Your questions answered: HVAC: Air separation in hydronic heating systems

Jim Swetye, technical training expert at Grundfos Pumps Corp., tackled unanswered questions from the May 20, 2015, webcast on air separation in hydronic heating systems.

By Jim Swetye, Grundfos Pumps Corp. May 28, 2015

Q: Where is the best location for an air separator, downstream or upstream from the expansion pump?

Jim Swetye: The air separator should be upstream from the pump, which is a low-pressure area. Downstream from the pump is a high-pressure area, which is not conducive to air separation.

Q: What is the recommended minimum water velocity to move air pockets?

Swetye: The minimum velocity is 2 fps. Obviously, a larger pipe diameter requires a larger flow rate.

Q: Can you please recap CV (flow coefficient or flow capacity rating of a device) values and the impact on air separators and the system?

Swetye: The CV value is an indicator of the flow rate that will result in a 1-psi pressure drop across a device. This value has no direct impact on the ability of a separator to remove air, but it will impact the system by increasing total pumping head, which can be a waste of energy if the CV value is too low. And obviously, because of related sizing issues, the CV value certainly has an indirect impact on separator performance.

Q: When should micro-bubble separators be used versus the tangential type?

Swetye: Tangential separators remove bubbles, but not micro-bubbles. Only the micro-bubble type separator can remove the micro-bubbles. Because micro-bubbles are less of an issue in chilled water systems, they are not applied as often in those systems.

Q: Are large systems more prone to air problems than small systems (all things being equal)?

Swetye: They are both equally prone to air problems when improperly designed, installed, or maintained.

Q: How reliable are automatic high-point air vents compared to manual high-point air vents?

Swetye: Manual-type vents, such as the coin vent bleeder, have a reputation for very high reliability. This is because they are seldom used in a properly-functioning system, so there is little wear on them. They can be somewhat prone to jamming in the closed position if they have not been used for years and if there is a mineral buildup. Conversely, float-type vents also provide long trouble-free service in a properly functioning system. However, mineral buildup can lead to the float sticking or the vent hole becoming clogged. Also, either of these types of devices can provide long, trouble-free service when the system is properly designed, installed, and maintained.