we don't close the cover only with tongue and groove, do we? The box is lined inside with 2cm of Styrofoam, that should be enough as thermal insulation. But a gap is still a gap ;)
Yes, that's how it is with ours too. (I've had to remove them quite often due to motor replacement for the pipe etc...):cool:
But, the tongue and groove is only on the front long piece, the two short sides on the right/left and the other long side should rest all around. So no draught can come through this gap.
In addition, the covers on ours are screwed all around. So on all four sides.