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actual energy savings by setting your hot water heater to 110  RSS feed

 
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I just spent the better part of an hour going over this with shawn.

First, shawn agrees with me that the only significant energy savings is with heat loss from the hot water heater itself.   So a 140 degree hot water heater will "leak" more heat than a water heater set at 110.  That's pretty much all of it.  

We found some blogs that talk about savings of something like $3 per month by turning it down, but when we tried to follow the links to see where they got that number, that information was gone.   And I felt that $3 was way too high.  My first guess was that you would save about ten cents per month.  

In the end, I did some really quick numbers in my head ....   how warm does the hot water heater feel ...   how many watts of loss is that ...  ... ...  ....   50 cents per month.   Shawn ran some numbers in a similar way and came up with $1.00 per month.   I felt that both of our numbers should be about 20% or 30% lower because we aren't even taking into consideration the loss of heat for 110 yet.  Plus, I kinda think that if your water heater is in an insulated space, like mine is, it becomes even more efficient.  

In the end, I was thinking about $5 per year would be maximum energy savings.   And Shawn was not willing to go lower than $10 per year.   At this moment, this part of the book reads as $10 per year.  

We both agreed that there could be some extremely carefully measured stuff out there - but we couldn't find it.   All we could find is blogs referencing something that isn't there anymore.  

If anybody wants to play this estimation game ...    Measure your hot water heater, or feel it on it's many sides.   It probably feels about five degrees warmer than air temp.   Measure it if you can - what is the external temp of many different points of your hot water heater.  

Shawn and I are both engineers.   So we do wacky estimating all the time.   And you might think that this technique is terrible (say so below if you think this is solid or too loosey goosey).   Take all the heat coming off of the hot water heater and concentrate it, in your mind, to the surface of a light bulb.   A 40 watt light bulb will burn you.  How many watts is your light bulb?

Mine is 7 watts.   Shawn's is 15.  What is yours?

7*24*30 = 5.040 kwh; at 10 cents per kwh, that's 50 cents per month
15*24*30 = 10.800 kwh; at 10 cents per kwh, that's $1.08 per month.  Rounding to one significant digit is $1.00 per month.

That's before knocking off some for the 110 numbers.

I'm hoping to hear from dozens of people and their way of measuring, the research they dug up, their numbers, etc.





 
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I came to the same conclusion years ago, but just by feeling it for hot spots and figuring it was pretty well insulated.  I never quantified it, and I accept that you lose more heat with a higher delta, but I never bothered with working it out.

Insulation gives diminishing returns and I figured the cost of adding more insulation would take years to pay back.

When I lived in an old farmhouse with an oil water heater I put the control on a programmable timer.  I saved money by only firing it up once a day for showering, washing, and dishes.
 
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The nipple on top "hot out" is warm. 90.8 degrees. Hot water travelled through it so it's skewed The brass valve (drain)at bottom is less warm. 78 degrees.. Top of heater is 75. Side of heater is 73. I would expect these differentials. Brass drain has no thermal break to the metal drum. Direct contact.

A bare wall away from heater is 69.

Heater is in closet in house. Electric. 220v
 
paul wheaton
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Let's hear your final calculations fellas.  How much do you think you might save per year if you were foolish enough to lower the temperature on your hot water heater from 140 (safe) to 110 (legionella party zone).

 
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I just looked up a 40 gallon water heater.  The outer dimensions are 20" diameter and 48" tall.  I'm going to GUESS that there's 2" of good insulation around the tank.  Thus the cylinder of water is 16" diameter by 44" high.  That actually gives a volume of 38+ gallons but it's pretty close.  That gives a surface area of about 17 square feet.

Assuming the room is 60 degrees, the delta T is either 50F or 80F in the two scenarios mentioned.  Let's assume a good R value of 4 per inch on the factory insulation so R8 in total.  

Heat loss in BTU/hr is (Area x delta T)/R value.  Sorry metric people...

So in this case the hotter heater will lose (17 ft2 x 80)/8 = 170 BTU/hr of loss = 50 watts

The colder water heater will lose 17x50/8=106 BTU/hr of loss = 31 watts

Wrapping the hotter heater with a thin fiberglass blanket (R2 on a good day) would bump the overall insulation to R10 which drops the loss to 136 BTU/hr.

In real heat loss math, I believe the outside diameter matters as well.  If you used a R100 blanket you'd somehow lose more heat due to the larger surface area.  That's scary math that is beyond many of us.

I may have messed something up or misplaced a decimal point so feel free to double check me.....
 
