We don't need no water!
June 11, 2010 6:26 PM Subscribe
I'm going to be out of town for three weeks. Is it safe for me to turn off the water heater in my high rise apartment? Each unit has its own and I'd like to save water while I'm gone.
Response by poster: Oh, I meant electricity, not water... lol
posted by Aanidaani at 6:37 PM on June 11, 2010
posted by Aanidaani at 6:37 PM on June 11, 2010
If it is relatively new there should be a setting called "vacation"--otherwise I would suggest you search the manufacturers web site for guidance and instructions. Personally, I would leave it as is.
posted by rmhsinc at 6:42 PM on June 11, 2010
posted by rmhsinc at 6:42 PM on June 11, 2010
Just turn the breaker off. Not a problem at all to leave it off for three weeks or more. Turn it on when you return. You will save electricity. I always turn the breaker off for our water heater when I will be away for a few days.
posted by JayRwv at 7:05 PM on June 11, 2010
posted by JayRwv at 7:05 PM on June 11, 2010
mmmm Legionnaires' disease
an ubiquitous aquatic organism that thrives in temperatures between 77 and 113 °F (like vacation mode). Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria (like in a shower).
posted by malp at 7:06 PM on June 11, 2010 [1 favorite]
an ubiquitous aquatic organism that thrives in temperatures between 77 and 113 °F (like vacation mode). Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria (like in a shower).
posted by malp at 7:06 PM on June 11, 2010 [1 favorite]
Also, I haven't done the math, but it could easily be true that shutting off the heat and then turning it back on requires more energy than simply leaving it on.
Nope. Energy has to be added to the system to replace the energy lost through the walls of the water heater. Energy is lost (and must be replaced) proportionally to the difference in temperature between the inside and the outside of the water heater, so the higher the water temperature over one's vacation, the more energy that will be used. This is a very simple situation and there is no way you can "do the math" so that leaving the water heater on uses less energy.
posted by ssg at 7:07 PM on June 11, 2010
Nope. Energy has to be added to the system to replace the energy lost through the walls of the water heater. Energy is lost (and must be replaced) proportionally to the difference in temperature between the inside and the outside of the water heater, so the higher the water temperature over one's vacation, the more energy that will be used. This is a very simple situation and there is no way you can "do the math" so that leaving the water heater on uses less energy.
posted by ssg at 7:07 PM on June 11, 2010
ssg, that's presuming perfect thermal conductivity from inside to outside. With an insulator, that proposition does not necessarily hold.
It's a question of whether the energy cost to replace energy lost is greater than the energy cost of heating 80 gallons of water from 72°F to 140°F. Also factored in here are various electrical and mechanical losses in the heating element itself. Pure thermal energy is insufficient to calculate the electricity required.
posted by Netzapper at 7:21 PM on June 11, 2010
It's a question of whether the energy cost to replace energy lost is greater than the energy cost of heating 80 gallons of water from 72°F to 140°F. Also factored in here are various electrical and mechanical losses in the heating element itself. Pure thermal energy is insufficient to calculate the electricity required.
posted by Netzapper at 7:21 PM on June 11, 2010
No it isnt. Its assuming any thermal conductivity. And it holds.
posted by ihadapony at 7:59 PM on June 11, 2010
posted by ihadapony at 7:59 PM on June 11, 2010
I don't know the specifics of your water heater but there's no inherent problem with turning the water heater off. My family has a summer cottage and we never keep the water heater on when we're not using it. Last person to leave turns off the hot water breaker (and the breaker for the well pump), first person to arrive turns it back on. Been doing it ever since they put in a hot water heater (in the 70s?) and none of us have succumbed to Legionnaires' yet, although that may be because it's being outcompeted by other nasty things living in the water (the distances between the well, the lake, and the leach field really do not bear thinking about).
posted by mskyle at 8:13 PM on June 11, 2010
posted by mskyle at 8:13 PM on June 11, 2010
No it isnt. Its assuming any thermal conductivity. And it holds.
