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Radiant Slab Comments To the Editor: I have some comments on "Radiant Slab Techniques" (8/92). The author explains that if different slab areas are covered with materials having significantly different thermal resistance, additional mixing valves are required. This is not a must, because the floor surface temperature, and thus the energy output, may also be controlled by varying the hose spacing. Regarding oxygen-diffusion barrier hose, only about 20% of the oxygen content in a hydronic circuit can be directly attributed to the oxygen permeation through a "nonbarrier" tube. Sources of oxygen ingress include a failed or undersized diaphragm expansion tank, loose pump flange gaskets, and micro air leaks at screwed/compression joints, etc. The German Standard DIN 4726 states two more ways of dealing with corrosion,