All closed-loop hydronic systems are intended to operate with insignificant amounts of air mixed with water. When excessive air is present, the system’s circulator(s) will not provide proper flow. There will likely be gurgling or swishing sounds in the piping, especially within the circulator, which is an immediate clue to the troubleshooter that an air issue is present. There might even be an “air binding” condition, where a large amount of air collects in higher portions of the system and completely blocks flow.
Modern methods for eliminating air from a system involve two concepts:
1. Forced water purging to eliminate bulk air.
2. Use of a microbubble air separator to capture and eject initially dissolved air.
Forced water purging relies on high-speed water flow through piping from some external pressure source (i.e., a hose connected to a pressurized water system, or a purging pump). The water is forced through the system to push bulk air ahead of it and eventually out the side port of a purging valve. This step is especially important when commissioning a new system, or when filling a system that has been substantially drained for service. Forced water purging eliminates the vast majority of the air from the system.
Microbubble air separation can be thought of as an ongoing process for capturing smaller residual amounts of air in the system, especially air that is present as dissolved gas molecules (e.g., oxygen, nitrogen, etc.). It’s done using one of several high-efficiency air-separating devices, such as the Caleffi DISCAL® or SEP4™ separators. These devices are designed to encourage dissolved gas molecules to coalesce into tiny bubbles that can then be captured, routed to an automatic air vent at the top of the separator, and ejected from the system by internal water pressure.
For more detailed information on air separation in hydronic systems, see:
Recurring air problems are often the result of improperly designed, installed or maintained systems.
Causes of recurring air problems include:
• Improper placement of the expansion tank relative to the circulators.
• Insufficient static pressure in the system.
• Anything that causes a sub-atmospheric pressure at the location of a float-type air vent.
• Loose valve packings or circulator flange gaskets.
• Lack of backflow prevention, combined with a drop in plumbing system pressure.
• A clogged screen on a feed water valve, allowing system pressure to drop to a point where a slight vacuum develops at the high points of the piping.
• The feed water system is completely turned off, preventing any water from entering the system to make up for minor water losses.
• A waterlogged expansion tank that allows a negative pressure to occur in the system when the water cools.
• Unnecessary draining and refilling of the system. Adding “fresh” water to a closed-loop system on a periodic basis is not necessary or beneficial.
• Use of polymer tubing without an oxygen barrier.
The suggested procedure for correcting air problems in existing system is as follows:
1. For hydronic systems connected to cold water plumbing, verify that the screen in the feed water valve is not clogged and that any valves needed for makeup water to enter the system are on.
2. Ensure that the water pressure in a system serving a low-rise building (three stories or less) is at least 12 psi in the mechanical room. If it’s not, boost the pressure in the system.
3. Look for any signs of leakage or weepage, such as water stains on the floor or piping components, or buckets placed under the discharge piping from a pressure relief valve. Correct any such leaks, snug valve packings or circulator flanges that show evidence of weepage.
4. Check the location of the system’s expansion tank relative to its circulator(s). The expansion tank should connect near the circulator inlet. This allows the differential pressure developed by the circulator to add to system pressure. If the expansion tank is located near the outlet of the circulator(s), it should be relocated because it can be the underlying cause of the recurring air problem.
See idronics issues #16 and #12 for more information on proper placement of expansion tanks:
idronics #16 idronics #12
5. Check to see if the system’s air separator is working. Old-style “air scoop” separators provide marginal performance. Modern microbubble air separators, such as the Caleffi DISCAL, provide high-efficiency air separation. On systems with recurring air issues, consider replacement of air scoop separators with microbubble separators.
6. If the system exhibits signs of significant internal air, use forced water purging to eliminate as much of the bulk air as possible.