Darro wrote in message news
I'm looking for comments and suggestions on whether it's safe and
practical to run 20 feet of 4" metal exhaust duct horizontally from my
relocated clothes dryer to tie into an existing 4" dryer duct which
runs about 20 feet vertically. The existing vertical duct is
uninsulated and is inside an always-warm house without air
conditioning.

Thanks in advance for your replies.
Darro

The problem with extra-long runs is the fan inside the dryer may not be able
to push the air that far, especially after a year or so & the tube gains
some wind resistance in the form of lint that catches on things on the
inside of the vent (this is why you don't screw dryer vent tube sections
together). Wind resistance is also one reason why the flexible vinyl hose
is not recommended (the other being it can catch on fire).

Check with the mfgr, see what they say about maximum vent length.