Actually the house is under 10 years old. I suspect it's more down to the way it's used, and whether the logs are properly seasoned. My point, however, was that it's clearly not needed to heat the property, or else the windows would presumably be shut tight, so the crap coming out of it is in any case unnecessary.
You may be able to make log burners that are relatively clean, but you can't guarantee the owner will use it in the way it's designed to be.