lWe plumbed the tank into the system and as the system was filled it compressed the air and that gave it the pressure charge. If you look hard at the green tank that SDRobertson put up you will see some markings on the side. They were calibration marks I made as I filled it with cold water to pressure test. I could see the water line from condensation moving as the water filled and I marked it to corespond to the gage. If for some reason you would have to add air you can just add air with the air valve and let out the amount of water you need to . We don't use a makeup system but you could. We don't as you will never have to add water if you don't have a leak and this way we will know if we have a leak. Remember we are both using 2000+gals so if we have a leak that makeup water is trying to make up to keep the boiler wet we have a HUGE problem. We would rather know if a small leak develops.