It is a well known fact that the dual cell model depends on the resolution of the core meshes. If the core mesh is very coarse, then accuracy is severely affected. Make sure you first enable beta feature access, as described in Introduction, then activate the alternate formulation for heat transfer using the following text command:

define → models → heat-exchanger → dual-cell-model → alternate-formulation?

The results obtained using the alternate formulation is mesh independent and gives a reliable solution even on very coarse meshes. Please note that the default formulation and alternate formulation results are comparable on a sufficiently fine core mesh. Also the alternate formulation should not be used for non-matching core meshes.

For background information about the dual cell heat exchanger, see the section on using the dual cell heat exchangers in the Fluent User's Guide).