Use this guide to understand how water meters work, the types you will meet and what goes wrong with them. It finishes with one of the most common questions, why a meter can appear to run backwards.
Not sure which guide you need? Start at Meters Not Reading or Poor Signal: Where to Start.
What a water meter is
A water meter measures the volume of water that passes through a pipe, in cubic metres, so that each property can be billed for what it uses. In a block of flats each property usually has a cold (potable) water meter and, where there is a communal hot water system, a hot water meter too. Meters can be read from the display, or remotely through a pulse output, MBus, wireless MBus or LoRaWAN.
How water meters work
| Type | How it measures | Common use |
|---|---|---|
| Single jet and multi jet | Water hits a small wheel, and the wheel speed follows the flow. The wheel turns a register, or a magnet that gives a pulse. | Individual properties |
| Turbine (Woltman) | A larger turbine in line with the pipe turns at a speed that follows the flow. | Larger pipes and communal supplies |
| Ultrasonic | Sound pulses travel with and against the flow. The time difference gives the speed of the water. There are no moving parts. | Properties and larger supplies, hot and cold |
| Electromagnetic | A magnetic field is applied to the water and the voltage generated gives the flow. The water must conduct electricity. | Large pipes and supplies |
Mechanical and turbine meters
- Simple, proven and widely used, but they have moving parts that can wear or jam.
- Debris, scale and sand in the supply can slow or stick the wheel, so the meter under reads or stops.
- They can spin on air, so air in the pipe after a drain down can make the reading jump.
- Turbine meters can be turned by reverse flow, which is why they can run backwards.
- They need a minimum flow to start, so very small flows such as a dripping tap may not be counted.
Ultrasonic meters
- No moving parts, so less wear, and they can measure small flows well.
- They need the pipe to be full. Air bubbles or an empty pipe give a missing or wrong reading.
- Deposits on the sensors can weaken the signal. Many meters show an error code for this.
- They are battery powered, so a flat battery gives a blank display.
- They can show reverse flow as a negative value or warning rather than spinning backwards.
Reading the meter remotely
- Pulse output: each pulse counts a set volume, set by the pulse rate printed on the meter. If the platform uses the wrong rate the reading is wrong by the same factor.
- MBus, wireless MBus and LoRaWAN: the meter or a module sends the reading in a telegram or message. See Wired MBus Overview and Diagnosis, Wireless MBus Overview and Diagnosis and LoRaWAN Overview and Diagnosis.
Before you start: collect the facts
- Whether it is a cold or hot water meter, and its type.
- What the meter display shows compared with the platform.
- Whether the problem is a stopped reading, a reading that is too high or low, a reading that goes down or no reading at all.
- Whether there has been plumbing work, a drain down or a change in pressure.
Good to know: Always compare the meter display with the platform first. If they match, the meter and the communications are fine and the question is about the water use. If they do not match, the fault is in the communications or set up.
Where issues usually lie
| What you see | Likely cause |
|---|---|
| Reading does not move though water is used | Isolation valve closed, strainer blocked, jammed turbine, flat battery on an ultrasonic meter, or an empty or air filled pipe. |
| Reading too high | Air passing through a mechanical meter, a leak after the meter, or a wrong pulse rate or multiplier on the platform. |
| Reading too low | A meter too big for the flow, worn or jammed turbine, a meter fitted the wrong way round, or reverse flow subtracting from the total. |
| Reading goes down | Reverse flow. See the section below. |
| Water use at night when no one is using water | A leak, a dripping tap or a running toilet after the meter. |
| No readings arriving | A communication fault. Check the pulse cable or module, its battery and the signal. |
1. The meter has stopped or does not move
- Run a tap and watch the meter. A mechanical meter should turn, and an ultrasonic meter should show flow.
- Check the isolation valves either side are fully open.
- Check the strainer is not blocked with debris.
- Check the arrow on the meter matches the direction of flow.
- For an ultrasonic meter, check the display for a battery or error message and for air in the pipe.
- Replace the meter if a mechanical meter is jammed or worn.
