The process of dipping steel pipes into molten zinc to galvanize them has been around since the early 19th century, and galvanized pipes still are common in outdoor and industrial water systems. Plumbers don't use them for residential plumbing, however, because they have better alternatives in copper and CPVC pipes. If your old house has galvanized pipes, replacing them is a good idea. The process of removing them is more labor intensive than complicated.
Without knowing the age of the pipes, and the hardness of the water, and without having a visual of the piping configuration, it is difficult to advise whether or not to replace. Some of my clients who own older homes have replaced galvinized piping some have not, (yet). The best advice would be to interview master plumbers and get a variety of opinions. there’s a weekend radio show “Tom Tynan Show” and on Tom’s website there is a wealth of information about plumbing and list of plumbers and building contractors he endorses. I hope this helps you. I’m happy to chat with you if you need more specific info. to contact me use the contact form on HAR. Thx, v
The possible use life of the galvanize pipes are 50 years. And most houses in last 50 years dont have these pipes. Big possibility their use life is ended. With years the pipes get brittle and repair is tricky. Once there is a leak, galvanize pipe is more prone to fail other places as well. If it goes with second and third repair, it will be costlier than replacement. Considering all these, I recommend replacement.
First of all, what is that pipe transporting? How old is it? Why does it need repair/replacement? This is a plumbing question. If that is the water line, it's old and rusted and/or it has sediments/calcium deposits that restrict the water flow I would replace it with a very well insulated copper line. If that doesn't fit your budget, then replace it with a PVC line. Again, because you mentioned it is under the house, don't forget to insulate it very well, every inch of it. Copper is the best solution, but it is more expensive. The most common solution is PVC for cold water and CPVC for hot water. The best thing you can do is to hire a good plumber to get it done for you. This way you'll sit rest and assure that the job is well done (by the code) and backed by the plumber's insurance. If that's a gas line, you better hire a plumber and have him replace it with black piping which is the standard for gas lines.
source:
My source is a very good Master Plumber who happens to be one of my closest friends.