diff --git a/Source/wall.f90 b/Source/wall.f90
index 2db8178762a..ca703fcc636 100644
--- a/Source/wall.f90
+++ b/Source/wall.f90
@@ -3536,7 +3536,7 @@ REAL(EB) FUNCTION HEAT_TRANSFER_COEFFICIENT(NMX,DELTA_N_TMP,H_FIXED,SFX,WALL_IND
    BCX => BOUNDARY_COORD(LPX%BC_INDEX)
    DN = SFX%CONV_LENGTH
    R_DROP = LPX%RADIUS
-   IF (R_DROP>TWO_EPSILON_EB) CONV_LENGTH = 2._EB*R_DROP
+   IF (R_DROP>TWO_EPSILON_EB .AND. SFX%GEOMETRY/=SURF_CARTESIAN) CONV_LENGTH = 2._EB*R_DROP
    TMP_G  = P1X%TMP_G
    RHO_G  = P1X%RHO_G
    ZZ_G(1:N_TRACKED_SPECIES) = P1X%ZZ_G(1:N_TRACKED_SPECIES)