Grant R290 inbuilt monitoring vs MID meters

TrystanLea · 1 July 2026 17:00

Continuing on the theme of inbuilt vs MID metering. See also:

Here’s an interesting result from a Grant R290 unit I helped install back in February.

Performance using the Level 3 bundle: 4.33 so far: https://heatpumpmonitor.org/system/view?id=1088

Running SCOP as reported by the local Econet API is settling to ~7.4!

So why are we seeing such a difference?

Zooming in on a short run period around the 25th of March where conditions are fairly stable:

Axioma Heat meter:

Flow temperature: 36.02C
Return temperature: 32.91C
DT: 3.11K
Flow rate: 15.01 L/min
Heat output: 3232W
Assumed specific heat = 3232÷((15.01÷60)×3.11) = 4154 J/kg.K (expected result)

Grant R290 inbuilt sensors

Flow temperature: 34.80C
Return temperature: 31.82C
DT: 2.98K (~4% less)
Flow rate: 18.27 L/min (~22% more)
Heat output: 4387W (36% more)
Assumed specific heat = 4387÷((18.27÷60)×2.98) = 4834 J/kg.K (strangely high!?)

Electric power is much better only a 0.2% difference!

really remarkably good only 0.25% difference here over a longer period!

The Econet API reported a COP of 6.3 over the 5:20-6:20am period selected above, even though it’s heat output / electrical input measurements suggest: 4387÷746.7 = 5.9. Perhaps it’s doing a running average or something. The MID metering COP for the same period is 4.33.

Cross checking the % of ideal Carnot using the MyHeatpump app gives 54.8% for the MID metered performance, which is towards the higher end of the range we would expect (but within the expected / realistic range). A COP of 6.3 would be 80% of carnot which is clearly too high.

I think @glyn.hudson spotted that Grant may have removed the COP/SCOP metrics from their app and so they may well be well aware of this accuracy issue, but it’s an another interesting data point on the variability in the quality of built in metering - and as I’ve said before - I do want the quality of built in metering to improve as I think higher accuracy built in metering is the right way to scale heat pump monitoring longer term - even though that would reduce the need for our product

TrystanLea · 1 July 2026 17:35

@glyn.hudson’s 9 kW Grant R290 https://heatpumpmonitor.org/system/view?id=748

Axioma Heat meter:

Flow temperature: 29.65C
Return temperature: 27.5C
DT: 2.15K
Flow rate: 18.31 L/min
Heat output: 2697W
Assumed specific heat = 2697÷((18.31÷60)×2.15) = 4111 J/kg.K (expected result)

Grant R290 inbuilt sensors

Flow temperature: 29.22C
Return temperature: 26.73C
DT: 2.49K (~16% more)
Flow rate: 18.33 L/min (~0.1% more)
Heat output: 3809W (41% more)
Assumed specific heat = 3809÷((18.31÷60)×2.49) = 5013 J/kg.K (again very high..)

It’s interesting here that the flow rates are very similar but it’s now the DT and again the specific heat that’s off the mark.

On Glyn’s system the electrical measurement has also got a greater difference, e.g in the same window above SDM120 498W, grant in-built: 460W (7.6% less). Zooming out over a longer period of time the grant is reporting 9.4% less electric consumption in this window:

Here’s a comparative screenshot that Glyn took a while back that compared SCOP as reported on Econet with the SCOP/COP based on the MID meters.

Dave_Storey · 7 July 2026 09:40

Do Grant/econet measure the electrical power of the MMI / ethernet/wifi interface, given that is not fed from the heatpump itself?

glyn.hudson · 17 July 2026 00:19

No, but it only uses a few watts. It’s not really worth including. Unless any secondary pumps are installed, they should be included.

Sparkie · 17 July 2026 16:15

Unfortunately I don’t have an Open Energy Montior setup, but I can provide a data point for your survey.

My 9 kW Grant R290 was installed with an electricity meter attached, so it has been counting the kWh since installation. I have some semi-regular readings, about once per month.

I have also been recording the power consumption reported by the heat pump using Home Assistant, and integrating that to get a measure of daily energy.

I can compare the differences in the electricity meter readings against the HA data.

The agreement is better than I expected:

Electricity meter total: 1561 kWh

Integration of Grant’s reported power: 1528 kWh

Dates	Elec. meter	Grant’s own reporting
6/1 - 23/1	355	334
23/1 - 6/2	295	289
6/2 - 4/3	387	354
4/3 - 16/4	390	393
16/4 - 6/6	134	137

glyn.hudson · 19 July 2026 21:20

The onboard Grant electrical metering accuracy is pretty decent, it’s the on board heat metering is where the error is.

Sparkie · 9 October 2026 11:10

Another imperfect but hopefully useful test of the inbuilt Grant metering system, this time looking at flow and return temperature sensors.

Aim

I wanted to determine whether the flow and return values reported by a Grant 9kW HP290 were accurate, or at least internally consistent, since this could be affecting the calculated COP.

Method

I used the EcoNext Home Assistant integration to record the flow and return temperatures reported by the Grant unit.

I used a Sonoff Zigbee temperature probe to monitor the flow temperature on the radiator circuit. The probe was placed lengthways along the main flow pipe, underneath 13mm foam insulation which was sealed with tape.

When the Grant heat pump is on standby, the primary circulation pump and radiator circulation pump continue to run. This should lead to uniform water temperature throughout the primary pipework and radiator circuit. Two time-series were collected, one when the heat pump had been in standby for several hours, and one where the water was cooling down following a heating cycle.

Results

Reported below are the flow, return and circuit temperatures for the standby experiment, and also the calculated difference between flow and return:

Temperatures dropped gradually during the experiment, since the heating water was losing energy during the round trip to the outdoor unit. The radiator circuit temperature agreed closely with the return temperature, but flow temperature was 0.18 ± 0.08 °C higher than return.

When the experiment was repeated following a heating cycle, the temperatures started higher and cooled back towards the stable internal temperature of 21 °C.

Once again, the reported flow temperature was 0.1 - 0.2 °C higher than the return, and the radiator circuit probe tended to agree with the return temperature.

Findings

I can’t independently calibrate any of these sensors, so the findings are all relative. However, data from the Sonoff probe on the radiator circuit suggests that the accuracy is reasonable.

It seems that the flow temperature sensor is not perfectly paired to the return sensor, it is consistently reporting values that are 0.18 °C higher. These results are slightly better than those from Glyn’s data above, where the dT reported by the heat pump was 0.34 °C greater than that measured by the heat meter.

Small offsets like this seem quite good when compared to some of the reports from the corresponding Vaillant thread, where differences of 0.9 °C or more are reported (e.g. Vaillant inbuilt monitoring vs MID meters - #194 by CH_18 ).

Effect on COP

My reported dT is higher than reality by 0.18 °C, which I believe will lead to over-estimates of thermal output and COP of 4 - 8% depending on the actual dT during operation (reported dTs are ~5 °C during a DHW run, ~2.5°C during a heating cycle at the moment).

From the posts above, it seems that flow rate and specific heat estimates from my Grant HP290 are likely to be larger sources of COP uncertainty than the temperature sensors or electrical power.