A heat meter measures how much heat a building or flat has taken from a heating system. It is the basis of heat billing, so accuracy matters. This article explains what is inside a heat meter, how it calculates energy and which meters Sycous supplies. If a meter is faulty now, go to Quick Approach to Heat Meter Fault Finding and Heat Meter Error Codes and Common Faults. For the basics in a few lines, see What is a Heat Meter?.
What a heat meter is made of
| Part | What it does |
|---|---|
| Flow sensor | Measures how much water passes. Most modern meters are ultrasonic: they send sound pulses with and against the flow and use the tiny time difference to calculate flow speed. Older meters use a mechanical impeller. |
| Temperature sensor pair | Two matched sensors, one on the flow pipe and one on the return pipe. They are usually Pt500 or Pt1000 platinum sensors whose resistance changes with temperature. |
| Calculator (integrator) | Combines volume and temperature difference into energy, keeps the totals, shows them on the display and passes them to MBus, wireless MBus or pulse outputs. |
| Power | A lithium battery, mains, or power taken from the MBus line. Battery meters switch their display and radio off to save power. |
How the energy figure is worked out
Energy is calculated from the volume of water that has passed, multiplied by the difference between the flow and return temperatures, and by a factor that accounts for the heat held by water at that temperature. The meter totals this continuously.
Energy is roughly volume multiplied by temperature difference multiplied by the heat coefficient of the water.
This is why a heat meter needs three things to be right at the same time: the volume, the flow temperature and the return temperature. A problem with any one of them changes the energy reading.
| Quantity | Typical unit | What it tells you |
|---|---|---|
| Volume | cubic metres (m3) | Total water that has passed through the meter. |
| Flow rate | m3 per hour | How fast water is moving right now. |
| Power | kW | How quickly heat is being taken right now. |
| Flow and return temperature | degrees C | What the system is delivering and returning. |
| Temperature difference (delta T) | K | The heart of the energy figure. A low delta T means little heat is being used. |
| Energy | kWh or MWh | The billing figure. |
Heat meters on the Sycous range
| Meter | Notes for fault finding | Where to look |
|---|---|---|
| ZENNER C5 IUF heat and cooling meter | Ultrasonic. Powered from the MBus line or battery. Numeric error codes that add together. Can be woken from sleep mode and have its MBus address changed. LoRaWAN and wireless MBus options. | Zenner C5 IUF |
| Diehl Sharky 775 | Ultrasonic. E code errors, with an error memory for faults lasting more than 6 minutes. Leak detection codes. | Sharky 775 |
| Diehl Sharky 774 | Ultrasonic. Self diagnostics with error codes shown in the main display loop. | Sharky 774 |
| Axioma Qalcosonic E4 | Ultrasonic heat and cooling meter. Includes a guide for programming the mounting position. | Qalcosonic E4 |
| B Meters Hydrosonis ULC | Ultrasonic heat and cooling meter. | Hydrosonis ULC |
| ZENNER VMT IUF flow sensor with WR3 energy calculator | Separate ultrasonic flow sensor and calculator. | Zenner VMT IUF and Zenner WR3 sections in Meter Hardware |
| Micronics Ultraflo U1000 clamp on meters | Clamp on ultrasonic meter that is fitted to the outside of the pipe. Check the mounting and pipe condition. | Micronics U1000 sections in Meter Hardware |
Good to know: Temperature sensors, sensor pockets, ball valves and fitting kits for these meters are in the Accessories category, so a replacement sensor can be matched to the meter.
Cooling and combined heat and cooling meters
A heat and cooling meter must handle a flow that is colder than the return. Check the meter is set for the right mode, and that cables are protected from condensation on cold pipework. A meter set for heating only will read zero in cooling mode.
Frequently asked questions
How does an ultrasonic heat meter measure flow?
It sends ultrasonic pulses along the water in both directions. The pulse travelling with the flow arrives slightly sooner than the one travelling against it. The difference gives the flow speed.
What is delta T?
The difference between the flow and return temperatures. It is the amount of heat the water has given up in the system.
What temperature sensors do heat meters use?
Matched pairs of Pt500 or Pt1000 platinum resistance sensors. The resistance rises predictably with temperature. Check the datasheet for the type your meter uses.
Where can I read more about heat meters?
The Heat Meters section has a short guide, What is a Heat Meter, and the error code reference and the fault finding guide.
Need a hand on site? Contact Sycous at info@sycous.com and have the details from the checklist above ready.