I will preface this by saying I looked in to these quite some years ago, but, flow rates were the problem with the tankless variety when I did. I'd pay close attention to those numbers and figure out your flow rate (of the hot water) carefully. If you're running through a mixing valve and you only use 20% hot water to get to 68 or 70 that is a very small amount of water volume, which might not be sufficient to trigger some units into action. I'm a fan of a well insulated hot water tank, and insulated hot water pipes.