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The Fine Art of Purging Air Air trapped in the piping is the natural enemy of hot water heating systems. Air circulating in the system makes noise. When large enough pockets of air collect, circulation of water (and heat) can be reduced or completely stopped. Air combines naturally with water as it falls from the sky as rain. Cold water holds surprisingly large quantities of air. When that water is heated it surrenders the air. You have seen this happen hundreds of times when you draw a glass of cold tap water and leave it on the counter. An hour later the water has warmed from roughly 50° F to room temperature and small bubbles cling to the side of the glass. A well designed hydronic system will purge air as it leaves the boiler. An air purger is usually installed on the supply side of the boiler, right after the expansion tank and ideally just before the circulating pump. Since the water is hottest at this point and the system pressure is lowest the air can be separated from the water before it gets trapped out in the heating system. Some older more complex systems may require purging vents out in the system but usually the arrangement shown and a careful startup routine are all that is required to purge the system and keep it free of air. See Flash Demo |
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