Tobes, Without inspection, I believe that it could be a pressure differential problem. The infinity is a high temp Hws, Hot water will always increase in temperature.
Are you in Australia?
The laws relating to the installation of HWS in Australia are quite clear, have you installed the mandatory Tempering valve? You should already be aware that Hot Water BURNS.
The implications for you as the installer are fairly onerous should any person get scalded.