Abstract: The modular heat exchanger system comprises at least a primary heat exchanger having two parallel manifolds with a chosen plurality of heat exchange tubes running therebetween, the manifolds defining a water pathway therein within which baffles are provided to produce a serpentine path through the manifolds and heat exchange tubes of the exchanger, one of the manifolds being an intake manifold and the other being a return manifold having a terminal return chamber, the chamber feeding into a return tube extending through the manifold and exiting the opposite end thereof, the tube being of smaller diameter than the manifold and creating an annular channel therearound through which liquid flowing through the exchanger is routed.