1st hr rating=deceptive - Tricky hot water heater sizing problem
It seems the 1st hr rating is overly simplistic since it doesn't really express how much the water heater is replenishing on a per minute basis.. which is the most important factor when water usage is near simultaneous.
Let us assume the following (actual) morning condition(peak usage):
Everyone gets into the shower at roughly the same time.
Length of each shower time is 10 min.
2 showers @ 2.5 gpm x 10 min = 25 gal x 2 showers (going at same time) = 50 gal
Master bath (multiple shower heads) runs 7 gpm x 10 min = 70 gal.
This comes out to ~ 120 gal initial utilization.. which is not accounted for by the "first hour usage" since we are talking about "first 10 min usage". It seems like you need to take the size of the initial tank and add in the amount it will recharge in the first 10 min ( ?? 1/6 of the 1st hour rating??)
But then the master shower is used again, 10 min @ 7 gpm for the other individual which comes out to another 70 gal.
So the usage pattern is (in minutes)
0-10 min = 120 gal
10-20 min = 0 (break as the MBTH users switch)
20-30 min = 70
30+ min = 0
so we need to recharge 70 gal by 20-25 min to not run out of hot water.. so what does that come out to?? 210 gal first hour usage rating?? With a 120 gal tank capacity ? Seems pretty crazy to me (oversized).
What am I missing in the analysis?