From my physics lessons I learnt that leaving the water heater on 24/7 is actually cheaper than switching it on before showering and off after showering. Is this true? Have any experiments been done in this regards. Water heater specs:1200W and 50 litre capacity
It’s easier to keep something hot than to get it hot.
No cites just experience.
No, it is not true.
I think you need to find a new physics teacher.
If (1) the water heater was perfectly insulated and (2) electricity cost the same 24/7 then it wouldn’t make any difference at all in the cost. However, (1) is never true and (2) is sometimes not true (check with your power company).
(1) Since the water heater isn’t perfectly insulated, there will be heat loss from the water heater to the surroundings. The amount of heat loss is greater when the difference in temperature is greater, so this makes it cheaper to heat up the water as needed.
(2) Many power companies have “peak” hours (typically during the day) and “off peak” hours (typically at night). They charge less during off peak hours to encourage people to balance out their power loads, which helps prevent overloading on the system. This means it is cheaper to heat up the water at night, then switch off the water heater, and hope that you have enough hot water to get through your morning shower.
I guess the bottom line for me is that its cheap as hell to keep the water hot all the time. How much money could you really save by leaving it off. My powerbill is 60 dollars a month. A significant chunk of which has to come from keeping my house at 65 and my lights on whenever I want, stereo on as loud as I want, I never turn off my computer . I mean if I have to turn the water heater on, wait 10 minutes, shower, and then turn it off that time adds up. I make 12 bucks an hour, about an hour a week wasted from turning on/off and waiting…
I’ve met people who actually live like this, but I think they’re crazy.
Electric hot water systems can be broadly separated into two types:
a) instantaneous, and
b) storage.
Instantaneous systems hold a very small amount of water, of the order of 5 litres. Storage systems hold more, like around 200 litres. Your 50 litre thing is a pretty small storage system, but I suppose not unheard of.
The thing is, instantaneous systems are designed to heat only a very small amount of water, as and when needed. Storage systems are designed to keep a much larger amount of water hot, whether it gets used or not.
So first off, from a practicality viewpoint, can you switch on the hot water just before your shower, and switch it off afterward? If you have an instantaneous system, yes. If you have a storage system, no, because the larger amount of water that it holds won’t get hot in time.
Which is cheaper? That depends on whether your electric utility has an off-peak rate. If it doesn’t, and all electricity use is charged at the same rate, then it’s cheaper to use an instantaneous heater. The instantaneous heater won’t waste as much electricity heating up the air around it. If your utility does have an off-peak rate, then it’s likely that storage heat will be cheaper, but it might not be. It depends on the size of the tank, what the thermostat setting is, and how much hot water you use each day.
This issue was discussed at some length 2 weeks ago … http://boards.straightdope.com/sdmb/showthread.php?t=292455
Perhaps it’s 50 gallons, which is a standard size tank.
Yea, I agree.
First thing is to have an insulated water heater. Insulated hot water pipes are good too.
If you live alone, bathe once a day or less and have no dishwasher or washing machine, then turn it of and leave it off until fifteen minutes before you bathe, then shut it off before bathing. If 15 min. doesn’t get the water warm/hot enough for you, add a few more minutes.
If you do have a DW and/or WM, then turn the heater on half an hour or more before bathing and do the dishes and laundry as well so you take advantage of, and use, all the hot water at one time. Shut it off sometime before the last use of hot water or at last use. You might have to experiment to see how long to leave it on to have enough hot for everything. If you want some hot/warm water for use after the chores, leave it on longer.
Even with two people, this can work if you bathe together or one right after the other. The point is to heat up enough water, hot enough for the job/s and then shut off the unit til next time. The longer the time between heats, the more you save. For normal households of three or more, forget it, just shut it off if you go away for more than a day or two.
Yeah, I’ll third that. I’m curious, though, as to exactly what physics principle the teacher thought was being illustrated by this example.
I’m not sure we need a new physics teacher. The lesson taught and the lesson learned are different things.
I have to disagree with the conventional wisdom on this, which is that turning the heater on before your shower and turning it off afterwards results in the greatest energy savings. Actually the most energy efficient way to do this is to turn the heater on before the shower, leave it on long enough to heat sufficient water for the shower, and then turn it off before the shower.
This results in the hot water tank being filled with cold water, as it displaces the hot water drawn from the top of the tank. As everyone knows, the heat loss is proportional to the log of the mean delta T between the tank and the surroundings, so the colder the tank is the less heat is lost to the surroundings.
If you take the shower with the heater on, the tank ends up full of hot water and the heat loss is much higher. In a 70 degree F room, the heat loss from a 140 deg F tank is 3.5 times that of a 90 deg F tank.
Of course, it’s inconvenient as hell, but from a physics standpoint much more efficient.
But these “surroundings” are the inside of your house. Which you are trying to keep warm by running a furnace in it. So this “loss” just serves to reduce the time that your furnace has to run. Here in Minnesota in January, we wouldn’t consider keeping our house warm a “loss” at all!
But you might consider the generally higher cost of electical energy a loss.
What are you talking about?
My water heater runs on natural gas, not electricity. As do most of them in this area.
But the OP’s is electric. He even gives the Power rating.
I have a gas heater in my house, but my furnace is gas, too. In the last two places I lived (both apartments), I had electric water heaters and furnaces.
And also, even in Minnesota, you’re only heating for what, half the year, and during some of the rest of it you may be using energy for cooling, and in colder climates the furnace and water heater is usually in a basement or cellar, which is usually not heated as well as the living area. Although you are correct about the heat not being lost it may not be where and when you would like it.
:smack:
Up until your post I wondered why no one mentioned what a pain in the ass lighting the pilot every time would be…guess it’s like them electric stoves you hear about. But isn’t there electricity involved in a gas water heater too? I know the dryer won’t start during a power failure despite being gas powered, anyway.
There is no electric typically involved in the operation of a gas water heater, the controls are all operated by heat or lack thereof. And if you wanted to there is a vacation or pilot setting on gas water heaters that essentially turns them off without needing to relight the pilot.
As to the OP, hey if you can knock ten bucks a month off your electric bill that’s a 120 dollars more in your pocket. that’s more than a days pay at your current salary. Some people spend a dollar to save a dime. I think in your case with that little water heater I wouldn’t waste my time. 50 gal tank with two 4500 watt elements definitely turn it off or get a timer