"bobby" wrote:
I'm considering replacing a 40 gal water heater with an Rinnai tankless
heater. The Rinnai has a flow rate of 8 gpm. The shower in the master
bath has a 2.5 gpm rate but there are six 2.5 gpm body shower spray
heads mounted in the walls. Will the Rinnai have enough capacity to
provide hot water if all heads are in use?

Since this is a new house I'm not sure how well the heads worked with a
40 gal tank.

Real unlikely. Assuming 50F cold water, 120F hot water, you're going to need
8.75gpm of hot just to get 85F water. Most folks prefer a shower closer to
body temperature (say 100F), so you're going to need even more gpm out of the
heater. And that assumes that you truly are getting 8gpm out of the water
heater. Check the output curves -- you'll often see a marked reduction in
output if the input water is particularly cold.