Hi all:

I've got a tankless water heater which I love - as I tend to take
extended showers sometimes and I love not worrying about when it will
run out. I recently have upgraded my shower, however, to include
multiple shower heads/body sprays, and now it appears the tankless
doesn't have the GPM output to support the flow of these devices (the
water is pretty much just a trickle when they are all on). I know
there are at least two choices on how to solve this, neither of which
I am particularly enamored with.

1) Add one or two more tankless water heaters in parallel in a
manifold - this is extremely expensive
2) Replace the tankless with a large (e.g., 75 gallon) traditional
water heater - I don't like this one since I may go back to running
out of hot water, although it would clearly provide the flow I need.

What I am wondering is about a third option - putting a 75 gallon
water heater in series with the tankless - the 75 gallon being located
after the tankless in the run. While I understand that once the 75
gallon is out of hot water the flow output would definitely be as low
as the tankless puts out, what I don't know is what happens before
that. Say the tankless puts out 5GPM while the pipes themselves (and
the tank) can put out 50GPM. If I use this set up, does that mean I
can get up to 50GPM until the tank runs out, and then the system will
only be able to put out 5GPM, or will the rate limiting of the
tankless limit the whole system throughout and the tank will only put
out 5GPM even when it is full of hot water?