Rheem RS120-64BP Use & Care Manual page 17

In a single tank system install a third thermometer
(No. 3) directly after the mixing valve above the solar
storage tank. In a two tank system you may install the
third thermometer either directly above the hot outlet
TILT MOUNT
on the solar storage tank or after the mixing valve on
the back-up water heater.
The circulation pump shall be the Grundfos model
fig.11
17