A 66 gallon water heater is an aberation. It is slightly larger than a 50 gallon one but costs twice as much, or more. It is also slightly smaller than a 75 gallon one, but can cost at least as much. The FHR is a combination of the usuable water in the tank, plus the amount the heater can produce in an hour. Therefore, a larger tank, with the same size burner WILL have a better FHR than a smaller tank or a smaller burner. YOUR FHR has nothing to do with the number of bathrooms or how many people live in the house. IT depends on how much water your shower uses AND how long you use the shower, or the size of the bathtub if you have a bathing pool sized one.