I am remodeling a secound floor bath room, the whole house has 3/4" galvenized pipe from the origanal remodeling from and old barn.
So that I don't have to tear up the new remodel if the old pipes go bad I am replacing from the basement up. My question is should I reduce the size of pipes to the secound floor to 1/2" or leave the same 3/4" all through the house.
Re: plumbing question
You'll want to reduce your pipe size to 1/2" at some point. This will make hot water get to the sink faster than if it was 3/4" all the way. Contrary to popular belief, a larger pipe doesn't necessarily mean water travels through it faster. The volume of water sitting in a 3/4" pipe is approximately twice that of the 1/2" pipe; all of this water has to be pushed out of a faucet that only flows 1.5 gallons per minute before the hot water gets there. Oversizing your pipe will be a waste of money in materials, and a waste of energy for hot water.
I don't remember the numbers exactly, but each fixture is assigned a certain number of "units" in the code. As I recall, a vanity sink is one unit, a shower or tub is two units, and a toilet is three units. (Or maybe a toilet is two units and a shower or tub is three units.) A 1/2" pipe has a rated capacity of seven units, so you can run a 1/2" pipe to a bathroom, even if it has two sinks, and there should be no appreciable pressure loss even if everything is in use at the same time. Capacity assumes that all fixtures in the house are in use at the same time. I don't remember how many units dishwashers, kitchen sinks, and washing machines account for. Icemakers may count as zero do to their minimal use.
If there's another fixture somewhere along the line, you'll probably have to run 3/4" to the point where it branches off. It all depends on the total number of units on each branch.