If done properly, no problem, at last with copper rads. It must be flushed with water before using. Here's how we generally do,
1. Drain Loop.
2. Remove rads.
3. Make a loop with the rads ... Pump ... Rad 1 .... Rad 2 ... Gallon Container for solids to drop out .... Pump
4. Fill container with vinegar solution .... run pump ... fill container again (as needed)
5. Next I drop the container and leave open ended discharge into sink .... instead of pump, use tap water ... There are no algae and spores in tap water or it wouldn't be tap water.... water supplier would be shut down, Leave tap water in a an open container for a bit and yes, your tap water will absorb algae, spores, years, etc from air. Note that distilled water if left in open container will do the same. While freshly distilled water has a PH of 7.0, if your container is left open, the water will absorb carbon dioxide, creating carbonic acid and pH will drop to acidic range of 5.8 or a little lower. I change the tap water temp and volume to shake any flux or whatever loose.
6. Finally will run freshly prepared DW on sealed container thru the loop for 4-6 hours, replace it with another filling and circulate again .... before putting in the final DW with desired algaecides and corrosion inhibitors.... is specially prepared manufacturer coolant depending on owner wishes.
It must be noted that vinegar is best avoided with aluminum rads and can cause problems with blocks and fitting with aluminum components or fasteners.
But if you want to do a more thorough job, I'd recommend the "Blitz Kit" prepared specifically for this purpose. I have used it after vinegar cleaning and was amazed at the aadditional amount of crap collected in the container.
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Mayhems Blitz is the most comprehensive cleaning kit ever released for liquid cooling computers. It has been developed to clean you radiator’s better than new and leave your system in perfect condition. Please remember you must use either with the Blitz pro Kit or the Blitz basic Kit. Do not use...
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Again not to be used in aluminum rads or in anything except copper rads, keep away from other metal parts in your system.