Redundant Water Heater – Never be without hot water

In the summer and autumn, the upstream heater is turned off (optionally even its pilot light), since the incoming water is much closer to room temperature than freezing in this half of the year.

In the winter, when incoming water temperature ends up reaching temperatures closer to freezing than room temp, both are operating. So the total BTU capacity doubles when the upstream "pre"-heater is put online again late October.

One variation I suggest would be the above system, except with the main heater (downstream) being a heat pump circulation driven tank heater. Something like 50-60 gal maybe. This way, you get some "free" air conditioning in the summer. You also have benefit of redundant fuel all year around.

The down stream gas pre-heater could optionally remain on in the summer (or on any of the settings, from vacation to medium to hot settings) depending on whether the number of people in the household calls for a higher capacity of hot water.

But most gas-fired water heaters fail when the tank rusts through and dumps the contents into the floor. This setup doesn't take that into account.