what is the difference between sealed and unsealed systems?

a unsealed system has a small f/e tank in loft. fed by a ball valve from the mains water. a sealed system has an exspansion vessel (like a red foot ball) instead of a tank and the system is filled by a filling loop of the mains water to a certain pressure. this system is under pressure. unsealed is gravity pressure

hi-spec plumbing
January 2007