One option would be ... 10:28 Jan 29, 2011
... "prone to clogging", I think: the gases contain "sticky matter" (soot) which remains on the surfaces of the heat exchangers, reducing the interior diameters and thus efficiency.
One problem is that even in German, it's not the gases themselves which are "prone to clogging", but the heat exchangers. So I don't know whether the expression would work in English. Maybe one would have to write: "have a tendency to clog the exchangers", or something along those lines. | 
The translation workplace 
