tankless
One reason is that the majority of tankless heaters, any brand, do not have T&P valves. They have "P" valves because the units are so well controlled that they cannot "run full blast" without a water flow, at least that is what the small print says. I, for one, do not believe it so I always add a T&P valve to the installation. But if it runs for ANY AMOUNT OF TIME without flow the heater WILL start hammering because it it generating steam. THEREFORE, if it is running and not banging there is a flow somewhere.