Work out the heat a circuit carries from its flow rate and the flow and return temperatures, to check a heat meter reading.
Heat delivered
227.6kW
Check the values to see a result.
Compare with the heat meter over the same period. A billing heat meter uses its own approved sensors and fluid model.
Q = V̇ × ρ ÷ 3600 × cp × (Tflow − Treturn)
10 m³/h with 20 K between flow and return carries 227.6 kW.
Tip: A 1 K error in ΔT is 5% of a 20 K difference. Matched temperature sensors matter more than the flow meter on a low-ΔT circuit.
A heat meter multiplies the flow through a circuit by the temperature difference between flow and return, and by how much heat each cubic metre of the fluid carries per degree. Doing the same sum from independent readings is the quickest check that a meter is reading sensibly: 10 m³/h of water at 70 °C flow and 50 °C return carries 227.6 kW.
ṁ = V̇ × ρ ÷ 3600
V̇ is the volume flow in m³/h and ρ the fluid’s density in kg/m³; dividing by 3,600 gives kilograms per second. Water at 70 °C is 977.8 kg/m³, about 2% lighter than at 20 °C.
Q = ṁ × cp × (Tflow − Treturn)
cp is the specific heat in kJ/(kg·K): about 4.18 for water across the heating range. With mass flow in kg/s the result is kW. When the return is warmer than the flow, the circuit is taking heat up, as a chilled water circuit does, and the calculator reports it as cooling.
E = Q × hours
A meter’s energy register is this power integrated over time. To check it, compare the change in the register over a period with the average power over the same period multiplied by its length.
For water, ρ × cp ÷ 3600 is close to 1.16 kWh per m³ per kelvin at room temperature and 1.14 at 70 °C, which gives a mental check: flow in m³/h times ΔT in kelvin, times 1.14, is kW. Glycol mixtures carry less: in Dow’s data for its inhibited propylene glycol at 30% by volume, ρ × cp is about 6% below water’s at 10 °C.
10 m³/h of water at 977.8 kg/m³ is 2.716 kg/s. Times 4.19 kJ/(kg·K) and a 20 K difference, it carries 227.6 kW. A heat meter on this circuit should add about 228 kWh to its register each hour.
Heat delivered 227.6 kW Open in the calculator
At 2.5 m³/h and a 30 K difference the substation takes 85.35 kW. District heating networks aim for a large difference, because the same pipe then carries more heat for the same pumping energy.
Heat delivered 85.35 kW Open in the calculator
A 30% propylene glycol circuit at 20 m³/h, 6 °C flow and 12 °C return, with Dow’s figures at 10 °C of 1,043.8 kg/m³ and 3.757 kJ/(kg·K), takes up 130.7 kW of heat. The same flow of plain water would read 139.7 kW, so a meter set up for water over-reads by about 7%.
Heat taken up (cooling) 130.7 kW Open in the calculator
Heat in kW for water at 70 °C (977.8 kg/m³, 4.19 kJ/(kg·K)) for each volume flow and flow-to-return difference.
| Flow (m³/h) | ΔT 5 K (kW) | ΔT 10 K (kW) | ΔT 20 K (kW) | ΔT 30 K (kW) |
|---|---|---|---|---|
| 0.5 | 2.845 | 5.69 | 11.38 | 17.07 |
| 1 | 5.69 | 11.38 | 22.76 | 34.14 |
| 2 | 11.38 | 22.76 | 45.52 | 68.28 |
| 5 | 28.45 | 56.9 | 113.8 | 170.7 |
| 10 | 56.9 | 113.8 | 227.6 | 341.4 |
| 20 | 113.8 | 227.6 | 455.2 | 682.8 |
| 50 | 284.5 | 569 | 1,138 | 1,707 |
| 100 | 569 | 1,138 | 2,276 | 3,414 |
Most often because the readings are not from the same moment or the same circuit, or because the temperature sensors are not matched. On a 20 K circuit a 1 K error is 5% of the result; on a 5 K circuit it is 20%. Heat meters use a matched sensor pair for this reason.
Yes. A meter calibrated for water applies water’s density and specific heat. On a glycol circuit it over-reads unless it is set up for that mixture, so check the meter’s fluid setting before trusting its register.
Often that too much water is flowing for the load, a problem called low delta-T. It wastes pumping energy and, on district heating and heat pumps, lowers efficiency. It also makes metering less accurate, which is why the calculator flags a difference under 3 K.
Many heat meters have an M-Bus port. A wired M-Bus master reads many meters on one cable, or a ZHM reads one meter and reports it wirelessly. See wired M-Bus commissioning for heat meters.
A ZHM reads a heat meter’s M-Bus port and reports its readings wirelessly, so this check can run on live data.
