"¿it's the same as running with the heater valve open, no?"

If you consider a radiator and a piece of hose to be equivalent, it is
the same. Close examination of a heater coil will convince you that it is
very like a radiator and does indeed cool the water. The loop of heater hose
I would consider to be counterproductive. Better to cap it off and let all
the water go through the radiator.

If the heater valve is open but the vents are closed, naturally the cooling
would be a great deal less, more like a loop of hose.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma