metal rusts. both the zinc galvanization and the vinyl coating are there to help inhibit and slow down that action. but it is a temporary fix... there are problems with both systems... 1) Zinc galvanization does rust and break down... it is a white powdering coating (zinc oxide). 2) the vinyl coating can crack, tear and rip... so that water can enter be trapped - rusting the steel wire from the inside. water, salts and acids can all help the rusting process along... ALL of which are present in a good healthy garden and compost pile. In welding we avoid zinc galvanization, because of the dangerous off gas that is created, but even that requires long term exposure and high quantities... do a topic search of: "zinc oxide poisoning" Vinyl arguably leaches potentially dangerous chemicals over time... do a topic search of: "vinyl leaching" sorry... too much information?

Given that you have galvanised steel piping and are installing something prone to eventual failure in an inaccessible location, use whatever works and don't worry be happy. Sorry if that comes across as snide. At the least, install good components with suitable-for-your-water materials in a way to permit future repair. You could transition the respective metals to a CPVC union and avoid galvanic reaction.

Gents, Thanks for the post and suggestion. I found this articles validates what you guys suggesting me http://www.familyhandyman.com/plumbing/repair/how-to-join-dissimilar-pipes/view-all . I'm very nervous right now with the time bomb dresser coupling ahead my kitchen. I have another question. I plan to change the pipe the whole house in a couple of months. I will do stop-gap solution by changing the PVC dresser coupling to a steel coupling per your advice. My other question is, what type should I change to when it's time to change for the entire house, there are a bunch of them out there. 1) PEX 2) CPVC 3) PPVC 4) Copper Please give suggestion what and why. Our home is over 40 years old, 2 adults and 2 dogs living in 1800 sqft home. Thanks.

Sooner or later this will be necessary. I owned a house for many years that was built in 1952. I remodeled and added to it in the 1980's and the new plumbing was copper. I wanted to replace the old galvanized, as well, I had a somewhat more difficult task than you, since my house was built on a slab and the galvanized was embedded in concrete, so I had to run lines through walls and ceilings where none existed before, which I did progressively over several years. I still remember when I ran new copper from the meter to the house. The galvanized I took out was about 40 years old, and when you looked down a 10' length of it you couldn't see daylight at the other end because there was so much corrosion. If it would have been a human, it would have been dead from arterial sclerosis! My advice is to go for it. You will save yourself a ton of money, compared to having a plumber do it. I don't know what your skills are, but this was the project where I first taught myself to sweat solder...and I still enjoy doing it. Your project sounds very doable, and you can do it over a period of time. You will probably need to cut into some walls to get at the various valves (toilet, shower, bath, etc.) that are plumbed to the galvanized, but it will be worth it. And when I sold the house, it made a difference to buyers that there was no galvanized left in the house. Regarding your waste lines, probably nothing is necessary.