Hi Tomo: I sell the AQ-170 but don't recommend it for residential use. It only modulates down to 57,000btu's and that can overheat low flows of water. That's the only down-side to the 170. I suggest the T-K1. At 165,000btu's, it's the same as the 170, but more efficient (almost 84%) so it produces more hot water, and modulates down to 37,000 btu. The T-K1 also has electronic ignition, so it uses less gas. If you need more info, please contact me at 800 851 9891. Regards, Brian. They are a piece of cake to install, heat water instantly, barely reduce pressure, and you'll never run out of hot water. Your tenants wil love you.