Due to a hot water shortage at peak demand time, I'm replacing my two gas-powered water heater system, combined storage of 80 gallons, with a two water heaters totalling 100 gallons.

I'm confused because of a disagreement between the two plumbers who gave me estimates concerning what is necessary to satisfy the needs of the building. Both agree on stepping up to 100 gallons, but one says the heaters should have improved recovery time - that is, increasing from 40000 BTU's to 65000 BTU's per heater - and the other says remaining at 40000 would be enough.

So to make a decision it seems that I need to know the FHR need. There are ten small apartments, one bathroom each, with 12 people, all adults. There are no dishwashers or washing machines. Can anyone help me with the FHR need?

I want to make sure that the new system resolves the shortage but I don't want wasteful overkill.