no big gain to shutting down power
What hj said.
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It's fine to have two in series. It's a good thing.
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The previous posts by the others, are all right, to an extent, but they are not seeing things like the way hj pointed them out, and that is what was missing in the big picture, in my opinion. Not that they said anything wrong. That is my friendly view, which i offer to you for consideration.
If you turn off power on the first one, your cost savings gain is minimal. Why?
1.) Heat energy needed to warm up incoming cold water has to come from somewhere. Most of that energy comes from the heating coils of the HW heaters.
2.) Some heat energy also comes from the surroundings (i.e. your basement, the subsoil, the entire house, etc.) since heat flows to colder places. The incoming cold water is colder than anything else. A cold tank next to a hot tank will "grab" most of its pre-warming heat from the hot tank. Why? Heat loss is higher the greater the temperature gradient between hot and cold places. A hot and a cold tank side by side will engage in a nice little heat transfer process, and the cold one always wins. Heat never flows the other way.
3.) If you turn off power on the first tank
, and if you place a big (high and wide) insulator that includes a radiant heat barrier, between the two tanks, your basement "area" will provide some more heat to the cold tank. This is a slow process, especially when your powered-off HW tank
itself is insulated.
4.) Heat loss is higher, faster and greater where the temperature gradient between the hot and cold places is greater. (Sounds like a mantra, repeated). So, some people set their first tank to a lukewarm termperature level, so it only "pre-warms" water in the first tank. When only one tank is hot, heat loss to the surroundings is lessened, to some extent. This is what hj mentioned already, in few words ("standby losses"). Some people lower the temperature on the second tank too, since heat loss is greatest where the temperature gradient is highest. No amount of insulation will stop heat loss; it only slows down the heat transfer process.
What I might do to save money:
Lower both tanks' temperature and make sure that the first tank is not hotter than the second. Keep lowering the first tank's temperature until you find you are not satisfied with the volume of hot water you can still get after hot water has already been used in greatest demand. Keep lowering the second tank's temperature until you find that you are not satisfied with the amount of hot water you get in general.
Executive Summary:
* Avoid really-hot hot water when you want to save money. Standby losses are greatest when your tank(s) are hottest.
* Having two tanks in series is a good feature, giving you advantages you could not acquire through any other means. This set-up prevents diluting the hot water in the tank with (perhaps extremely cold in winter) water after using the tank only a bit.
* To pre-warm cold water it is possible to use the ambient heat available from the ground and surroundings by installing a low-cost uninsulated tank, in series.
david