Managing Heat Meter Complexity: A Fundamental Guide

Greentherm Managing Director ENDA RUXTON demystifies heat meter selection, exploring essential components—from ultrasonic flow meters to LoRaWAN communications. This guide outlines critical sizing parameters, MID certifications, and installation best practices to ensure accuracy and longevity in energy efficiency applications.

Heat meters explained

Heat meters provide a valuable tool in the energy efficiency armoury, but they’re also widely misunderstood in terms of selection and application. To get an understanding of the fundamentals, we need to view a meter under the following four elements;

1. Flow meter
2. Heat Calculator
3. Temperature sensors
4. Optional communications interface

Certifications

Thermal meters sold within the European Union must have CE certification and should comply with EN 1434 parts 1-6 2022. When utilising a thermal energy meter for energy billing purposes, it should comply with the Measuring Instruments Directive (MID), a European Union directive that aims to ensure that measuring instruments used for legal purposes meet certain accuracy and reliability standards. Typically, MID class 2 meters are deemed sufficient for energy billing purposes.

There are two fundamental types of flowmeters: “clamp-on”, which are non-invasive in nature and are typically used for temporary installations; and “in-line”, which are mechanically installed in the piping circuit for permanent use. For the purposes of this article, I will focus on the in-line variety.

Flow meters

There are four common varieties of in-line flow meters.

– Single Jet:  Suitable for low flow applications. Fluid enters through one inlet, causing a single rotor to rotate, indicating flow.

– Multijet: Utilised for moderate flow applications. Multiple jets distribute the flow, causing a turbine to rotate; it is more sensitive to low flow rates.

– Woltmann:  Used for large flow applications. The fluid flow rotates a turbine housed in a chamber, using gears to translate speed into flow rate.

– Ultrasonic:  Applicable for small to large flow applications. Uses ultrasound travel-time measurement to derive flow rate with no moving parts.

Flow meters can be manufactured from brass, steel, or, in some cases, polymer materials and are generally suitable for temperature ranges of around 5C-150C ( 41F- 302F) and operating pressures of up to 16 bar.

Thermal calculator

This is an electronic device that comes in two varieties. The first are calculators factory-fitted to the flow meter. These calculators are often detachable from the flow meter and can be wall-mounted or similar in relative proximity to the flow meter. They are factory calibrated as a finished product with the flow meter and come with one temperature sensor embedded in the meter and a pre-wired sensor intended to be ‘wet-installed’ in the opposing circuit pipe.

The second variety are flow meters with a separate calculator and temperature sensors. These need to be wired on site during installation, but have the same operating principle as the pre-wired version.

Calculators can be configured for either heating or cooling mode.

There are three specific values of relevance that are important when selecting a heat meter:

– Qp: This is the nominal or standard flow rate where the meter performs reliably and accurately over time, often used for general designation and testing.

– Qi: This is the lowest flow at which the meter can measure within its specified accuracy.

– Qs: This is the highest flow rate the meter can handle continuously and accurately.

When sizing a meter, it is important to be mindful of the above parameters as these define the operating limits for accurate measurement.

Temperature sensors

Temperature sensors are available in a range of diameters, typically 5, 5.2 and 6mm. They are mostly the PT1000 variety with a normal length of 1.5 metres, but available up to around 3 or 6 metres, generally as a special order. This is an important detail when dealing with a detachable calculator, which is prewired from the factory as a calibrated item, as this limits the distance between the flow meter and the calculator.

Communication

There are generally various communication options available for heat meters. These can be divided into “wired” and “wireless” options.

The most basic wired communication method is a pulse output. This uses a volt-free dry relay contact that closes every time a set amount of energy is measured by the heat meter. This value is factor-yset in the heat meter calculator, so one pulse may correspond to one kilowatt-hour (kWh) or a multiple thereof.

Meter-bus, also known as M-Bus, is a basic two-wire digital communication protocol with a low data transmission rate (baud rate). It falls under the EN 13757 standard, which should but doesn’t always guarantee interoperability between hardware devices.

M-Bus can, in theory, have up to 250 node devices and be up to 1km in length, although the distance can limit the number of nodes and the transmission speed.

Modbus is a wired open-protocol communication method normally using an RS-485 interface. The protocol can have up to 247 communication nodes, such as heat meters and can have a maximum network length of about 1.2 km.

