I've cleaned out quite a few chinese heaters fitted to boats, IMHO the problem is because you can mess with the fuel settings they soot up, they really need running on full once a week to clean out the burn chamber, maybe the decent heater manufacturers didn't allow the owner to fiddle with the fueling for a reason
Don't get me wronge they work fine but need looking after more, not a fit and forget thing like a decent heater, TBH most heaters are way over sized for the application, most 40+Ft narow boats get on fine with 3KW (ish) sized units, fitting a 5KW in a van meens either too hot or you run it at low setting and soot up the burn chanmber ?