All heating systems must have....
a method of creating a minimum static pressure in the system...to bleed out air and ensure pump bearings run ok etc
a method of allowing the water to expand in volume as it is heated without an excessive pressure rise
a safety system in case of excerssive pressure rise
And there are two common and one not so common system setups to provide the above requirements.....
A conventional open vented system incorporating a feed and expansion cistern. The vessel height provides a static pressure, the open vessel accomodates expansion water (as the system heats up) via the feed pipe. And the safety vent pipe provides addtional safety when things go wrong. Some varients may allow combined feed and vent but generally only when other safety systems are incorporated ie overheat stats.
The sealed system. A vessel containing pressurised air (or nitrogen) provides the static pressure. The vessel also accomodates expansion water and a moderate pressure increase is allowed for as the air is further compressed. A safety valve provides additional safety as does the overheat stat on the boiler.
The third and rare method (by todays standards) is the "semi-sealed" setup. These are the Hydro Spartens, Primatic cylinders, some Elsons and Harton combined cisterns. Here the static pressure is created by the cold cistern, the expansion water is accomodated by compressing a trapped air bubble (within the units above) and to provide additional safety the system is open to the atmosphere via the cylinder safety vent pipe on the cylinder. Since primary and secondary water could mix (do to lack of air bubble etc) no inhibitors should be used since the hot water could become contaminated.