thanks for the reply.
When drained it looked like 7l. The added hoses are only 2x 30cm. or so, not much extra volume.
The old connection is used to go in and out of the circulation, so the in and out hose to the new pump are connected to the exact place where in and out of the old pump were, via custom built adapter flanges with hose connectors.
Can't check the circulation, but will try again with other pump, pully driven with impeller this time. Not sure what caused the damage, but the pump was leaking a lot. I disassembled and it turned out to be completely broken, the bearing loose and the aluminium impeller wheel
all edges. Could bee the debris of that is blocking something inside... but a water
hose gives good throughput.
oh yes, there has been some oil
in the system, (filled up in the wrong opening)
and the system has been frozen once, popped out a frost protection lid, not sure what it;s called.