About 3200 sf, one room really sucks the heat out as the circulator for it never shuts off. I had a rust issue early on between the clean out & the actual use water. This is just my opinion, and i'm not a chemist, but that 'clean out' process was a waste of time & money. A person can get inside one of these units and wipe & clean everything by hand. I had to with the slimy rust that formed. The clean out did nothing to stop that, actually made it worse [time between draining & refill] Just mt 2 cents. I'm rust free now, and happy with the Garn. What it replaced [moving wood into the basement, a chimney so plugged with creosote, smoke that would fill the valley here] I threw the chimney brush in the scrap metal bin. I don't know why one unit has leaks at the welds, there are many hundreds of Garns out there with no problems. Many OWB's have failed for leaks, even some stainless tank ones.Thanks for the responding. How much area are you heating? I assume you are following the water testing program. Any signs of corrosion?