sorry JohnMA didnt mean to hijack your thread...
but i did mean to say what temp is you flow and return water...
the internal stat on the 600s is set to 43 Deg C.
and as far as i know...its not adjustable.
so if your temp isnt getting that high...it wont ever kick in.
so you may need to...
thats funny, i've got 2 600s with a similar problem.
worked ok last winter..but
this winter they dont seem to work.
so i pulled them out and tried to bleed them...
i got a needle fine jet of water out...
which got hot enought to cause the stat to make the fan run.
so i know hot water...