3 ms·

reheating a stale tank to a target temperature would use exactly the same energy input as holding it at the target temperature the whole time anyway Essentiall

reheating a stale tank to a target temperature would use exactly the same energy input as holding it at the target temperature the whole time anyway Essentially this same line of reasoning was used during the 1970's energy crisis to argue that night setback of a thermostat wouldn't save energy. "You spend as much energy to reheat the house as you save by letting it cool down." Unfortunately, that argument is incorrect. Think about the overnight cycle, during which no water flows in or out of the tank, and the water ends up at the same temperature as it started. The total energy you have to put into the tank equals the heat lost through its insulation. That heat loss is driven by the temperature difference between the hot water and the surrounding air. Lowering the water temperature means the tank loses less energy over that time. Therefore you have to put less energy in. In other words, you were right to identify your assumption (a) as incorrect. The tank's heat loss does, in fact, depend in a very direct way on the water temperature.

adrianN 16y ago
There certainly are real energy savings. But without any numbers we can only speculate whether this whole setup is worth the effort. A simpler way to save energy would have been to wrap a thick blanket around the tank to improve the insulation. A better way would have been to replace the tank with a tankless heater.
crux_ 16y ago
Assumption (b) is probably incorrect too, you know. ;) That section could probably have been omitted entirely, since my point lay more in the following sentence: > it still seems likely that the 'stale tank re-heat' will use a decently large fraction of (2)'s energy. In other words, this project tackles the minority energy user (only 25% of all heating), then addresses the minority energy consumer within that (far more energy is used heating fresh cold water), and does so only partially (vs, say, switching to a fully on-demand heater).

adrianN 16y ago

There certainly are real energy savings. But without any numbers we can only speculate whether this whole setup is worth the effort. A simpler way to save energy would have been to wrap a thick blanket around the tank to improve the insulation. A better way would have been to replace the tank with a tankless heater.

crux_ 16y ago

Assumption (b) is probably incorrect too, you know. ;) That section could probably have been omitted entirely, since my point lay more in the following sentence: > it still seems likely that the 'stale tank re-heat' will use a decently large fraction of (2)'s energy. In other words, this project tackles the minority energy user (only 25% of all heating), then addresses the minority energy consumer within that (far more energy is used heating fresh cold water), and does so only partially (vs, say, switching to a fully on-demand heater).