Mike Jay
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Sorry, forgot to answer the question.  I'd save 19 watts an hour and at 12 cents per kwh that would save me $20 per year.  Once again, if I did the math right...
 
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I think there are other variables that, when considered, can change the equations for calculating energy savings in tank style water heaters. For instance, a real efficiency killer in heat transfer from the heat source, gas or electric, is mineral scale deposits on the inside of the tank. A new water heater will heat the same volume of water faster, using less energy, than one that's identical but 10 years old with a layer of mineral deposits lining the inside of the tank.
 
paul wheaton
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I once heard that the r-value per inch is something really high because they are using some pretty toxic stuff.   Something like r-10.   Something that is poured in and becomes rigid.   The wraps are going to be something safer with a r-value of 3 to 4 per inch.

This might be similar:

Typical insulation in a reefer truck is two inches of polyurethane which has an R-value of 12.  



http://www.kevothermal.com/transportation_refrigeration.html

 
Mike Jay
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What do you know, it's better than I thought.  Per the government, water heaters should be a R12 to R25. www.energy.gov.  

The water heater I looked at with a 2" shell is probably closer to the R12.  Thus the hot heater would use 33 watts and the cooler one would use 21 watts.  

And I looked up actual insulating blankets and found them surprisingly thicker than I remembered at R10.  So that would take the hot and cold heater down to 18 and 11.5 watts respectively.  My water heater says the warranty is void if an insulating blanket is put on it...
 
paul wheaton
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Mike Jay wrote:The water heater I looked at with a 2" shell is probably closer to the R12.  Thus the hot heater would use 33 watts and the cooler one would use 21 watts.



Delta of 12.

If your water heater is in a little, insulated closet, it might get a bit warmer in the closet.  Thus reducing the delta.

 
Mike Jay
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Yup, 12 watts ain't worth risking icky water over.  
 
paul wheaton
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I think the real number, for the average hot water heater in a america, to be turned from a proper 140 to a dangerous 110 is $4 per year.  I come to this conclusion because I think that the average water heater age is probably about six years, and the newer hot water heaters are better insulated than the old hot water heaters.  Further, I think that most hot water heaters are in a closet of some sort, which acts as an extra insulator.   Further still, I think most americans keep their homes at 72 degrees.   And for those that are saving money by turning the heat down, there is another person that is turning the heat up.  So this is all about the difference in temperature of the room and the outer skin of the hot water heater.  

If the home is 72 degrees and the water heater is in a closet, I suspect that the closet temp will be something like 75.   And if the water heater at 140 is putting off an average of 78 degrees - this is a pretty tiny delta.  

I'm tempted to drop my number to $3.  But I'm gonna hedge my bets and say $4.

Mike says 12 watts ...   12*24*30*12*($0.10 per kwh) = $10

So it seems we have:

paul:  $4 per year
shawn: $10 per year
mike: $10 per year

anybody else?





 
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Well, at my house the water heater is in an unheated space, and this is common.  At least one of my neighbors it is outside.  And, electricity is about .20/kWh.   So, the math works out different.  My water heater is in the pantry which is almost the same temp as the garage, in the winter it is likely 40'F, on average.  

Water heaters also have some losses thru convection currents in the water pipes coming into the Water heater.  You are supposed to use some flexible hosing and make a heat trap on the hot by having a loop, so that there is now a Loop above the level that then goes to the rest of the system.  

I turned mine down, but I also leave it off alot, so hopefully I never get any bad bacteria in there as it is often just warm. There is a timer for the electricity to it. That saved a not insignifigant amount of money each month, this electric hot water heater realy costs alot if left on.

I own a solar hot water system to it ( closed system antifreeze), but it is broke ( panel leaks) and so far I have not found someone who can fix it.  The hot water system, for the many years it worked, saved ALOT of money, and I had hot water more often.  We also experimented at first with a hot water loop thru the wood stove, put a water jacket in the wood stove, but this was a bit scary when the power went out in winter ( it needs to pump) even though there is a pressure valve, I dont trust that fully so then I would crawl under the house and manually open that valve or take the fire out of the stove.  Not too often, but the potential for a problem is real, in my mind.  Also, I think it pumped more than it should have when the fire was getting colder or may have thermal siphoned somewhat.  Then, the water would just get raised from teh 50's or 60's to about 90'F, which is helpful as then the electric only had to boost it up less. There is a maybe 25ft run from the wood stove to the water heater.  But, between the issues I decided to not replace the water jacket and it is unhooked.
 
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