No, it doesn't. There is a time factor that depends on the thermal conductivity and the temperature differential. Otherwise insulated coffee mugs wouldn't work.
It costs $x kw to raise the temperature to the setpoint. It costs $y per hour to maintain it. There will be a cutoff where $y * hours <> $x. I presume that cutoff is the amount of time it takes for the water in the tank to get to room temperature.
So yes, eventually, the temperatures will equalize. Whether it is worth it depends on how long that is.
posted by gjc at 8:23 PM on June 11, 2010
No, it doesn't. There is a time factor that depends on the thermal conductivity and the temperature differential. Otherwise insulated coffee mugs wouldn't work.
It costs $x kw to raise the temperature to the setpoint. It costs $y per hour to maintain it. There will be a cutoff where $y * hours <> $x. I presume that cutoff is the amount of time it takes for the water in the tank to get to room temperature.
So yes, eventually, the temperatures will equalize. Whether it is worth it depends on how long that is.
posted by gjc at 8:23 PM on June 11, 2010
Just flick your breaker off, it's safe and you'll save money. Not a lot but it'll be noticeable though maybe less than the lack of TV/computer/cooking/refrigeration consumption. And there isn't any risk of disease if your water supply is chlorinated and maintains pressure. If you are worried you should be just as worried about all the cold/room temperature water in your pipes.
Netzapper writes "Also factored in here are various electrical and mechanical losses in the heating element itself. "
Electrical water heaters are always 100% efficient. Any "losses" would express themselves as heat and therefor would heat the water.
posted by Mitheral at 8:50 PM on June 11, 2010
Netzapper writes "Also factored in here are various electrical and mechanical losses in the heating element itself. "
Electrical water heaters are always 100% efficient. Any "losses" would express themselves as heat and therefor would heat the water.
posted by Mitheral at 8:50 PM on June 11, 2010
I have a vacation cabin. I turn off the breaker whenever I am not going to be there for a week or more. Summer or winter.
posted by JohnnyGunn at 9:26 PM on June 11, 2010
posted by JohnnyGunn at 9:26 PM on June 11, 2010
It costs $x kw to raise the temperature to the setpoint. It costs $y per hour to maintain it. There will be a cutoff where $y * hours <> $x. I presume that cutoff is the amount of time it takes for the water in the tank to get to room temperature.
That is a common fallacy. It really makes no difference if the tank holds water or air or how long it takes to cool or reheat. By the law of the conservation of energy, the only thing that matters is the rate of heat transfer from inside the tank to the outside air. The energy that leaks out of the tank is exactly equal to the energy put into the tank. If you lower the temperature of the water in the tank for any period of time, the energy leaking out of the tank decreases and you save money. The fact that it takes a while to reheat the water to the set point is irrelevant. The amount of energy to reheat the water will always be less than the energy to hold the water at a steady higher temperature. This is because the rate of heat loss through the walls of the tank (which is the only thing that counts) is higher under the steady temperature scenario than the cool and reheat scenario.
posted by JackFlash at 11:33 PM on June 11, 2010 [7 favorites]
That is a common fallacy. It really makes no difference if the tank holds water or air or how long it takes to cool or reheat. By the law of the conservation of energy, the only thing that matters is the rate of heat transfer from inside the tank to the outside air. The energy that leaks out of the tank is exactly equal to the energy put into the tank. If you lower the temperature of the water in the tank for any period of time, the energy leaking out of the tank decreases and you save money. The fact that it takes a while to reheat the water to the set point is irrelevant. The amount of energy to reheat the water will always be less than the energy to hold the water at a steady higher temperature. This is because the rate of heat loss through the walls of the tank (which is the only thing that counts) is higher under the steady temperature scenario than the cool and reheat scenario.
