Darro wrote:
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

It is against most codes to "tie into" an existing dryer vent duct.
Why can't you run this independently?

Alisa LeSueur
Certified Dryer Exhaust Technician
http://CleanYourOwnDryerVent.com