webesto or espar work well, I have an 1990 espar that needed some work when I bought the boat but works well now. Mine is forced air, I believe they are less cost but alot of people are happier with the hydronic systems as they are quieter and can also double for hot water. Higher cost but more bang for the buck. I also have a seaward(?) heater box that works off engine heat while under way but no use while on the hook.