How do I calculate the head pressure in a 1/2" pex radiant system.
Did some looking on the web, but couldn't find anything conclusvive.
I would suspect that the pex would have so much head per ft. and then have to add connections etc.

The commonly used figure is about 0.4 pounds of pressure per foot of rise. Speedball knows the exact figure. I'm not sure what a "radiant system" is, so I might be off on my answer relative to what you are wanting to know.

Jlisenbe nailed it!
The coirrect way to calculate the head pressure of ANY column of water whether it's in a Pex system or a water tower is to multiply the height times 0.434
Hope this answers your question. Yom

That would get you the height calc. but what about the friction of the pipe and connections?
Does this also contribute to the head?

that would get you the height calc No! That would give you the PSI at the base of the column. Just what are you attempting to figure that you want to factor in line loss. Before we take this any farther let's have more details. Why the question? Tom

I looked up pressure loss for pvc one time. The bottom line is, it varies depending upon pipe size, the length of the run, and the volume being pushed through the pipe. For short runs such as I suspect you are talking about (tens of feet), it was, as I recall, insignificant.

You might find this site helpful.

Hazen-Williams Equation - calculating Friction Head Loss in Water Pipes (http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html)

OR this one

http://www.pondarama.com/html/friction_loss_charts.html