"David Martel" writes:
Ransley,

I don't believe that this is an apartment building, it is a house with 6
bedrooms. The water heater is undersized for the morning use.
Let's look at your claim for the apartment. You have 90 gals. of hot
water (125 degrees?). With 15 showers running and low flow restrictors in
place you'll probably draw 30 gals of water out of the heater in each
minute. Your water heater will be out of hot water in 3 minutes (a very
short shower) unless you can put a lot of heat into that unit very quickly.
You can post about Btu and temp rise but I don't find you credible.

Dave M.

Taking the numbers a bit further, the recovery rate (by my
calculations) would only give you about 7.2 gallons/minute or enough
for about 3.5 showers steady state after the first 3 minutes depleted
the existing tank.

I did:
(235000 BTU/hour)/(60 minutes/hour)/(8.33 lb/gallon of water)/(65
degree temp difference between 45degree source and desired 100 degree
shower)
= 7.23 gallons/minute

This is an optimistic case, assuming 100% efficiency and no issues of
mixing new cold water in at a faster pace than heat can be applied.