posted by JackFlash at 11:33 PM on June 11, 2010 [7 favorites]
We usually turn the water heater down to vacation mode when we're gone for longer than a week or two. When we come back we turn it back up and run the hot water in the shower and the kitchen for a while to flush things out.
posted by hat at 9:50 AM on June 12, 2010
- OSHA: Legionnaires' disease
- CAA-Québec - Flushing your water heater (see the part about leaving on vacation)
- Is it safe to turn down your water heater temperature? : TreeHugger
posted by hat at 9:50 AM on June 12, 2010
That is a common fallacy. It really makes no difference if the tank holds water or air or how long it takes to cool or reheat. By the law of the conservation of energy, the only thing that matters is the rate of heat transfer from inside the tank to the outside air. The energy that leaks out of the tank is exactly equal to the energy put into the tank. If you lower the temperature of the water in the tank for any period of time, the energy leaking out of the tank decreases and you save money. The fact that it takes a while to reheat the water to the set point is irrelevant. The amount of energy to reheat the water will always be less than the energy to hold the water at a steady higher temperature. This is because the rate of heat loss through the walls of the tank (which is the only thing that counts) is higher under the steady temperature scenario than the cool and reheat scenario.
I see what you are saying, and I think you've changed my mind.
At least with electric-fired heat. This all goes to hell if you are talking about a gas-fired tank, I think, because of the higher inefficiencies of gas heating.
posted by gjc at 11:22 AM on June 12, 2010
I see what you are saying, and I think you've changed my mind.
At least with electric-fired heat. This all goes to hell if you are talking about a gas-fired tank, I think, because of the higher inefficiencies of gas heating.
posted by gjc at 11:22 AM on June 12, 2010
This all goes to hell if you are talking about a gas-fired tank, I think, because of the higher inefficiencies of gas heating.
To the first order, it doesn't change anything at all. Efficiency just measures the amount of heat that goes into the water vs. how much goes up the flue stack. That doesn't change the fact that the amount of heat leaking out of the tank determines your energy bill. It just means that say, 20% of the heat goes up the flue when the heater is running.
To a second order effect, the gas heater actually favors turning off the heat when you leave. The efficiency of heat transfer between the flame and the inside of the heater depends on the temperature difference between the flame and the metal flue going up the center of the tank. If the water is cold, then the transfer of heat is more efficient. More heat goes into the water and less goes up the stack. This effect is small compared to the total heat bill but favors turning off the heater.
----
Here is another way to think about the problem. Assume that the water in the tank is kept at a steady 140 degrees. Assume that given the size and insulation on the tank it takes 1 kilowatt-hour of energy per day to keep the water at 140 degrees. 1 kwh goes into the tank each day and 1 kwh leaks out of the tank each day to maintain a steady 140 degrees.
In the steady temperature case, the heater uses 3 kwh of energy over a three day period.
Now consider the second case. The first day you leave the water at 140 degrees and use 1 kwh of energy. The second day you turn off the heater. The water starts to cool down gradually to, say, 100 degrees. Here is the key part. As soon as the water goes below 140 degrees, the heat loss rate begins to decline. So the important conclusion is that on day two you lose less heat than if you kept it at 140 degrees. So let's say that on day two you lose only 0.5 kwh of heat instead of 1 kwh. But you consume zero kwh of energy on day two because you keep the heater off all day.
Now on day three you turn the heater back on. It takes exactly 0.5 kwh of energy to heat the water back up to 140 degrees, exactly the same as the amount you lost on day two. Then it takes an additional 1 kwh of energy to maintain 140 degrees for day three.
So over the three days you use 1 + 0.5 + 1 = 2.5 kwh. This is 0.5 kwh less than you would have used to maintain 140 degrees for three days. This difference represents the reduced loss of heat on day two. It takes exactly the same amount of energy to reheat the water as you lost on day two. By reducing the heat lost on day two, you reduce the total energy required.
posted by JackFlash at 12:03 PM on June 12, 2010
To the first order, it doesn't change anything at all. Efficiency just measures the amount of heat that goes into the water vs. how much goes up the flue stack. That doesn't change the fact that the amount of heat leaking out of the tank determines your energy bill. It just means that say, 20% of the heat goes up the flue when the heater is running.
