baseboard radiator supply line

If you have hot water (hydronic) baseboard heat 3/4" is usually used. Unless you have separate "zones" of heat the baseboard heat is run in series. If you are using hot water radiators you may want to run each radiator separately on it's own zone if there is more than one room or heated spaces. Multiple radiators can be run off the same zone usually using a 1" main line system with 1/2" venturie tees so the hot water will get to the radiators at the same time. Hope this helps.

RPK1

I recommend each radiator on it's own PEX loop.
use 1/2" PEX
And each radiator has a thermostatic valve installed, similar to this: http://hydronicalternatives.com/html/oventrop.html
Use PEX manifolds that have a flow indicator, so you can adjsut flow to each radiator (balance the flow)

I think it depends on how much heat you are trying to move. 1/2" will easily move 10- 12,000BTU/hr. They do offer 1/2" baseboard for small, low load zones.

You can calculate the output of the second fin tube if it is piped in series by knowing the temperature drop thru the first board. Generally when baseboard is piped in series the last zones require more footage due to the decreased temperature they see.

All things considered I like piping each board as suggested with TRVs to regulate the temperature, if they are located in different rooms. I have also piped panel radiators with the "homerun" method using 3/8 PAP, if the load allows it. Bathroom towel bars or radiators are a classic example of load load requirements.

The best way to make a choice is to know what the load requires.

hr

My radiators are grouped into zones, which vary in demand. Each zone has its own thermostat, most zones are supplied by 1/2", some needed 3/4".