In choosing how to go about this another thing to look at is the piping lay-out. I can deal with a wait for hot water to the shower or lavs but what drives me nuts is waiting at the kitchen sink. Much of the problem is that while we have reduced the flow volume of the faucets we have not changed codes related to pipe sizes. I have suggested that when a tankless is put into a house replacing a tank the first thing to do is find the best location for the new unit which may not be where the original was. If you can re-locate yoou may not need re-circ. Back to pipe size. If you look at the cross section diameter of pipe respectively 3/8, 1/2, 3/4 & 1” it is .11, .20, .44 & .78 sq in. That directly equates to volume of water to move before hot water arrives. The kitchen being the source of frustration for many customers and myself I have solved flow problems by running a 3/8 pex line direct to the kitchen. It may be on the same route as the probable 3/4” main, but with 1/4 the vol. You can’t re-pipe the whole house probably, or don’t want to, but compare the current lay-out and give a thought to pipe volume and how to get over it.
