I thought most cold water heaters were both electric and propane. It uses little propane to keep it hot when you're plugged in. occasionally I hear ours kick in. The atwood Rep adjust flame to make it quieter. I don't know about soot. I've been wondering if the electric side is working properly, seems the propane should not have to kick in at all when plugged in. Have to get around to checking it. Some units are not supposed to use electric and propane, at the same time but our 05 says ok. The switches are confusing. The one for electric only glows if the propane goes out or vice verse.