Reaching the design quality factors and lowering the residual losses of the cavities at the superconducting electron linac in Darmstadt (S-DALINAC) is still an ongoing activity. After installation of an UHV furnace in Darmstadt two years ago, six cavities have now been heat treated at 850 C to remove residual hydrogen from the niobium cavity surface. A gas analysis during the heat treatment procedure showed that the cavities were strongly contaminated which might explain the low Q. We will report about the furnace, the heat treatment procedure and the results of subsequent surface resistance measurements. Prior to the heat treatment the field flatness of the 20 cell elliptical cavities was measured leading to unexpected results: After almost 10 years of operation, the field flatness of some cavities was heavily distorted. This might be an indication that the frequency tuning of the cavity, which is done by compressing the cavity longitudinally, does not act uniformly on each cell even though the cavity is only supported at the end cells.