Our monastery is going to install a Garn to heat the 14 zones of our radiant heating system. The people at Garn are recommending that we redo our current system (closed system with oil-fired boilers) so that the Garn is attached directly to the radiant zones (resulting in an open system, with oil-fired backup attached via heat exchangers). They say it is worth doing so we can take full advantage of the water storage of the Garn, and draw more btu's out of the tank than would be possible through a hx. My worry is our 14 cast iron Grundfos zone pumps, and 2 cast iron Grundfos Miximizers on the injection loops, which I think will not like being in an open system. The Garn people suggest this is not a problem. Am I worrying for no reason? I have always thought cast iron in an open system meant big trouble.