I have an oil boiler serving 6 zones (5 heat and 1 DHW). Each zone has a circulator on the supply side, but there is no circulator to push the return water through the boiler. Is this setup OK, or should there be a return side circulator to move the water through the boiler more quickly? If the latter, should there be some type of loop that circulates back to the return in the event the secondary loops are not drawing enough water to take the pressure off of the primary circulator? Thanks for your help.