Answer: -I wouldn't gang them together either way. Consider zoned hot water, e.g., a heater for the bathrooms and one for the kitchen / laundry, or some other combination based on your use pattern and how your house is laid out. You could then put each of the heaters closer to where they're needed, reduce heat loss through the pipes and provide quicker hot water. -Here's a question for you: Is there an option to 'split' the system? It may cost you more, but that's what we did. We asked for two water heaters. We have a 75 gallon heater for the master bath (and its thirsty Jacuzzi tub that we are using constantly) and a 50 gallon heater for the rest of the house (two conventional baths, a powder room, a kitchen.) We can always upgrade the 50 to a 75 later if we find out it's undersized. 1) Depends on what you mean by "go on." Do you mean "get switched into the line" or do you actually mean that the heater is full of X gallons of cold water? In the second case, yes it's too late. But if you have a spare tank of hot water that can get switched into the water line then this is a good thing. How would it get switched, I wonder? Some kind of sensor on the outflow temperature. 2) I can't see any reason for that. 3) Recovery time is the issue here. What kind of fuel is it? IIRC gas water heaters recover faster. If you empty a 75 gallon tank of all of its hot water it's going to take a couple hours, I'll bet, to reheat. However, if you empty a forty and then switch to another 40, it's entirely possible that by the time the second 40 is depleted the first one is recovered. Note that I have no experiential knowledge of recovery times -- most of this is a WAG. Even though it makes some kind of warped sense.