The whole point of putting a tankless in series with a tank type heater
escapes me. Once the 50 gals of hot water in the tank is gone, for
the tankless to be of any significant benefit, it's going to have to
have enough capacity to pretty much heat the incoming water up to full
temp. This means a gas fired unit of sufficient capacity. Once you
have that, what's the point to having the tank heater too? It just
defeats one of the main features of the tankless, which is no standby
loss of heat from the tank.

And forget the greywater heat exchanger, as you're never going to
recover enough heat from tepid water going down the drain to heat
incoming fresh cold water anywhere near to the point of it being
usable. Just more pie in the sky ideas, without regard to the real
world.

Well a small single faucet 200 buck unit probably wouldnt help at all


but any raise in incoming water temp will yield more hot water...

witha new hot water heater adding a tempering valve and setting btank
to max will help some

the OP should of purvhased a high BTU tank for greater recovery.

if theres space adding a second regular tank in series should fix his
problem./

frankly i HATE running out of hot water!