Heat Meter Overview and Diagnosis

A heat meter, or thermal energy meter, measures the amount of energy consumed in Wh. It is made up of a flow measurement device, at least two temperature sensors and a calculator.

How a heat meter works

The flow tube measures how much water passes through, the two temperature sensors measure the difference between the flow and return pipes (delta T), and the calculator turns the two into energy. Most meters on the Sycous range measure flow with ultrasound, sending pulses with and against the water and using the time difference to find its speed. The reading is shown on the display and can be sent out over MBus, wireless MBus or LoRaWAN so that it can be read remotely. For the full detail, read How Heat Meters Work. For what the codes mean, read Heat Meter Error Codes and Common Faults.

The parts of a heat meter

• The flow tube. The flow measurement device, normally called the flow tube, is often installed in the pipe and measures the flow rate of water through the meter, using a mechanical, ultrasonic or other measurement technology.
• The temperature sensors. The two temperature sensors or thermocouples are installed in the flow line and return line, taking a measure of the temperature of each line.
• The calculator. The calculator or integrator then calculates the difference in temperature between the two lines, multiplied by the flow rates and a pre programmed coefficient value over a period of time to provide an energy reading. This calculator normally has a digital display which will then display this energy reading in kWh.

Installing a heat meter

Important: heat meters must be installed to manufacturer requirements. Check the minimum straight length of pipe work required before the flow tube to avoid erroneous flow readings.

Temperature probes should be installed to correctly measure all the flow and return lines, as well as being in a standardised location, such as both in a pocket or both in a direct sensor adaptor.

Diagnosis and quick approaches

Use this guide when a heat meter shows an error, shows no energy, or cannot be read. Work through the sections in order. For how the parts work, read How Heat Meters Work, and for the full code lists see Heat Meter Error Codes and Common Faults. If the meter reads on the display but is not read by the data logger, also use Wired MBus Overview and Diagnosis.

Not sure which guide you need? Start at Meters Not Reading or Poor Signal: Where to Start.

Before you start: collect the facts

• Meter manufacturer, model and serial number.
• Any error code or symbol on the display, with a photo.
• The current energy, volume, flow rate and flow and return temperatures from the display.
• Whether the heating is running and whether other meters on the same system are reading.
• The date the problem began and anything that changed: system drain down, boiler work, a new HIU or a battery swap.

Warning: Heat meters sit on live pipework that can be hot and under pressure. Isolate and make safe before you touch a sensor, and use a competent engineer for any work on pipework or electrical supplies.

1. The display shows an error code

1. Write down the code and take a photo of the display.
2. Check the code in Zenner C5 Error Codes, the Sharky 775 Error Codes, the Sharky 774 Error Codes, or the manual for the exact model. Codes are not the same between manufacturers.
3. Use the table below to find the likely cause, then follow the section for that cause.
4. Clear the error only once the cause is fixed. Some codes clear themselves and some need a button press.

2. No flow reading, or a flow sensor not filled with liquid

1. Check the system is full and at pressure and that the pump is running. The C5 shows 20 when there is no water in the measuring tube, which points to low operating pressure.
2. Bleed air from the pipework and the flow sensor until the flow reading is steady.
3. Check the flow sensor position. Avoid the top of a pipe where air collects. Many manuals recommend rotating the sensor about 45 degrees away from the top.
4. Check strainers and valves for blockage.
5. Watch the display for several minutes. If the error clears and the flow reading is stable, the fault was air.
6. If the error returns, suspect a trapped air pocket in the pipe layout or a failing transducer, and move to section 4.

3. Temperature sensor fault or a strange temperature reading

1. Compare the displayed flow and return temperatures with what you expect. A flow temperature far above any sensible value suggests an open circuit. A value near zero or below suggests a short.
2. Check each sensor is fully inserted into its pocket and that the retaining screw or seal is in place.
3. Check the cable for crushing, cuts, kinks and damaged connectors, especially where it passes behind pipes or through a cupboard door.
4. Check the sensors are on the right pipes. Flow and return swapped will give a small or negative temperature difference.
5. If you have the tools and the meter allows it, measure the sensor resistance with the meter isolated. A Pt1000 reads about 1000 ohms at 0 degrees C and rises by about 3.9 ohms for each degree. Pt500 reads half of those values.
6. Replace the sensor pair if one is open, shorted or out of line with the other. Sensor pairs are matched, so replace both unless the manual allows a single sensor. Matching temperature sensors and pockets are in the Accessories category.

