Have you tried to reduce the speed of the circulator to the lowest, see if that changes the duration of the cycle which is about 6 hours, according to your curves?
You write here waterpumpingfiltration / transformingasolarDHWtankintothermodynamicwaterheaterwithasmallpoolt17035160.html # p481584
My calculation may not correspond to anything, but by multiplying flow rate by deltaT, you transmit roughly the same temperature to the tank:b) Estimate of the real flow at 3kW of heat exchange: P = q * 1,16 * deltaT
Q (m3 / h) = 3 / (1,16 * deltaT)
2.5 ° C delta: 1,03 m3 / h
1.9 ° C delta: 1,36 m3 / h
 1.03 x 2.5 x 6 = 15.45° for 42 WH
 1.36 x 1.9 x 6 = 15.5° for 132 Wh consumed by the circulator.
The 3 kW announced are theoretical, the power transmitted at 3 m3 can be calculated: Pt (kWh) = delta T tank * 1.162 x 3
To obtain it, it would be necessary to know exactly the temperature of the volume of water in the tank before and after, but with stratification, it would be necessary either to have several probes, or to mix the water before and after.
A probe placed under the insulation of the balloon could perhaps give a good idea of this deltaT
