I purchased a water heater from my brother-in-law's plumbing supply company. I thought I needed a 40-gallon heater, but realized afterward that I should've bought a 50-gallon tank. They will not let me return or exchange it.
I have a 2700-square foot, two-story home. It has 3 full bathrooms. There are four adult family members living in the house, but only my wife and I take showers in the morning. Do you think a 40-gallon gas water heater will be enough to replace the 20-year old 50 gallon gas water heater we currently have?
It's not the answer you're hoping for, but I think a 50-gallon water heater is the ideal size for a home with three full baths. However, some silver lining: In your particular case, if you only have two family members showering at same time, a 40-gallon tank should be sufficient. Be aware, though, that if hot water usage increases, or if you sell your house to a different family, your undersized unit could affect your comfort and even your home's value.