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Old 04-10-2010, 06:48 PM
 
Location: Hockley, TX
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Someone once told me that it takes more energy to heat the water tank up from cold than it does to keep the water hot. Is that right?
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Old 04-10-2010, 07:03 PM
 
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Quote:
Originally Posted by CaroleF View Post
Someone once told me that it takes more energy to heat the water tank up from cold than it does to keep the water hot. Is that right?
As I understand it, yes. However, if you leave for more than 24 hrs it pays to turn down the water heater to low or vacation, IMO. I do this with an electric water heater and it really lowers the electric bill to turn it off at the breaker if I am going to be gone for several days at a time.

When I had a gas water heater I would turn it down to the lowest setting if I was going to be gone for more than a day or two.

If you are the only occupant and your water heater is electric you might try a month turning it off at the breaker when you are not home and turning it on about 1/2 hr. before you need to use it. See what happens with your elecvtric bills as a result.
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Old 04-10-2010, 07:19 PM
 
Location: Las Flores, Orange County, CA
26,346 posts, read 80,642,654 times
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Quote:
Originally Posted by CaroleF View Post
Someone once told me that it takes more energy to heat the water tank up from cold than it does to keep the water hot. Is that right?

That is not correct.
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Old 04-10-2010, 10:08 PM
 
2,843 posts, read 4,003,539 times
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No- it is less energy to heat water as needed. However people tend to use less water if they have heated water on a circulating loop so they don't waste the first minute or so of water waiting for it to get warm.
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Old 04-12-2010, 06:21 PM
 
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Default Yes and no.

Quote:
Originally Posted by CaroleF View Post
Is it more cost effective to heat water as needed or to keep the water heater on all the time?
Quote:
Originally Posted by CaroleF View Post
Someone once told me that it takes more energy to heat the water tank up from cold than it does to keep the water hot. Is that right?
You asked two questions - the answers are not the same.

Second question first:
Does it take more energy to heat up from cold than it does to keep it warm? Per hour?
Yes - but that isn't really the important issue. When you hold water at a constant temp, you aren't using energy as quickly, but you are using it for a much longer time.

Think of it like this: Does it require more energy to lift a heavy weight or to hold it up? Lifting, of course, requires much greater energy inputs. It doesn't take long, though. Holding the weight up can take a long time, and your are constantly having to provide energy at a small rate. If you aren't going to use the heavy thing right away, you are probably better off putting it down until you need it for something.

We have a similar situation with water heaters, with the added factor that when you turn the heater off, it doesn't immediately fall back down to the floor. In fact, it returns to room temperature no faster that the rate of energy flux that would be dealt with if the temperature was being held constant. (And that rate slows as the temp in the tank approaches room temp.)

First question:
Is it more cost effective to heat water as needed or to keep the water heater on all the time?


Heating 10 gallons of water is less expensive than heating 10 gallons of water, then keeping it warm until the time you need it. (At least for energy costs.)



All that said - I have a tankless water heater, and I think it is more or less break even. I am not paying to maintain water at high temp, but I end up leaving the hot water on while washing dishes, because if I turn it off, it takes 6 seconds for it to get warm again. Also, since I never run out of hot water, my showers are occasionally longer than they should be.
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Old 04-13-2010, 05:51 PM
 
Location: Eastern Washington
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Pigankle and Charles are correct, from a heat transfer point of view. The heat required to raise one pound of water from 59 to 60F at normal atmospheric pressure is the definition of a BTU.

You pay to heat the water and you pay for the heat lost through the sides of the water heater.

The amount of heat lost through even an old water heater is not that great, though, roughly equivalent to a 60-W light bulb.

If you heat your house with electric resistance heating, the heat lost by an electric tank-type water heater, provided it's in the heated part of your house, is a wash.

I'm always amazed at the number of people who think you use more energy somehow if you turn the water heater off for part of the day, or a couple of days, whatever. I guess these people did not major in Physics in college (if they have any college!) I think it's a mantra chanted by people who CBA to do anything to try to reduce their own electrical consumption...
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Old 04-13-2010, 05:54 PM
 
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Installed a tankless water heater last year and reduced gas use by between 30-40%.
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Old 04-13-2010, 07:37 PM
 
Location: Hockley, TX
759 posts, read 2,452,454 times
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Thanks everyone. My ex-husband told me that it was cheaper to keep the water on. I was a language major in highschool and did not study science after I was 14! I had no way of arguing the point with him, and in those days there was no Internet to run to for answers!

On the other hand, growing up in Scotland with my parents, we had an immersion heater for water, and we turned it on only when we were going to use the hot water for dishes or baths (no showers in those days either). Nice to know that my prents had it right and my ex did not .
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Old 04-13-2010, 09:10 PM
 
25,756 posts, read 49,570,622 times
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It might not be cost effective to heat water only when you need it if the location is subject to freezing... freeze damage could be very costly.

Grandparents would turn the 30 gallon gas heater to pilot when they would be away... the water, just from the pilot would be warm even when the utility room would be around 38...

A neighbor, with the same home, turned off the heater and had a pipe burst at the heater connection... the water damage when it thawed caused the floors to buckle...
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Old 04-13-2010, 10:36 PM
 
9 posts, read 40,311 times
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Quote:
Originally Posted by M3 Mitch View Post
The amount of heat lost through even an old water heater is not that great, though, roughly equivalent to a 60-W light bulb.

If you heat your house with electric resistance heating, the heat lost by an electric tank-type water heater, provided it's in the heated part of your house, is a wash.
Excellent point. It reminds me of something I have thought a lot about - CF bulbs. In my part of the world, we have the lights on only when it is dark out. Up here in Seattle, darkness tracks cooler temps very closely. Since all of the "inefficiency" of incandescents is due to their tendency to produce more heat than light, it seems to me like having them instead of CF bulbs is functionally the same as having distributed low-watt heaters in every room, but without the mercury disposal issues.

And before anyone points out that they high up in the room, let me just say that at least they are in the room - I rented an apartment once that had electric coil heating mounted in my ceiling - my upstairs neighbors sure were delighted when I cam back after X-Mas vacation!
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