Caution: Do not splice or extend temperature sensor cables unless the manual allows it. Changes to cable length or joints can alter the reading and break the approval.

4. Hardware error in the ultrasonic measurement

1. Power cycle the meter if the manual allows it, by removing and refitting the battery, and note whether the error returns.
2. Check the cable between the flow sensor and the calculator for damage or strain, particularly where it was wrapped around the pipe or trapped during fitting.
3. Check for water in the connection or around the transducer seal.
4. If the error returns after a power cycle and the cable looks sound, the transducer or its cable has failed. The flow sensor, or the whole meter, needs to be replaced under the manufacturer procedure.

5. Flat battery or blank display

1. Press the display button. Many battery meters switch the display off between uses.
2. Check the battery date or the battery code in the meter menu.
3. Check the calculator is not too hot. Many manuals recommend keeping it below about 45 degrees C for best battery life.
4. Replace the battery or the calculator using the manufacturer procedure. Some meters must not be opened because of the calibration seal.
5. Check the radio or MBus polling interval. Fast polling or a fast radio transmit interval shortens battery life.

6. Reverse flow or a negative reading

1. Check the arrow on the flow sensor body against the actual direction of flow in the pipe.
2. Check the calculator is set for the pipe it is fitted on, flow or return.
3. Check the temperature sensors are not swapped. The Sharky 775 reports this as E3.
4. Refit the sensor correctly. For an ultrasonic sensor, check that the seals are seated and the pipe is clean at the joint.

7. The meter cannot be read by the data logger

1. Check the meter is not in sleep mode and that the display shows readings. For a ZENNER C5 see Waking a Zenner C5 from Sleep Mode.
2. Check the MBus primary address and make sure no other meter on the same bus has the same address. For a ZENNER C5 see Changing Primary MBus Address on Zenner C5.
3. Check the baud rate matches the data logger.
4. Check the MBus voltage at the meter terminals. Use a qualified electrician. The expected bus voltage is 21 to 24V or 36 to 42V depending on the MBus master, such as an Elvaco CMeX10S to CMeX13S or a CMe gateway.
5. Check the wiring polarity and the connections for loose or corroded terminals.
6. See the wired MBus guide for the full bus checks.

8. Energy reads zero, but the heating is running

1. Check flow is registering. If not, go to section 2.
2. Check the temperature difference. If flow and return are almost the same, the meter may be correct to show little or no energy.
3. Check the heating control. A bypass, a stuck valve or a failed HIU may be sending hot water straight to return.
4. Check the meter start up limits in the datasheet. Flow below the minimum, or temperature difference below the cut off, is not counted.
5. If everything looks right and energy is still zero, contact the manufacturer or Sycous with the meter details.

Frequently asked questions

Why is my heat meter not counting energy?

The usual causes are air in the flow sensor, a temperature sensor fault, flow below the minimum start up level, a very low temperature difference, or a meter in a fault state that stops the calculation.

What does error 40 mean on a ZENNER C5?

It means there is air inside the medium. Vent the system and watch the display until the flow reading is steady. Error 20 means no water in the measuring tube, so check the operating pressure.

What does error E4 mean on a Sharky 775?

It is a hardware error in the ultrasonic measurement. The manual lists a defective transducer or a short circuit in it as the cause. E7 is the air in the measuring path code.

How do I check a heat meter temperature sensor?

Measure the resistance with the meter isolated and compare with the sensor type. A Pt1000 reads about 1000 ohms at 0 degrees C and about 1096 ohms at 25 degrees C.

How do I know if the flow sensor is fitted the right way round?

Check the arrow on the body points the same way as the water. Reverse flow or a falling volume register means it is fitted backwards.

Need a hand on site? Contact Sycous at info@sycous.com and have the details from the checklist above ready.