TRV's on a closed loop system??

It can be done with various different bypass schemes. Requires careful calculation and a quite well-balanced system to begin with.

Years ago someone here posted a link to a VERY good document by Danfoss: "Design of Radiator/Convector Configurations in a One-pipe and a Two-pipe Installation". Unfortunately I've lost the link and can't find it by searching Danfoss US. Perhaps someone will re-post the link.

Here's an interesting quote from the document:

"In two-pipe installations a small heat load leads to high temperature drops, while a small heat load in a one-pipe installation involves small temperature drop, since all the water is led through the bypass. This means that the flow temperature in a one-pipe installation must generally be as low as possible--depending on the local requirements--since otherwise the return temperature becomes too high, leading to excessive heat loss from the pipes. Regulation of a one-pipe installation thus requires the use of a highly accurate, central regulation valve, which in practice is very difficult because of different time constants. A two-pipe installation, on the other hand, has a positive feature in that if the flow temperature is too high, this simply results in the radiator thermostats regulating even faster and thus even more energy-economically."