I have an oil fired boiler for heating my home and hot water. Recently the backflow preventor has started leaking (has atmos vent). So I have found a new preventor, and in prep I was making sure I could close off the water in the boiler system to keep what's up in the radiators IN there. The spigots to shut this part of the system off are "corroded" and test turning one of them resulted in it beginning to leak. I have never replaced any of these but I am assuming there is no repairing this as it is very severely corroded all around the valve, stem, nut the works... Any tips on replacing these? And more to the point since this job will drain my boiler system tips on refilling it. It is piped into my home water supply to top itself off hence the backflow preventor. But I would imagine air in the system is going to complicate the refill process correct?

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The spigots to shut this part of the system off are "corroded"

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Assuming these are brass valves they should not corrode.

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around the valve, stem, nut the work

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Assuming that the leak is around the stem, try tightening the bonnet nut, the nut around the stem, about 1/8 to 1/4 turn.

Your system should have a tank connect to it, it will have a valve (like a bike tube valve ) on the bottom
This tank needs to have air in it so that when system expands, the (expansion tank) takes up the extra pressure, if the tank is bad, it will be full of water, you can check this by testing the air pressure in the tank, any tire air gauge can be used, the rated pressure should be listed on the tank( avg. is 13 pounds for residential units , if water comes out of the valve you will need to replace,

The tank is not the problem.. it has air, the backflow preventor does leak. My question is more one of how do I go about ensuring the system completely refills with water or do I just need to continually bleed at the radiators?

Yea, that is why I had corroded in quotes :) All around the valve is blueish whiteish greenish, I have hard water and the water in the boiler system is not treated by a softener. Will attempt to tighten the nut this evening

A little confused here. Only a little familiar with these systems.
A back flow preventer sounds like just a check valve, shouldn't have an atmospheric vent. Could this be a pressure reducing valve with a screw for adjustment? Can you post a picture of this device and valves?