Switching from 1/2" to 3/4" copper supply lines

Hi-

I'm in the middle of a second floor bathroom remodel and I'm getting ready to plumb the tub. I removed most of the wallboard, so I currently have access to the basement through a thick wall that carries HVAC and electrical through the core of the house. The house is 40 years old and is 1/2" copper throughout. I have a tankless water heater in the basement in close proximity to the HVAC wall. I was thinking of re-plumbing the supply line to the water heater with 3/4" and then running 3/4" up to the second floor through the HVAC wall and branching off to 1/2 once there. It would be a short run of pipe, maybe 15 feet total, so I could do it with a minimal effort. In the future I may want to upgrade the shower in a second floor bathroom adjacent to the one I am working on now, so if I'm going to upgrade the upstairs supply line to 3/4", now is the time to do it while I have the HVAC wall open.

The specs on the water heater say it can handle a minimum of 1/2" on the supply side, and they indicate that 3/4" inlet piping is fine. However, the actual inlet piping and the copper coil in the heater are 1/2".

My concerns are several fold. My heater actuates on demand with a .5gpm flow rate required. If I increase the pipe diameter to 3/4", will I decrease the actual flow rate through the unit because there will be more capacity in the line? Also, will the heater impead the flow through my hot water supply line on the downstream side of the water heater? Will having the heater in the line create a pressure imbalance such that there will be more cold water flowing to my shower valve than hot water. I plan to neck down to 1/2" at the shower valve, but I'm not sure if that will make a difference.

Thanks for any thoughts.

Unless you are installing a shower with multiple heads, up-sizing 1/2 copper to 3/4 copper to the bath would not be a very good investment. No common bath fixtures require a 3/4" supply.

In addition, unless you are running a recirc, you will waste a lot of additional water flushing the cold from the larger pipe while waiting for the hot to get there.

If this line is going to feed two bathrooms in the future, do it now. The size of the pipe doesn't affect the flow of a restricted outlet device, but if there's a tub there, larger flow will fill the tub faster with a larger pipe, at least it can, depending on the valve.

It would also minimize flow fluctuations is someone flushes a toilet or uses some water in the bath. WIth the required anti-scald technology in the vavles, it won't change the shower temp much, if at all.

It WILL however take longer to purge the extra volume of cold water in the pipe from standing there overnight before you get hot to the end point. Won't make any difference once it does get there, though.