Hi Brian & everyone else looking at tankless options. We offer "AquaStar" with an "Ergomax" tankless coil combo. We can guarantee more than 4gpm hot water from our units, by teaming them with 23-109gallon, 99% efficient "Ergomax" heat exchangers. This means, that between 23 & 109 gallons of hot water is available immediately, at whatever flow you need, then continuous hot water at between 3-4gpm thereafter. AquaStars may be piped in parallel to supply as high a flow as is needed. The beauty of this arrangement, is, that neither the tankless unit or the heat exchanger will ever scale up (#1 killer of water heaters). This is because the AquaStar heats the same water over & over, and the coils in the "Ergomax" are heated indirectly, so they don't scale up either. This setup is also perfect for radiant heating, and all other water heating applications. If we can help anyone with their water-heating needs, please call us at 1 800 851 9891, or e-mail us at email@example.com. Sincerely, Brian Wood.