I'm told that I should boil any water freshly added to my boiler to avoid excessive rusting. For how long? I'm looking to add a manual bypass valve around my automatic feeder this summer, and I imagine that testing the work will mean pumping in fresh water. Also, I expect that my automatic feeder will be adding extra water each time I do the blowdown test.
The issue is you need to make sure it boils all of the water you added. If your setup is like mine then it also adds water to the return line which means you need to wait for all of that to get fed into the boiler. Mine feeds a few inches from the boiler on the boiler side of the hartford loop so it shouldn't take terribly long but I still give it a good 5-10 minutes of boiling.
I usually start feeding water right after the burner lights at the beginning of a heat cycle.
Weil-McLain EG-45 connected to 392sqft of radiation via two 2" risers into a 3" drop header and 2" equalizer. Using Rectorseal Steamaster water treatment to greatly reduce corrosion in the boiler.