In any hydronics system, the point where the thermal expansion tank joins the piping is called the “Point of No Pressure Change.” Since water cannot be compressed or stretched, no water can leave the hydronic loop when a pump starts/stops. Circulator pumps create a pressure differential when running, which upsets the pressure balance and causes fluid to flow. The pump’s differential pressure will be added to the loop if it is installed after the thermal expansion tank – Pumping Away.
The added pressure causes air bubbles to shrink, which makes them less buoyant and helps carry them through the loop where they can be removed by an air elimination device such as an automatic air vent or separator. Pumping Away helps eliminate air, enhances quiet performance, and helps systems run more efficiently.
Warning: Pumping towards a thermal expansion tank has the opposite effect: pressure will decrease at the systems’ highest elevation; air bubbles will expand and gather together; noise will be created during operation; customers will complain; nuisance no-heat calls will occur; and the potential to damage system components will be present.
(from Grundfos Handbook)
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