Where does it come from?
When an empty system is refilled, all the air in the pipes is compressed and pushed into the highest points in the piping system. With cast iron radiators, that often means the top of every single cast iron rad. The fresh water filling the system also contains up to 10% dissolved air. This explains why after the system is filled and bled, a few radiators have air in them again the next day.
As the heating system operates throughout the year, every valve stem and pump seal is slowly leaking at a microscopic level. There is evidence of this with treated systems as a white crust develops around all these tiny leaks. This is normal. When water leaves the system through all of these tiny openings more fresh water is fed into the system through the boiler feed valve. This fresh water is of course full of dissolved air.
The dissolved air becomes a problem when the water pressure is lowered. The fresh water comes from the building supply at approximately 80psi. The boiler feed valve delivers the water to the boiler at 25psi. This water is heated by the boiler and ends up in the fourth floor radiators at less than 20psi. The lower the pressure the bigger the air bubbles get, and the easier it becomes to block the flow of heat. For a demonstration shake a can of carbonated pop and open it fast. High pressure inside the can moving to low pressure outside the can allows the bubbles to expand and blow sticky pop all over your floor.
How do you get the air out?
Many buildings have the standard automatic air vents on the top floor radiators or on the roof. When a system is filled, these vents let air pass right through and stop when the water reaches them and a small float rises and shuts it off. Unless enough air builds up in the system this air vent will remained closed. Air vents are not effective in removing the air that is dissolved in the water.
An air separator is a small tank-like container filled with a special mesh. This mesh creates a low-pressure area for the air bubbles to adhere to, and be “scrubbed” from the system. The air separator is piped in the heating system as close as possible to the boiler and at what is called the low-pressure side of the heating pump. The reason for this is dissolved air bubbles come out of solution at the highest temperature and the lowest pressure. The separators will remove entrained air but will not reach up to the top floor and remove trapped air. Using separators and vents together is most effective.
Location Of Heating Pump, Boiler Feed And Relief Valve
We have solved air problems in buildings simply by relocating the heating pump to the supply side of the boiler. Many heating pumps were installed on the cooler boiler return pipes to protect them from being overheated. This is no longer relevant with modern, high-temperature pump materials. Moving the heating pump to the correct location allows all the energy created by the pump to add pressure to the highest points in the heating system. This higher pressure keeps the accumulated air bubbles as small as possible. Correct pump location puts the boiler, feed valve and expensive safety relief valves on the low-pressure side of the pump. This helps these components last longer.