I had a central air system installed last year with hydro heat(coil in air handler), so far it seems to be pretty good although it hasn't been real cold yet. The original system was a gas steam system. The guy left all the steam pipes as is. The system was modified so much that I can't figure out how to fill the boiler. It has a sight glass gauge for the water fill that the guy shut the valve off on the top and bottom( i opened them back up). I started draining the system to clean out all the rust like i did when it was steam . I drained about 3 gallons and never saw the sight glass drain? I didn't want to drain too much. How much water does a boiler hold and what should the level be? When it was steam I would fill it to the middle of the sight glass. I tried to trace the lines so I could fill it with more water with no success. Where does the supply line go to fill the burner?