My tankless water heater manual suggested the recirculating set up shown here: diagram for recirculating system

It appears there are 2 air vents, but one is attached to an air separator.

  • Do I need both an air vent upstream of my pump and and and air separator with air vent after the recirculation pump? Why?
  • Should my boiler have a similar setup?

The air separator is in the wrong place (it should be right after the heater/boiler) in the loop, as hotter water holds less air/gasses in solution than cooler water, so that's where it's most easily/effectively separated. The cooler return side is a poor choice of location.

The high vent serves to trap bubbles that escape the separator out of the loop, since circulating pumps are poorly suited to pushing large bubbles around.

I need to check, but I'm pretty sure the expansion tank is in the wrong relationship to the pump, as well. Yes, indeed. The arrangement shown increases the liklihood of cavitation, which accelerates wear and damage to the pump, shortening its life

  • So the air separator should be before the high vent? Do I need an air separator if I have a high vent? – virtualxtc Feb 16 '18 at 19:21
  • Thank you for your answer to my question. However, I was a bit confused as your advice about the expansion tank placement is at odds with logic about air separator placement (the hot water is also likely to increase cavitations), and the the manufacture's recommended placement of the expansion tank (outside of the hot water loop). Then I realized the tank is outside the loop. – virtualxtc Mar 27 '18 at 22:27

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