Virtual power meter from boiler water inlet and outlet temperatures? How-to?

I’m trying to design virtual sensor for metering heating power of the wood fired stove, that is connected to 3 m3 boiler. The stove is nominally 40 kW unit and has no extra instrumentation for power metering, just internal wiring and controls needed for daily operation and system uses normal European 230AC, so know fiddling there. Actual heating power depends of course on wood used (quality, moisture, species of tree burned etc.), inlet air temp and density and many other factors and as such, using some fixed value is not reasonable.

Also, boiler has internal separation plates for thermal layering and energy is used all the time, so simple water temp measurement multiplied by volume and mass ends to be inaccurate.

Stove gets its cold feedwater from the bottom of boiler and returns heated water to top of boiler and there’s pump for circulation. Pump is controlled by water temperature using thermostat. There’s also mixing device with thermostat, that keeps stove water in closed circuit until it reaches at least 72 °C. When stove is fired and killed (automatic shutdown based on exhaust temp by thermostat), it has to be concluded from the factors like derivatice of feedwater temp, exhaust gas temp (this can be measured) and inlet water temp derivative. These two derivatives change rapidly, when pump is running, if derivative sensor has short enough timespan (minutes, not hours).

Heating power itself is quite easy to calculate from massflow (stamped on pump), and inlet and outlet temp difference and time pump is running. This must be calculated only over the timespan pump is running.

I have all the temp sensors needed (inlet temp, outlet temp and exhaust gas temp) and I think exhaust temp is actually not really necessary. Sensors need to be placed properly, as not to measure internal circulation during initial heating phase.

In theory, this is quite simple, I think.

And now the questions:

  • Has anyone created virtual sensor like this and willing to share it?
  • Some other solutions for calculating power on wood fired water heater with lineary changing heating power?
  • Something else worth mention when planning this?

Virtual sensor itself should be quite simple and with few extra definitons it should be possible to integrate it with HA Energy metering services, already usig it with heat pumps and Open energy meter+SmartMaic pulse reader. Sensor data from stove is fed into HA using MQTT and is not problem.

You may take a look at powercalc