To a second order effect, the gas heater actually favors turning off the heat when you leave. The efficiency of heat transfer between the flame and the inside of the heater depends on the temperature difference between the flame and the metal flue going up the center of the tank. If the water is cold, then the transfer of heat is more efficient. More heat goes into the water and less goes up the stack. This effect is small compared to the total heat bill but favors turning off the heater.
----
Here is another way to think about the problem. Assume that the water in the tank is kept at a steady 140 degrees. Assume that given the size and insulation on the tank it takes 1 kilowatt-hour of energy per day to keep the water at 140 degrees. 1 kwh goes into the tank each day and 1 kwh leaks out of the tank each day to maintain a steady 140 degrees.
In the steady temperature case, the heater uses 3 kwh of energy over a three day period.
Now consider the second case. The first day you leave the water at 140 degrees and use 1 kwh of energy. The second day you turn off the heater. The water starts to cool down gradually to, say, 100 degrees. Here is the key part. As soon as the water goes below 140 degrees, the heat loss rate begins to decline. So the important conclusion is that on day two you lose less heat than if you kept it at 140 degrees. So let's say that on day two you lose only 0.5 kwh of heat instead of 1 kwh. But you consume zero kwh of energy on day two because you keep the heater off all day.
Now on day three you turn the heater back on. It takes exactly 0.5 kwh of energy to heat the water back up to 140 degrees, exactly the same as the amount you lost on day two. Then it takes an additional 1 kwh of energy to maintain 140 degrees for day three.
So over the three days you use 1 + 0.5 + 1 = 2.5 kwh. This is 0.5 kwh less than you would have used to maintain 140 degrees for three days. This difference represents the reduced loss of heat on day two. It takes exactly the same amount of energy to reheat the water as you lost on day two. By reducing the heat lost on day two, you reduce the total energy required.
posted by JackFlash at 12:03 PM on June 12, 2010
hat writes "Legionella specifically is not a huge death risk unless you're immunocompromised, very young, or very old, but a PubMed search indicates that a not-insignificant percentage of the 10,000-50,000 cases in the United States each year may be attributable to water heater contamination in homes and group residences. Chlorination is not enough to keep it down. Also, it's not the only disease-causing bacterium that grows in domestic hot water tanks."
Lowering the temperature of your tank to 40-50C can increase the risk if your tank is already contaminated with the bacteria because it is the ideal temperature for the growth of that bacteria. However there is no greater risk posed by turning your hot water tank off than than that posed by the cold and uncirculated hot water lines that run from the street to and through out your house.
Running the water after a long absence is a good idea anyways to flush the system of dissolved lead from brass fixtures.
posted by Mitheral at 3:25 PM on June 12, 2010
Lowering the temperature of your tank to 40-50C can increase the risk if your tank is already contaminated with the bacteria because it is the ideal temperature for the growth of that bacteria. However there is no greater risk posed by turning your hot water tank off than than that posed by the cold and uncirculated hot water lines that run from the street to and through out your house.
Running the water after a long absence is a good idea anyways to flush the system of dissolved lead from brass fixtures.
posted by Mitheral at 3:25 PM on June 12, 2010
This thread is closed to new comments.
Also, I haven't done the math, but it could easily be true that shutting off the heat and then turning it back on requires more energy than simply leaving it on. The thermal mass of 80 gallons of hot water is substantial. Letting it get cold and then heating it back up might not actually save anything over letting it put in the energy required to maintain the heat that's already there.
The insulation in a modern water heater is excellent. We regularly lose power, and yet maintain hot water for several days if we're sparing with its use.
posted by Netzapper at 6:31 PM on June 11, 2010