If the heater works poorly you probably have no circulation, if it does
work well you might have a blockage somewhere. Drain the coolant system
and stick a gardenhose in the various hoses and see what happens. I once
found a freezeplug which would jam the thermostat shut. On cooling down
the thermostat shut completely causing the freezeplug to drop down again
in the cylinderhead!

Thanks for the water-pump advice, I was kind of hoping it wouldn't be
that, since I do not have a good description how to change it (I foud
one description but for gasoline engines and I expect it's not the same
on the diesel).
I was searching the internet high and low for a Haynes manual wich
would talk about the Volvo 240 DL with Volkswagen engine but it does
not seem to exist or I did not search well.
My second thought is to try to find a Haynes for Volkswagen and apply
it with the Volvo but I have no clue wich Volkswagen would have the
same motor than my Volvo so I'm kind of stuck.
The heating works perfect we have a  sauna  on wheels if I put it on
MAX.
The radiator did show some signs of overheating in the summer
(trafic-jam on the highway for 4 hours) or if I drive above 100 km/h..

In any case I have to take out the radiateur even if I want to get to
the waterpump, isn't it? (so I might start for hunting for the blockage
with the radiator?)
Wouldn't it make some kind of noise if the waterpump is out of work?
Would it harm the car if I drain the system take out the radiateur and
it is staying on the street (around zero or bit bellow freezing)
without antifreeze meanwhile I'm hunting for blockage and charging
around for parts?

Perhaps I'm overworried about my car but I'm the second owner of it,
the previous one had it for 20 years and kept it in avery nice general
condition so I try not to ruin it.

Andrea