2. The reading is too high or too low
- Compare the meter display with the platform. If they differ, check the pulse rate or multiplier set on the platform.
- Turn off all taps and appliances and watch the meter. If it still moves, there is a leak or a running toilet after the meter.
- Check for air after any drain down. Run the tap until the water runs clear.
- Check the meter is the right size for the flow. A meter that is too big may miss small flows.
- Check the meter is fitted in the right direction and is allocated to the right property.
3. The reading goes down
- Check the arrow on the meter and the direction of flow, and look for a reverse flow warning on ultrasonic meters.
- Check for a missing, stuck or leaking check valve, also called a non return valve, on the hot water line.
- Check whether the reading only goes down when no one is using water. That points to the causes below, such as pipes cooling or pressure changes, rather than a meter fitted backwards.
- Read the section below for the causes and fixes.
4. No readings are arriving
- Check the meter display is working and counting.
- For pulse meters, check the pulse cable and the device that counts the pulses.
- For MBus, wireless MBus and LoRaWAN, follow the matching guide linked above.
5. When to call for support
Call for support if the meter display and the platform match, the meter is moving and the consumption still looks wrong, or if you cannot find the cause of a reverse flow. Have the meter type, the readings and what you have checked ready.
Need a hand on site? Contact the Sycous Project Lead, or email info@sycous.com, and have the details from the checklist above ready.
Frequently asked questions
What is the difference between a turbine and an ultrasonic water meter?
A turbine meter has a wheel that turns with the water, so it has moving parts that can wear or jam. An ultrasonic meter measures the water with sound and has no moving parts, but it needs a full pipe and a battery.
Why does my meter read when no tap is running?
Water is going somewhere. Check for a leaking tap, a running toilet or a leaking pipe. A mechanical meter can also spin on air or reverse flow, and an ultrasonic meter can show a false reading with air bubbles.
Why can air make a reading too high?
A mechanical meter cannot tell air from water. After a drain down, air rushing through the pipe turns the wheel and adds to the reading. Run the water until it is clear.
Can a hot water meter be used on a cold line and the other way round?
No. Hot water meters are made for higher temperatures, and cold water meters should never be fitted on a hot line.
What is a pulse meter?
A pulse meter gives a small electrical pulse for each fixed volume of water. A counter adds up the pulses. The pulse rate printed on the meter has to match the rate set on the platform.
Why a water meter can run backwards
There are situations where a hot water meter can clock in reverse. This guide explains what can cause it, what it does to the readings and billing, and what can be done to fix it.
Why a hot water meter can run backwards
- The water meter is installed in reverse. This is common for normal turbine meters.
- Thermal contraction of cooling hot water pipes can pull water backward and slowly spin the meter in reverse.
- Natural convection can create a small circulating loop that moves water backward when the system is not flowing.
- A missing or leaking check valve can allow reverse flow through the hot water line. This is also known as a non return valve.
- Pressure fluctuations in the plumbing, such as cold water use elsewhere, can temporarily push water backward.
- A meter without anti reverse protection may register even tiny back and forth pipe movements as reverse rotation.
Good to know: if the meter clocks upwards but then slowly goes backwards, one of the causes after the first is more likely than a meter installed in reverse.
What does this mean for billing?
When a turbine hot water meter spins backward, even slowly, it effectively subtracts from the total recorded usage, meaning the resident is billed for less water than they actually consume. Over time, this can result in undercharging, which not only affects revenue for the building or utility but also skews consumption data used for managing utilities or detecting leaks.
What can fix this issue?
It is always recommended to fix the problem on the hot water supplying system itself where possible. That is not always practical, so here are a few ways to fix a hot water meter going backwards.
- Install or replace a check valve on the hot water outlet or recirculation line to stop reverse flow. This is also known as a non return valve.
- Add a thermal expansion tank to absorb pressure changes that cause backflow.
- Insulate the hot water pipes to reduce cooling and prevent reverse convection currents.
- Install a heat trap (or heat trap nipples) on the water heater to block thermosiphon flow.
- Replace the meter with one that has anti reverse features if accurate metering is required even with tiny flow fluctuations.