: We want to replace a 5 gpf, (vintage 1965) two piece
: wall hung toilet with another wall hung unit. I have
: some questions - thanks for help on any of them.

: 1. I've read that there are problems replacing a 5 gpf
: with a 1.6 gpf because the the 1.6 needs a narrower drain
: with smooth surface. Is this true, and if so, is it
: possible to replace enough of the piping to work well?

The piping for all toilets is the same size and material.

: 2. The few wall hung units I've found often have the
: tank in the wall. How much space does this take and
: how do you prevent condensation in the wall?

Only a couple of companies make a residential (1.6 gpf) toilet.

: 3. I haven't been able to find many wall hung models
: that don't look like they are for industrial use.
: Also, I don't have much space - about 6" inside my
: current wall plus 27" beyond. Any recommendations?

They look like commercial units because that is what they are. Most only come in a 3.5 gpf model that is rated for commercial use only.

: 4. Does anyone know how the Gerberit shower toilet,
: which has a wall hung version, would do in my
: situation?

Most Gerber-it items are plastic, and if that is what their toilets are, I would not recommend them.