I am about to reroute leaking pipes from under the slab through the ceiling bay of my two story home, but first I am trying to grasp where everything runs, but cannot pin down why there are three 3/4" copper trunk lines going to the side of the house with the leaks. (The other side of the house had its trunk hot and cold trunks rerouted through the ceiling a decade ago.)

The basic design of this part of the house is there is a manifold in the wall between the house and the garage. Out of the mud seal comes a 1" cold that branches off into two 3/4" before it goes up the wall to feed the hot water tank. The two 3/4" lines feed each side of the house. A 3/4 then runs from the hot side of the tank, and branches into a 3/4" line to feed each side of the house.

What stumps me is what seems to be an extra 3/4" line. I know the single 3/4 feeding the manifold in the downstairs laundry was leaking and it has been isolated. there are four 1/2" lines running from that manifold. #1` ran back into the slab and emerged from the mud seal of the perimeter wall, which it traveled up to feed the two sinks in an upstairs lavatory. It was not leaking, but has been isolated. #2 also ran back into the slab, under it and came up under the pedestal sink in an adjacent half bath. It was leaking and has been isolated. #3 then feeds hot water to the washing machine box. #4 continues up the wet wall to the lavatory where it feeds the bath/shower mixer valve.

What doesn't make sense, and I want to try to have everything sorted out before I start to reroute everything, is the design of the cold water system. As I understand it, there should be a single 3/4" copper pipe from the main manifold that feeds cold water to this half of the house. However, a 3/4 emerges from the slab to feed the fixture nearest to the primary manifold via a 1/2" line; a toilet. That line then goes back into the slab. A second 3/4" copper line also emerges from the slab, but on the perimeter wall on the far side of the serviced area. As well as I can see so far, noting branches off of it and it travels upstairs to the lavatory above. (It is possible that this is a hot water line!)

Whether it is a hot or a cold, I cannot figure how it can connect to the primary manifold, unless a connection was made under the slab and would anyone do that.

Anyway, forgive the ramble. Can someone explain the design. I am going to try to see if the temperature of the 3/4 to the upstairs changes when I run cold water. I can't run hot as the line has been capped at the primary manifold.

Possibly plumbed as a future return line for a hot water recirculation system ?

Problem is that they are clearly cold water lines. Anyway, I think I got it figured out. It is a single 3/4" trunk line running from the supply line manifold into back into the slab. It emerges out of the slab into the nearest curtain wall of the downstairs powder, where a 1/2" line comes off to feed the toilet, before it travels back into the slab. It then travels under the slab to the far wall of the laundry room. It then travels up the wall where it feeds cold water to two sinks and another toilet. Likely off of a submanifold in the wall of the upstairs lavatory. Another 1/2" branch line returns from the upstairs lavatory manifold down the same far wall and into the slab. It then travels over to the cold water manifold located behind the hot water submanifold of the downstairs laundry. That feeds cold water to the washing machine box and the sink in the adjacent powder room before up goes up that wall and feeds the mixer valve of the upstairs tub/shower.

I am pretty sure I got this right, now. What confused me was that someone said that one of the lines in the wall behind that firs toilet was 1/2" and the other 3/4". If so, there would have had to have been a joint in the feed pipe under the slab and I don't think anyone would have done that.

If I am wrong, so be it. I hired a licensed plumber to repipe this half of the house and am confident in him knowing what to do.