I had no idea there was a drain up tight in behind it. To bad it was set
into the concrete 5cm above the floor height.

Fortunately it was a relatively slow leak only causing a small pool of
water and although it soaked the sill plate it did not wick through and
get to the drywall on the other side.

I decided to cut the drain off with a saws-all with a long blade. Then I
added a pan with the pipe taking a 90° elbow right into the open drain.

In addition to the lousy job on the drain the gas line was run down from
the ceiling in front of the furnace and old water heater in such a way
as it all had to be taken out to get the old water heater out.

Had my bro-in-law (a plumber) move the main line in behind and added
some new flex line so it will be the last time it has to be take apart
like that again.