Saludos.

The primary circulator moves the boiler water through the primary circuit. When the flow of cooler water returns from the secondary circuit, the hot primary water, ...
http://www.t4engr.com/understanding_primarysecondary.htm

Immediately after these two actions the pump will operate producing a flow of primary water within the boiler through the calorifier and main heat exchanger.
www.icontraining.co.uk/TM10.aspx

The Tank-in-Tank is connected to a boiler via the primary circuit. The hot primary water generated by the boiler is circulated through the outer tank and heats the ...
www.acv-uk.com/tech.htm