Both M-Bus and Modbus can provide a digital telegram containing readings for Kilowatts (kW), Kilowatt- Hours (kWh), flow rate, and flow and return temperatures. This data is quite valuable to provide diagnostics to control systems that can help to identify system inefficiencies and fault analysis around Low Pressure Hot Water (LPHW) circuits or heat sources, such as heat pump alarms. It is also possible to get BACnet communication modules for some meters.

The most common non-subscription, unlicensed wireless communication options are Wireless M-bus and LoRaWAN.

Wireless M-bus operates on the 868 MHz frequency, and, as the name suggests, is the wireless version of wired M-bus. It is a low data, low power, shorter range technology capable of transmission up to 100 metres in urban areas, but this is greatly reduced inside buildings. It is common to use repeater devices to ‘bounce’ the signal onward until it reaches a gateway device that brings it to the cloud. Alternatively, these devices can be read using a handheld device in ‘walk by’ mode, such as the hallway outside apartments.

LoRaWAN is a newer, reliable technology that operates on the 868 MHz frequency and has a range of around 2-5 km in urban areas, reducing to hundreds of metres indoors. It is also low-data, low-power technology, with less reliance on repeaters to get data to the cloud. This results in much less cost risk around the rollout of metering inside buildings, making it a more attractive option.

Installation considerations

Pipe sizing: Pipe diameter is often misunderstood in terms of heat meter selection. Sizing a meter solely by pipe diameter can often lead to poor accuracy and frequent oversizing, adding cost. It is important to examine the maximum flow rate and meter pressure drop, and to ensure that these are the two key parameters steering selection.

Pipe positioning: Meters configured for heating are typically factory-configured for installation on the return line of an LPHW circuit, because this element of the circuit is cooler, which maximises the operating life of the meter. It is possible to configure a meter for flow pipe installation, but this is usually only allowable as a one-time event, ie, once selected, it cannot be reversed.

Different types of flow meters have different pipe length requirements, sometimes on one side and other times on both sides of the meter. These requirements are outlined by manufacturers, and, generally, ultrasonic meters require less linear length as opposed to certain mechanical varieties

As a general rule, one temperature sensor is factory-installed in the flow meter as a wet sensor, ie, in direct contact with the liquid, and the second sensor is inserted into the opposing pipe work. One frequent installation mistake is when the second temperature sensor is inserted into a dry pocket that is welded or threaded into pipework. This results in one wet and a ‘dry’ reading, respectively, leading to an inaccurate reading.

In order to solve this problem, it is possible to specify and install wet pocket valves or weldable threaded adapters, allowing temperature sensors to be directly tapped, measuring the liquid within the opposing pipe.

Cooling circuits

Using meters for cooling circuits can require careful selection, depending on the application, as not every meter is suitable for cooling. Where glycol is used for anti-freeze protection creates an additional challenge. Some meters can only be configured in 5% concentration increments, which can result in measurement inaccuracy; others offer glycol concentration measurement with automatic volumetric flow compensation. The latter also caters for variations resulting from automatic system pressurisation and glycol top-up during maintenance.

It is essential to note the Ingress Protection (IP Rating) of a heat meter device as well as the ambient operating temperature of the electronics. It would be expected that a heat meter would at least meet a minimum IP 65 rating that would withstand ingress to the level of protection shown below.

Battery life

Battery life is generally defined by the communication options and frequency of interrogation of stored measurements. The following is a guide to battery life expectancy across various scenarios.

– 20 years (without a communication module);

– 16 years (M-bus, readout interval one hour);

– 15 years (M-Bus, readout interval 10 minutes);

– 10 years (with others, eg, wM-bus, Modbus, LoraWAN)

 

In conclusion

While heat meters are indispensable for modern energy management, their effectiveness hinges on more than just pipe size. By understanding the interplay between flow measurement technology, calculator calibration, and accurate temperature sensing, engineers can avoid common pitfalls like dry sensor readings or oversizing.

Furthermore, selecting the right communication protocol—whether it’s the long-range reliability of LoRaWAN or the established standard of M-Bus—ensures that the data collected becomes a powerful diagnostic tool for system efficiency. Prioritising these technical fundamentals and adhering to MID standards will ultimately guarantee a reliable, long-term return on investment for any thermal energy project.