As clean yes they are required to but they usually aren't as efficient at transferring heat to the house so more heat is wasted up the chimney. They also generally use cheaper baffles attachments for the tubes latches etc. You can still get a decent life out of them but they usually don't hold up as well as the more expensive modelsSo...are the cheaper ones really as clean and efficient as they claim? They claim to burn just as clean as most of the expensive models.