if you want off the shelf you could try this one
you could repurposed a couple of sensors the say the in house line circulation for the cold return to the collector. then use the network connection to collect data of the sensors, and either infere the flow or addin a flow sensor use the top cell senor as the hot input temp, the line circulation temp as cool return
or you could try my per existing versions heatpump /solar thermal
but I used the same of monitoring solar thermals – but instead of using DS I would switch it to anlogue - NTC 10K if you wish higher accuracy
the arduino code for that is
#include <math.h>
double Thermistor(int RawADC) {
double Temp;
//Temp = log(10000.0*((1024.0/RawADC-1))); // adjust 10000.0 to get accurate temp - use other inline if temperate move in opposite direction
Temp =log(11500.0/(1024.0/RawADC-1)); // for pull-up configuration
Temp = 1 / (0.001129148 + (0.000234125 + (0.0000000876741 * Temp * Temp ))* Temp );
Temp = Temp - 273.15; // Convert Kelvin to Celcius
//Temp = (Temp * 9.0)/ 5.0 + 32.0; // Convert Celcius to Fahrenheit
return Temp;
}
void loop()
float tempD = ((Thermistor(analogRead(0))));