In message ,
writes
hi all, i have a danfoss hsv3 valve on my central heating that is
malfunctining... it is fine when ch is on or hw and ch is on but not when hw
only should be on...it stays in mid position. other than draining down and
fitting a new one or changing the powerhead...are they repairable? i have
some skill with a soldering iron so would be happy to give it a go! anyone
help me out with some tips?

There are two possibilities
Either the valve shaft ha become stiff through water ingress
or
the return spring has lost some of it's springiness

The former can be resolved by dismantling the valve (prolly a drain down
job), the latter will only be reliably resolved by buying a new actuator
head.

Going from closed to mid or open position, it is driven under power by
the synchron motor, from open to mid or closed, the spring is under more
tension and generates momentum. From mid to closed, the spring is no
longer strong enough to return the valve to the closed position

for closed, mid and open, read HW, HW/CH and CH - I'm thinking in terms
of testing them

--
geoff