5 ms·

So ... any actual measurements of how much energy was actually saved? Neat hacking for sure, but it seems somewhere along the line thinking clearly about the go

by crux_ 16y ago

So ... any actual measurements of how much energy was actually saved? Neat hacking for sure, but it seems somewhere along the line thinking clearly about the goal of "saving energy" got lost in the details. Although I'm not exactly experienced in the field, I'm going to guess it's quite unlikely the water heater's energy consumption was reduced by anything close to 2/3, or even by a significant amount. Most obviously, the author completely overlooked that there are two types of energy use by a tank-style water heater: 1) Heating incoming water to the desired temperature. 2) Holding the tank at the desired temperature. This setup does nothing to address the first energy need, which is the larger of the two. And, unless the timing is spot-on, it will heat a tank which is not immediately used -- which then cools off, and finally needs reheating later. If you make the (incorrect) simplifying assumptions that (a) a tank loses energy at the same rate regardless of its current temperature, and (b) the burner is instant-on/off to full efficiency, then my pre-coffee mind arrives at the conclusion that 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. Without those simplifications, it still seems likely that the 'stale tank re-heat' will use a decently large fraction of (2)'s energy. tldr; To take large chunks out of your consumption of heating energy, insulate your home better instead. Bonus: It saves you cooling energy during the summer, too!

joezydeco 16y ago
I looked into a project like this a while back and my research came to the same conclusion: holding a modern insulated tank at temperature takes a minimal amount of energy compared to heating the water. Recovery time of hot water heaters isn't the same as something like air conditioning a house where it's useful to run your house at a higher temperature when the home isn't occupied. Unless you radically change your consumption of hot water, you're gonna need the same amount of energy to heat it no matter what temperature control method you use. Something that is possibly way more efficient is waste heat recovery from a shower/tub drain. There are devices that do it, but they aren't cheap (price of copper is the killer)
- crux_ 16y ago
  Low-flow showerheads could have a decent impact as well. So would other ways of reducing hot-water use (turn the shower off when soaping & shampooing, etc), though I doubt they'd see widespread adoption.
tocomment 16y ago
So why does this method supposedly save energy with a house heater? E.g., you should turn down the heat when you go away?
- crux_ 16y ago
  Quick answer: Because in a water heater (2) is very small compared to the energy cost of (1). In a home, which is infinitely less insulated than a good tank, maintaining temperature is a lot more expensive. Edit to add: I'm not claiming zero energy savings -- I'm just saying that, compared to the other opportunities for savings available, this project's energy savings are likely to be tiny.
dmlorenzetti 16y ago
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).

by crux_ 16y ago

joezydeco 16y ago

I looked into a project like this a while back and my research came to the same conclusion: holding a modern insulated tank at temperature takes a minimal amount of energy compared to heating the water. Recovery time of hot water heaters isn't the same as something like air conditioning a house where it's useful to run your house at a higher temperature when the home isn't occupied. Unless you radically change your consumption of hot water, you're gonna need the same amount of energy to heat it no matter what temperature control method you use. Something that is possibly way more efficient is waste heat recovery from a shower/tub drain. There are devices that do it, but they aren't cheap (price of copper is the killer)

crux_ 16y ago
Low-flow showerheads could have a decent impact as well. So would other ways of reducing hot-water use (turn the shower off when soaping & shampooing, etc), though I doubt they'd see widespread adoption.

tocomment 16y ago

So why does this method supposedly save energy with a house heater? E.g., you should turn down the heat when you go away?

crux_ 16y ago
Quick answer: Because in a water heater (2) is very small compared to the energy cost of (1). In a home, which is infinitely less insulated than a good tank, maintaining temperature is a lot more expensive. Edit to add: I'm not claiming zero energy savings -- I'm just saying that, compared to the other opportunities for savings available, this project's energy savings are likely to be tiny.

dmlorenzetti 16y ago

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).