Harry Everhart wrote:
I am putting a tankless gas water heater in my home. All of the copper
is in the concrete slab so I am going to run an insulated copper tube
"up and over" to the kitchen. The water heater will be a foot from the
two bathrooms and showers. the "up and over" tube will feed the kitchen
sink and dish water - nothing else.

How much water is in a 100 foot by 1/2 inch copper tube?
How much water is in a 100 foot by 3/4 inch copper tube?
How much water is in a 100 foot by 1/4 inch copper tube?

I am too lazy to look it up - I am wondering if any of you experts on
here have the info off-hand. I am thinking of putting thinning tubing to
the kitchen because less water would be in it to cool down etc.

Right now I am running two 50 gallon electric water heaters in a house
for two adults - I am wasting alot of energy keeping all that water hot
- and the tanks are far away from where the hot water is needed anyway
running thru a cold concrete slab.

I made up a little spread sheet. It assumes 1/32" wall on the pipe.

OD ID Area cu-in cu-ft cu-ft
per ft per ft per 100 ft
0.500 0.469 0.173 2.071 0.001 0.120
0.750 0.719 0.406 4.869 0.003 0.282
0.250 0.219 0.038 0.451 0.000 0.026

I had to replace the copper pipes in my slab. I ran the new ones in a
channel chiseled in the slab to get passed doors. For the heating
system, I used 3/4" thick-wall copper pipe and wrapped it in duct tape
to prevent future corrosion. The fresh water runs are 3/4" to laundry
and kitchen, and 1/2" for the rest. The run to one bathroom is
approximately 75 feet. It takes about 30 seconds to start getting the
hot water at the end.