In residential systems the radiation flow requirements are often reasonably aligned with the boiler's min/max flow range, and as long as the delta-T at the boiler is never out of range it's possible to dispense with primary/secondary configurations. But you have to do the math on all of it, and select the right pump. Nobody is going to do the math for you gratis (and I wouldn't trust the result of anyone that would)- this sort of stuff is the heart & soul of good hydronic design, and it takes time, experience & judgment.
Some mod-con boiler manufacturers won't honor the warranty if the system isn't configured with hydraulic separation between the boiler & radiation.
The hydraulic separator need not be an expensive purchased component- in modest flow systems (nearly all residential systems) building it up out of sections of pipes & fittings is just fine. The center pipe just has to be fat enough that the velocity is fairly low, and long enough to promote good mixing between the radiation and boiler flows. The air vent & drain valve found on most commercial units aren't necessary for achieving the separation, but aren't tough details to add to a built-up version, if desired.
