I am installing a long clothes dryer vent in the crawl space under my house to replace the old one. I am using a combination of both flexible metal pipes and rigid stove pipes to try to reduce the friction as much as possible. Unfortunately the length of the pipe is 20 feet and there is really no alternative. In several spots the pipes almost come in contact with the copper hot water pipe for the house--it is a very tight area and I am doing my best to avoid touching the pipe. Is is a problem if both the flexible and rigid pipes come into contact with the copper hot water pipe in just a couple of small places?
This is not likely to be a major problem, but I can think of two ways it might cause trouble. One is noise, which can occur when the pipe moves due to thermal expansion or rapid water shutoff. Another is galvanic corrosion, which can occur when non-copper metals contact copper water pipes.
Either of these issues can be easily addressed by placing a section of foam pipe insulation around the copper pipe in a way that prevents contact with the duct.