I thought I had it fixed by replacing the expansion tank, looks now like the pressure is going up to 40 psi, it started out at around 30 with the new tank. I manually opened the autofill and thought it may have too much water and it was opening to get rid of the excess . Help Thanks

I'm a little confused about how the water gets into the system. I have an oil burner. I followed the main water line coming into the house and I see that it goes into the boiler. How does that water enter into the baseboard system? I thought the baseboard was a closed circuit... The reason I'm asking is b/c I added a room and Im adding baseboards to that room. I posted the other day and have done some research already. I used the Slantfin program to figure out how much i need. I have one zone that splits at the boiler w/ 3/4" pipe and returns w/ 1" pipe. So I'll be fine w/ the extra baseboard. So, this is really what I need to know. 1. How do I drain the system to add the extra pipe. 2. How do I fill it back up/bleed the system when I'm done. Thanks!