Abstract: A tertiary heat exchanger having three sets of adjacent ducting, two stacked within the third, all of which carrying heat exchange mediums so that the heat carried by one medium in any set of a ducting is efficiently transferred to the other two. Two of the three sets of ducting are tubing and this tubing is spirally coiled in a container within the third set of ducting. The two sets of tubing are in alternating layers and the fluid flow of the heat exchange mediums are in opposite directions in each adjacent layer of the tubing. Outlet channels are provided at various points along the helical path of a ducting so as to alter the effective length of a ducting.