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I recently purchased a large fixer upper with a 135 gallon AO smith gas water heater built in 2001. Only two people are living in the house so we need a fraction of that amount.

Do water heaters always store their full capacity or is there some way to limit what they store or reduce their cost other than turning the temperature down?

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4 Answers 4

up vote 5 down vote accepted

A larger tank does not cost more energy. The only thing that costs energy is when the water cools - and that depends only on the surface area, not the amount of water in the tank. (Mathematically volume increases by the 3rd power, but surface area only by the 2nd power.)

So to save energy add extra insulation around the tank. Reducing the amount of water in the tank will do nothing since it won't save all that much surface area (and potentially none at all).

(Yes, it takes more energy to heat the water in the first place, but after that it doesn't matter - and you already heated it.)

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You're partly wrong: larger tanks have larger surface area and therefore radiate more heat and cost more to operate. (You are correct that if you could put bricks in the tank or something to reduce the amount of water stored, that would not affect the heat loss since the surface area is the same.) –  Henry Jackson Feb 7 at 1:00
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@HenryJackson You are correct from the theoretical point of view. I was speaking strictly from the practical point of view. And also letting him know that a smaller tank doesn't help all that much - they usually make them just as tall, but narrower and the surface area is very similar, so it hardly makes a difference. –  Ariel Feb 7 at 4:55
    
Note: Check with the manufacturer before adding insulation. Some have objections. My own solution, when I replaced my old water heater, was a highly-insulated tank heated indirectly from my high-efficiently heating boiler, sized for the house's theoretical occupancy despite having having the place to myself most of the time. –  keshlam Jun 8 at 21:15
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Water tanks store the water under pressure, just like the rest of the piping in your house, so there's no way to keep it partially full. Turning down the temperature would be a great way to save money if it's higher than the recommended 120º.

If you can't or won't turn it down, you might just consider replacing the tank. 13 years is getting up there for a hot water heater... you might only have a few more years left in it. Plus a new one is likely to be more efficient, regardless of size.

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-1 as your answer is mainly about turning the temperature down, when the OP specifically restricted that as an answer. –  virtualxtc Feb 6 at 23:51
    
@virtualxtc: Fair enough. I still think it's a good suggestion, since there's no way to do what the OP asked. –  Henry Jackson Feb 7 at 1:01
    
Also, I'm not saying you are wrong, but I'm not convinced that the amortized cost of a new unit can be justified by the difference in operational costs. You might to add some analysis of how savings would justify an early purchase. –  virtualxtc Feb 7 at 1:41
    
Actually, the amortized cost of a new unit after a 13 year old water heater springs a leak that damages the house is pretty neat to see in action. You have the repair costs and cost of a new heater to pay for. Pretty easy to justify the new heater by the damage you averted, just ask my neighbor who had his bedroom, garage below and living room destroyed. Lots of theoretical on the answers to this question that just doesn't add up, including false economies. –  Fiasco Labs Jun 8 at 22:55
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Actually, the math seems to contradict parts of the top-voted, accepted answer.

Assume that you decide to cut the volume of the water tank in half, by reducing, in proportion, the dimensions of the actual internal tank, while leaving the shape the same, and leaving the thickness of the insulation the same.

This would require each linear dimension to be reduces to 79.4% of the original value (cube root of 0.5).

This in turn means that the any area of the tank would decrease to 63.0% of the original volume (79.4% squared). So, theoretically, this reduced tank would have heat conduction losses reduced to the same 63.0% of original.

This said, there is no real-world way to achieve this reduced size without replacing the tank. As others have pointed out, reducing the water temperature and adding insulation are both cost-effective ways to reduce stand-by costs

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I think that you and Ariel are using different assumptions about how one would reduce the volume of the tank. Perhaps Ariel was considering the 'brick-in-the-toilet-tank' displacement approach, rather than the 'build-a-smaller-tank' approach. Neither is right or wrong. –  mac Feb 6 at 20:50
    
@mac so,if you substitute a displacing balloon for a brick, than this answer would would be better, would it not? –  virtualxtc Feb 6 at 23:55
    
My answer was about what someone could actually do. Not about theoretically making a smaller tank - if you wanted to do that, you could just buy a new one. –  Ariel Feb 7 at 4:53
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Turns out, the surprising answer is you can reduce the thermal energy stored in a fixed size tank water heater.

At rest, water in a tank stratifies or even 'stacks', an effect well known to solar installers. Hot water floats up top and cold water sinks.

A typical older water heater has just one temperature sensor low down in the tank. Move that sensor higher on the tank, and it won't react to cold water at the bottom of the tank. The top of the tank will have hot water, the bottom cold water, pretty much like you wanted.


Now there is a significant downside: vigorous flow can mix the water in the tank, giving you the average between the hot water on top and the cold water in the bottom. If you could carefully draw the water off the top there would be no problem, but cheap tanks are going to introduce some turbulence and run the stratification pretty quickly.

And note this works better in electric heaters compared to gas heaters (with an electric you can disable the lower element, and regardless there is less mixing with the heat is on).

Great discussions on these matters can be read in United States Patent #6880493 or http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-water-heaters

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It sounds like moving the temperature sensor up just causes you to take readings at a slightly warmer part of the temperature gradient, which has the effect of cycling on the heater less frequently... exactly the same effect as turning the temperature down. The amount of mixing doesn't change based on where the sensor is. –  Henry Jackson Feb 7 at 0:57
    
You don't "need" the full capacity of the tank, so heating the upper half is "enough". It's not the same as less frequent cycling, as each cycle will attempt to heat the entire tank compared to the top half of the tank. It is not a PRACTICAL answer because of the limitations noted, but it is meant in the spirit of learning, and stratification is a REAL effect. –  Bryce Feb 7 at 3:01
    
I think for your strategy to work you'd have to move the element up. Otherwise the element will heat the cold water at the bottom and convection will cause it to rise and mix. –  Henry Jackson Feb 7 at 4:26
    
@Henry Some solar tanks do exactly that (move the heating element up) to keep the stratification. Remember this is an "in theory" not a "do this now" answer, and should be treated as such. –  Bryce Feb 7 at 6:23
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