Because the tank's air cushion fixes the pressure at its connection, the circulator's pressure rise is added downstream of that point and its pressure drop subtracted upstream. Put the pump just after the tank and the whole system gains pressure when it runs; put it before the tank and parts of the system drop toward vacuum, pulling in air and causing pump cavitation and noise.
Tank's on the return and the pump's on the return pumping into it, the point of no pressure change is in the wrong place, that's your air problem.
Also heard as
- PONPC
Related terms
Pumping away
Placing a hydronic circulator just downstream of the expansion tank so it pushes water away from it.
Expansion tank
A closed tank with an air cushion that absorbs the expansion of heating water as it warms up.
Air separator
A hydronic fitting that pulls air bubbles out of circulating water and sends them out through an automatic vent.