Size of Supply/Return pipe and Branch pipe on a two pipe system

Eric_7

I'm doing my first hot water system and have a couple of questions. The job is a small (1500 sq. ft) cape cod rental property that has an ancient, oil fired floor furnace. The owner wants to install a boiler with cast iron baseboard/panel radiators. I'm thinking of installing a two pipe system with TRV valves for the most control. The house sits just above grade with the exception of a small cellar under part of the kitchen and dining room. I can run PEX/AL/PEX from the copper supply over to the baseboard/panel radiator and PEX/AL/PEX back to the copper return. The copper supply and return will be next to each other and be about 30 feet long. The shortest run of PAP to a baseboard/panel radiator is 6 feet and the longest is 32 feet. Most of the runs will be 20 feet or so. Also, most of the baseboard/panel radiators will be in the 2000 - 2500 BTU range with three of them being around 5000 - 6000 BTUs.

The house has a 45,000 BTU heat loss. I'm trying to size the trunk lines and the PEX/AL/PEX branch lines to the baseboards/pane radiators. Can I run 1/2" PEX or would I need 3/4"? What about the size of the supply/return lines? 1"? 1 1/4"? Also, does anyone have/know a heatloss for insulated PEX running through an unheated crawl space? Any help would be appreciated. Thanks!

Eric