If you have an existing boiler and want to add a radiant loop or loops to it you will need to lower the water temperature for the radiant loop you will need to incorporate a mixing valve into the system. Cast iron boilers operate at 180 degrees to operate hydronic base boards or cast iron radiators, they can not be operated at less than 160 degrees without damaging the boiler, lower water temperatures will also prevent the radiators from putting out enough heat. Although staple-up radiant will work in some cases at 180 degrees with out problems any other method of installation will require a lower water temperature. The drawings below are the best and easiest way to provide tempered water to the radiant portion of you heating system.

One way to change baseboard hydronic heat over to radiant in one area such as a bathroom or kitchen is to simply remove the baseboard and  reroute the 3/4" copper tubing under the floor between the joists. One run suspended about 4" under the subfloor in each joist cavity will work very well. The big advantage is that it does not require an adding a pump or lowering the water temperature and does not  interfere with the flow in the loop as using smaller pex tubing does.