Although it would be nice to evaluate the pump ....maybe not ....
Given your concerns, think about simply putting the hot water service line into the CAC via a throttle valve and have your flow meter, tempIN and 0-10psi pressure gauge up stream as well (at the same level/height) and then
have a tempOUT and the outlet of the CAC to atmosphere ie: 0psig (bucket?).

Its important any static elevations are taken into account and hoses are >1".
500mm = 0.8psig and a 1" hose with 5gpm is still a low enuf velocity to prevent any measureable dP.

Feeding/capturing the waste water and feeding it back to the loop (if you must) needs to be done after this (use now spare johnson pump?)
That way you can effectively measure your dP and dT across the CAC without fear or favour.

I think your numbers may have provided some valuable info just the same - will post shortly