Can a circulator that is usually used in hydronic heating systems be used to improve free water flow, coming from a tank? Can it replace a pump. Pressure on the outlet of the pump is not important, but the flow rate is. I have a grundfoss circulator that is capable of circulating 13 qubic meters per hour, but to my knowledge a circulator can only be used in closed systems. The water would be coming from a tank and static pressure available at the circulator suction side would be 0.2 bar when the water level in the tank is the highest. It should pump the water through an heat exchanger and out to drain line.
Circulator pumps can be used in open loop systems (though I think these are typically aluminum or brass rather than cast iron since the latter will corrode in an open loop system).
There is no way to know if your particular pump will suffice because we don't know the head loss of the hoses or the heat exchanger. I'm assuming the 13 M^3/hr figure is pumping against no head. You will of course flow less the more back pressure involved. You also have not said what flow you actually need. If you know the model number of the Grundfos, it should be a simple matter to download the pump curve. That will tell you the flow at various head pressures.
You could measure the head loss of the heat exchanger by putting pressure gauges on the input and output of the exchanger. The difference is the head loss. Bear in mind, however, that the head loss will depend on flow rate.
Probably the easiest thing to do is just connect the pump as you envision and see if you get enough flow.