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How long will hot water heater stay hot?
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I need to know how long a 40 gallon electric hot water heater will keep its water hot without power. Assume the environmental ambient temps are around 70 degF and no water is being used.

If I heated the water to 190 DegF, how hot would it be after 24 hours?

Or if you have any information that would help also. Like your heater is set at 130 and you lost power and it kept the water hot until the next day? Or something?


Thanks!


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Registered: March 09, 2006Reply With QuoteReport This Post
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Murph,

You're going to have to find out what the R value of the tank's insulation is. That will determine the rate of heat loss from the tank to the ambient environment.


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Location: Utopia Planitia | Registered: February 25, 2005Reply With QuoteReport This Post
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Hello Murphy
Hmmm. I could give you some good information from personal experience but where is the fun in that?

Besides the R value, You will probably find that total surface area compared to the total volume of the container will make a difference. Whether the wind is blowing is likely to be important. Also whether the heater sits in the sun or the shade will make a difference.

Let us know what you come up with.

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Location: ลึก ประเทศอินเดีย | Registered: March 03, 2001Reply With QuoteReport This Post
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This will give you part of the answer:
http://www.thermalinc.com/math/insulation.htm
1 watt-hr= 3.4 Btu
One BTU is the amount of energy needed to heat one pound of water one degree Fahrenheit
water 8.33 pounds/gallon



 
Location: coldest N.America | Registered: May 03, 2005Reply With QuoteReport This Post
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Thanks for that link, John. Bookmarked.


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Location: Utopia Planitia | Registered: February 25, 2005Reply With QuoteReport This Post
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Murphy,

If you have a new water heater, the manufacture should have the energy cost to operate the water heater. Think energy balance, goes in equals goes out.
 
Location: The Deep South | Registered: December 06, 2004Reply With QuoteReport This Post



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The math and formula for figuring out heat loss is pretty basic. It's also pretty irrelevant to my question unless you can verify all the characteristics of any given heater unit that I may use.

How many 40 gallon water heaters out there will really hold 40 gallons of liquid?

So, I'm looking for real world information. I was hoping there might be a comparable chart or some other consumer rating or something for them or maybe someone has direct experience.


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Registered: March 09, 2006Reply With QuoteReport This Post
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Do you have a "40" gal DHW?
Note the temperature at a convenient location like the DHW outlet.
Note the time, turn it off.
Note the temperature and time about an hour later.
Repeat until you have enough points on a temperature vs time graph to estimate when it will cool to 'room' temperature.
It shouldn't take much more than 4 1-hr data points to get a reasonable estimate.



 
Location: coldest N.America | Registered: May 03, 2005Reply With QuoteReport This Post
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quote:
Originally posted by john galt:
Do you have a "40" gal DHW?
Note the temperature at a convenient location like the DHW outlet.
Note the time, turn it off.
Note the temperature and time about an hour later.
Repeat until you have enough points on a temperature vs time graph to estimate when it will cool to 'room' temperature.
It shouldn't take much more than 4 1-hr data points to get a reasonable estimate.


No I don't have a 40 gallon hot water heater to test. What is "DHW"??


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Registered: March 09, 2006Reply With QuoteReport This Post
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DHW = Domestic hot water


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Location: Vermont | Registered: April 26, 2006Reply With QuoteReport This Post
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When I was doing some initial testing on a 40 gallon water heater once I noticed the following:
1) Heated 37 gallons of water up to 150 deg. F
- (I've also never seen a 40 gal water heater that'll actually hold 40 gallons).
2) Ambient room temperature was about 70-75 Deg. F
3) I killed the heat & let it set
4) 3 days later when I went to empty the water heater, the water temperature was still at 90 deg. F.

Not too exact, but I've noticed that if the room temperature is about 70-80 deg. F, they can hold heat for a couple of days.

-Graydon




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Location: Utah | Registered: October 08, 2004Reply With QuoteReport This Post
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Thanks for the approximations. I can work with that.


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Registered: March 09, 2006Reply With QuoteReport This Post



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