[Rescue11]

1/2" pex will just about fit inside 1/2" galvanized btw...

Truth be told 3/4" pex has only slightly larger ID than 1/2" galv. for a few reasons. First being that the I.D. of 1/2" galv is larger than 1/2". Its actually closer to 5/8" but not quite. The OD of 1/2" hard drawn l,m, and k copper, cpvc AND PEX is 5/8".

SECOND...Cross linked polyethylene is pretty robust, however it needs more sidewall meat for expansion and general durability than copper does. Because pex was put in the CTS family for easy compatability with hard drawn copper tubing and cpvc, it has to maintain the o.d. to work with various fittings such as compression and shark bite. So.... thicker sidewall + same out side diameter = reduced i.d. for pex tubing.

When sizing, depending on elevation and f.u. 3/4" is used at very minimum for the main. Most of the time we use 1". 1/2" definitely has a place, just use enough sense to know what/where it is.

Edit, never understood the logic behind a hot water heater. I mean, why buy a water heater to heat already hot water? I myself have only seen regular old water heaters meant for heating "cold" water, but people always talk about these hot water heaters and I have never seen one. Its a mystery to me