends on where the meter is installed: periodic re-verification intervals for water meters are set by national rules, and hot water meters are sometimes treated differently from cold water meters. Check your local regulations for periodic re-verification requirements.

What is the difference between a cold water meter and a hot water meter?

The key difference is the water temperature the meter can withstand: Klepsan cold water meters are rated up to 50 °C and hot water meters up to 90 °C; Zenner's cold water model ETKD covers 0.1–50 °C and its hot water model ETWD 0.1–90 °C.

What size is a DN20 water meter in inches?

DN20 corresponds to a 3/4-inch pipe size. The meter body has a G1 thread, and the meter connects to a 3/4” pipe with unions. The DN20 models on our site have a body length of 110, 130 or 190 mm.

What do Q3 2.5 and R160 mean on a water meter?

Q3 (permanent flow rate) is the highest flow rate at which the meter operates within its maximum permissible errors in normal use: 2.5 m³/h. R160 is the ratio of Q3 to the minimum flow rate (Q1), so the meter measures flows down to about 15.6 liters per hour within its maximum permissible errors.

What does the M24 marking on a meter mean?

It is the supplementary metrology marking next to the CE mark on MID-compliant meters: a capital letter M followed by the last two digits of the year in which the marking was affixed. M24 means 2024.