3 heating loops in the bathroom, 1 not working, what to do?

Unfortunately, nobody really thought ahead there...

It is optimal to have the same pressure loss in all heating circuits so that no throttling is needed anywhere (of course, the heat load of the individual rooms must have been properly calculated in advance) and thus the maximum lowest flow temperature is achieved.

The problem here: circuits of similar length, yes... but the pressure loss of a circuit with a 5 cm pipe spacing is significantly higher than one with a 20 cm pipe spacing... this directly results in hydraulic balancing that, to achieve sufficient flow in the bathroom, all other circuits must be throttled down.. which in turn means that the flow temperature has to be senselessly raised -> loss of efficiency and higher heating costs.

Aside from that, it has already been mentioned... 20 cm pipe spacing is actually no longer used nowadays because it costs efficiency due to the higher required flow temperature. 5 cm in the bathroom is also pointless because the heat output compared to 7.5 cm is hardly better with higher pressure loss. Better to increase the surface area (for example through wall heating), the effect is much greater.

As has been said many times: contact the contractual partner, they should at least properly adjust the poor planning. This will come at the expense of a higher flow temperature and possibly the lifespan of the heat generator (more frequent cycling due to suboptimal hydraulics), but at least it will be warm everywhere.

OK, VA 20cm is intense, 10cm would be better. But anyway, your main contractor has to solve the problem for you (unfortunately). You pay for all other attempts to solve it yourself.

Technically, I would disconnect the non-working radiator from the supply and return manifold and blow it through with compressed air. Start thinking about an argument against the main contractor if the problem is in the pipe and the main contractor wants to brush you off with a small monetary reduction.

Question on the side : Does the pressure loss depend on the VA? As a layperson, I assumed the cross-section and length as influencing factors.

Question on the side : Does the pressure loss depend on the VA? As a layman, I assumed cross-section and length as influencing factors.

I join the question.

Can one of the experts here break down the interpretation for me?

Does the pressure loss depend on the VA? Naively, I assumed cross-section and length as influencing factors.

With a narrower spacing between supports, the bending radii will probably be smaller and their number increased, so that the pipe is bent more and more often, resulting in a higher pressure loss.