In article ,
"Christian McArdle" writes:
If I join the ends, I assume the path of least resistance will mean
the water won't be keen to run through the rads at all, just keep
going round the loop. But if there is no loop what happens if all the
TRV's are closed and there is no return route? Is this accounted for
in boiler design so it just stops?

There are several ways of doing this. Just connecting flow to return will
entirely short out the radiators and isn't one of the ways.

Well, this isn't electricity. A radiator will present lower
resistance than going all the way to the end of the pipe run
and returning. This is used in some industrial installations.
However, as you and others point out, this might not match
the design of the rest of the system, particularly for a
modern domestic installation.

--
Andrew Gabriel