Question: I know a lot of posts have been made on this topic, but, I have tried everything. I wanted to move my hot water tank from the first floor of my house to the crawl space. My old unit was performing well, but, the plumber I hired recommended a new unit (mine was 12 years old). Prior to replacement, my hot water worked perfectly!! We could take three showers back to back with no issues at all. Now, with a new unit, I can't even make it through one shower before it gets cold. During the one shower, I constantly turn the hot know to increase the temp until it just gets cold on me. I have taken the following steps:

* Replaced the new HW unit with the same size (38 gallons), number of elements (2), and wattage as the old unit
* Checked the dip tube in the new unit
* Replaced the elements (with genuine factory parts)
* Adjusted the thermostat to 120
* Replaced all the stems in faucets with a one handle (mixing stems).

I have already dropped 1500 dollars on the unit and stems including labor costs. Please advise.

Answer: 
PEX is a dimensionally equivalent product. That means the OD outside diameter is equivalent to the OD of copper. The ID inside diameter is less than the ID of copper. The PEX fittings fit inside the PEX tubing acting as flow restriction and thus volume of water. Do not think that the rules of thumb for copper are equivalent for PEX. Normally a 1/2" copper line will feed three shower heads or three faucets with a static line pressure of about 50 psi. You state problems with water pressure due to plumbing. Some people experience extreme variations in shower hot water temperature and equate it to a water pressure problem. It is normal to have hot water temperature variations due to more than one person living in a home. There are several work arounds for this problem. Install a hot water recirculating system and/ or install anti scalding shower valves. Both in good working order should reduce hot water temperature variations.
Personally I would just live with the trunk and branch. It will be very expensive to replumb the house.