I'm doing the same at my home now. I am going pex all the way, except for the stub outs for the fixtures. Pex to copper is fine, but I would suggest that you eliminate the galvie entirely. You do not have to run the "home run manifold system". With my lay-out it is better. YMMV. You can pipe it conventionally. What I like about the manifold system for me is that I am running small diameter tubing to each faucet. That means I will not have to wait as long to get hot water to a faucet. If you look at the cross sectional area of 1/2, 3/4 and 1" tubing, they are .19, .44 and .77 sq in respectively. That all equates to volume that you have to move out before you get hot water. Just something to think about.

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.

You can cut off and cap the copper pipes coming out of the floor, but I'd first try to figure out where they are coming from. You don't someone turning a shutoff in the basement or crawl space (I wonder what this does?) and finding out it controls the water line running to this pipe. Smaller sizes of cast iron pipe usually have threaded fittings, while large waste pipes may use oakum and lead for the seal. Old cast iron pipes are usually fine for vertical runs (vent stacks, etc.), but horizontal iron pipe runs trap moisture and form rust in the bottom (looks like stalagmites). This rust catches on whatever is draining through the pipe (hair, soap, etc.) and creates a blockage over time. A plumbers snake becomes your friend. The rubber fittings are usually called Fernco couplings, since its the most common manufacturer of this type of coupler. They are used to connect dissimilar pipes during renovation (iron pipe to PVC, etc.), or to reconnect a pipe that has been cut. There is nothing wrong with using them if the pipe is adequately supported on both sides of the rubber coupling. Bruce