Most of the boiler callbacks our customers receive are caused by residual air left in the system. The plumber thinks he has purged it right the first time, but he ends up wasting time and money doing it again, as his once happy customer stands over his shoulder with crossed arms. Sound familiar?
Air left in a hydronic system can cause a lot of problems, such as noises in piping, low flows, corrosion, uneven heat patterns, and more. This is why it is important for all hydronic systems to be properly purged before the system is placed into operation. In this article, weíll cover purging a hydronic system with homerun distribution piping; in an upcoming issue, weíll tackle the more involved process of purging a primary-secondary system. A fast and effective way to accomplish this is with fast-fill purging. Fast-fill purging is fast-moving water, starting near the outlet of the boiler, moving through the system piping, and then out of the system through an open boiler drain valve installed upstream of a full-port ball valve located near the inlet of the boiler. This is a good location for the ball valve because it lets you flush any other foreign matter out of the system before it can enter the boilerís heat exchanger. It also ensures that we have gone through the entire system, from beginning to end.
The first step in fast-fill purging is connecting one end of a garden hose to the open boiler drain valve, with the other end of the garden hose placed into a five gallon bucket. The five gallon bucket is to help us see air bubbles as they are expelled from the system. The automatic make-up valve is then placed into fast-fill mode. Some automatic make-up valves donít have this fast-fill feature; in this case, a neat trick is to install and open a full-port ball valve in parallel with the automatic make-up valve as a bypass. As the system begins to fill, water flows out to the distribution system, pushing air through the piping and eventually out into the five gallon bucket, as shown in the figure. The purge procedure should continue until the water being expelled is free of any visible air bubbles for at least one minute.
Systems with more than one heating zone should be purged one zone at a time. Each zone being purged should also be purged one circuit at a time. This will allow the maximum flow through each circuit, forcing as much air out of the system as possible. This can only be done if the return manifold has isolation valves installed on each circuit. Start by closing all of the circuit isolation valves on the return manifold, except for the one you are purging. If you are zoning with zone valves you will need to manually open one zone valve at a time, starting with the zone you are purging. After a circuit has been purged, and has no visible air bubbles for at least one minute, open the next isolation valve on the distribution manifold and close the previous one. Once you have purged each circuit on a zone, itís time to move on to the next zone. After all zones have been purged, the boiler drain valve and (if installed) the bypass valve can now be closed. The automatic make-up valve should then be placed back into normal operation mode. Donít forget to open the full-port ball valve near the inlet of the boiler. The last thing to do is set the system pressure at 12-15 PSI. If the system pressure is higher than you want, just bleed some water through the boiler drain valve. Any small or dissolved air bubbles left in the system will be pushed through the piping back to the air separating device located near the boiler, where they can be expelled from the system. If you follow this procedure, you should have a quiet and smooth-running system.