Replacing galvanized water pipe under house on piers

Pex just keep it out of direct sunlight.. if you go the plastic route use Cpvc, it is rated for hot water.. either way insulate the lines..

DIY project or hiring a plumber?

This will be a DIY job. I don't mind crawling around under there. I have heard Pex from some folks and high grade pvc from others. I have never used Pex and don't know anything about it. Would I need to insulate Pex?

Pex. We did a home renovation project a couple of years ago, and I hired out the plumbing since I would need a new washing machine hookup, three new sinks, putting a toilet back after I put new floors in, a freestanding tub, a stall shower, and a new toilet, and all of the associated drains that would tie back in to the main drain in the basement. Also had him replace a leaky outdoor faucet while he was there. I watched them do some of the Pex work. I probably should have just watched some YouTube videos and done that myself. The only real difficult part to me looked to be hooking it to the existing copper lines. But at that point after I had done a ton of the demo work, new walls, and other stuff, I was ready to just write a check and be done with it, and the price was hard to beat knowing they would have it all complete in about 2-3 days. But Pex is great. Really easy to work with, and it will expand and not bust. You probably won't need to insulate it.

I'm in the PVC camp.

Pipe, fittings, glue/primer. That's it. Prime the joint for a better seal than just using the blue stuff.

Pex has not been trouble free despite what people say. Some fittings have failed, rodent damage is a real possibility. Also, the crimping tool for 3/4 and up is not cheap. If you use the Uponor expanding rings those tools are even more costly.

Funny how material/labor costs have gone down sharply with the use of pex but plumbing pricing has not followed.

i'm in the pex camp. the first time i used it, my brother and i replaced all of the water lines in my rental (kitchen, 2 baths, laundry room and two outdoor faucets) in about 2 hours. no open flame like sweating copper and no glue drying time like pvc.

your bigger problem will be getting the old line out. after 74 years, the galvanized pipe will not simply unscrew from the fitting. the weakest point is apparently right at the fitting, and the pipe will twist off, always on the wrong side of the fitting.

i had a water main rust through on my 1945 capital heights house, and coburn supply suggested a fix for me that has worked well. cut the bad pipe off 6 inches from each end. use a compression fitting and a 6" nipple to restore threads to each end, then screw on a transition piece for whatever new material you use (pex, pvc, etc.)

each end would have a combination of this
compression fitting
galvanized nipple
transition from iron pipe to pex

attached by compression fitting to the remaining 6" of the cut-off pipe. your connection on each end would then be old pipe-compression fitting-iron pipe nipple-transition from iron pipe to new material.

you can make up both of the connections on the workbench, then all you have to do under the house is cut the pipe, attach the connections to the cut-off pipe and run the new material. while you're under there, you can T off the new material and add a new run for an outdoor faucet.

Cpvc for ease and Cooper is the best but pricey. PEX will require more specialty tools and rodents seem to gravitate to it and cause damage.