The required extraction rate must be guaranteed even in the worst conditions, i.e. lowest possible temperature and lowest dry weather value. The same also applies if, in extended periods of dry weather, the extraction rate is reduced due to deposits and adhesions (biofilm) on the surface of the heat exchangers. In the latter case, the reduction is estimated, using complicated calculations that take various factors into account, to be around 40 % normally. This reduction in performance is made up for by having a larger transfer surface.
