The steel tank is simply a tank that is connected to the system water, and which also contains a sufficient amount of air that when the water is heated and expanded there is enough void in the tank that the water never completely fills it. The buffer compartment of air is merely compressed like a giant spring. But there is a disadvantage with this system. The air that is in the tank can become dissolved in the water and carried away through natural convection currents. The dissolved air can then become undissolved when the water condenses at the radiators, allowing the formation of bubbles that are carried through the system as entrained air, causing problems elsewhere. These problems can be solved by a special check valve installed at the tank so that the tank water does not move back into the system. The tank that has dissolved too much air and become overly filled with water is said to be waterlogged.



The second type of tank solution is the diaphragm tank, which uses a rubbery type elastomer barrier between the water and air so that the two never actually come into contact. As the water heats and expands to compress the air, the elastomer moves with it. As it cools back down, the elastomer again moves with it.



The problems with entrained and dissolved air are eliminated. Of course, a ruptured barrier means the tank has to be replaced. Regardless of which tank type is used, tank air is pre-charged at installation to a certain pressure, depending on the size and vertical height of the system, and that pressure is then maintained.

Beside the compression tank, systems also include pressure relieve valves, air purgers, purge valves, high limit temperature cut off switches, zone valves, circulators and pressure gauges. Tanks use some of these other components to control the pressure inside the tank. The ASME Boiler and Pressure Vessel Code is where the standards for systems can be found. It is the standard reference for modern day HVAC systems (heating, ventilation, and air conditioning). Hydronic systems with baseboard heat exchangers provide a much better vertical temperature profile within a room than other central heating systems. A home can be retrofitted with such a system, but home owners who chooses not to install their own systems should have basic knowledge about them before retaining an experienced contractor with expert knowledge to actually do the work.