Just because a hydronic system appears to be completely filled, often times there is more in the system than what we think. Water in a hydronic heating system should be free of air bubbles and dissolved gases, such as oxygen and nitrogen. Dissolved gases in a hydronic systems come together to form microbubbles. These microbubbles are very difficult to remove due to their low velocities in the system. If we don’t removing these dissolved gases, many problems can arise. These dissolved gases tend to accelerate corrosion in a system. They also can eventually form larger air pockets causing noises in the piping and heat emitters, or can cause a system to have reduced flow and heat output. The best place to remove all air from a system is where the system has the highest temperature and lowest pressure. This happens to be between the boiler supply and the system pump, commonly referred to as the point of no pressure change. All air eliminators come with a taping on the bottom to allow the expansion tank and boiler water fill system to attach.
Now that you know why we need to eliminate air in a system and where to install it, the question is “What should I use?” Selecting which air elimination is best for the application does not always mean it is the cheapest option.
Cast iron air scoops have been around for many years and became known as the air eliminator to use. They do their job when installed properly. This means a full 18” on straight horizontal pipe is installed on the inlet side, to allow the water to settle and air to travel to the top of the pipe. Without the 18” of pipe in front of the air scoop, it becomes an expensive Tee for the expansion tank. Although many of these had elbows installed on the side, the old cast iron boilers seemed to perform “good enough”. Now that we have high efficiency boilers, with smaller passageways, it is even more critical to have all the air out of the system.
This brings us to the modern air separator. These are known by many different names from air scrubber to air eliminator to micro-bubbler. The main thing to know is these came around to make sure not just that the air was out, but to create an area for the microbubbles in the system to coalesce and vent out. These air separators have a larger chamber where a scrubber or mesh is located. When water flows through this chamber, the water is thrown around to allow the microbubbles to be released from the water. As more of these microbubbles are released, they gravitate to the top of the air separator and vent off. This style of air separator is the most practical solution for modern systems, with models available for vertical or horizontal applications.
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