Some advice in this thread: Radiator Balancing - is there an idiots guide?
…but heed this warning in post #15:
Obviously from a heat pump point of view, as I’ve found to my cost, you need to ensure there is enough flow going through the rads to ensure you have enough working system volume in the system.
I’d throttled down my rads so bad chasing DT5 across the tails I’d restricted by working volume to barely nothing.
My COP has shot up this last week since opening the rads up. I’ve now given the heat pump more system volume to work with. So excuse the pun, but there is a balance to be had here.
So, it may be better to keep all rads open and only balance them a little if needed; i.e. some rads or rooms aren’t getting warm enough, and some flow needs to be redirected.