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Old 03-14-18, 11:39 PM   #11
jeff5may
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If the water in the tank wasn't going anywhere, the naysayers here might have a valid argument. Unfortunately, the water passes through the tank in a finite amount of time. The scale insulates the heat source from the water, so both the raw btu throughput and the recovery time are directly affected by the layer of scale. The thicker the scale layer, the worse the performance. They don't call it resistance for no reason.


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Old 03-15-18, 07:02 AM   #12
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Heating time is a measure of performance (e.g. capability) not efficiency. I'd argue here that efficiency is energy in/versus energy out to accomplish a task.
https://en.wikipedia.org/wiki/Electrical_efficiency
Since the end goal is heating, and all the electrical input energy is converted into heat, and the system is essentially closed with respect to external heat loss, I challenge the assertion that scaling results in decreased efficiency.

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Old 03-15-18, 08:25 AM   #13
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Ok so if you have an uninsulated tank with a scaled up heat exchange surface, it doesn't affect your equation at all. The heating element is still burning away like an incandescent light or an Amish hearth heater. 95 percent efficiency at best due to natural losses. Current in wires and crap. The losses due to no insulation add to this overall bleeding of energy outside of the tank.

I bet you a buck a new insulated one would cost at least 20 bucks a month less to operate, even if you never draw any hot water out of it. Once the water starts flowing, the losses increase due to run time and lag. In fact, the best way to get more hot water out of the tank per watt input is to super insulate the tank. You can't tell me about the scale issue not affecting performance. Less run time is less run time, the end.

With a layer of scale, the heat transfer is impeded and a delta T increase is set up between the water and the element. This reduces raw BTU transfer rate and run time increases. The house wires and circuit breakers and thermostats and terminal connections and such all suck power while the element is running. The time lag also relaxes the thermostat controller loop and that lazy thermostat factor also acts as a vampire loss. That's mostly how your electrical efficiency is reduced from perfectly on paper form. The more scale, the longer the time lag and the higher the delta T between everything every heating cycle. The Q flow follows the change. Q spent over Q in the tank is your reference efficiency spoken of.

So from a math on paper perspective, one could relate the impedance of the scale layer to the parasites in the electrical system, and to the constantly exiting heat transfer through the tank surface area. Time is not your friend in this argument. Ten percent impedance in heat transfer rate equals 11 percent more run time, 11 percent more electrical vampire loss, and 11 percent more envelope heat loss, while the element is energized. I'm sure the geniuses at Whirlpool have to do the homework on this one every product cycle, and the industrial engineers do this as part of figuring out the budget.

I'm not that guy, but I can think that way when I try. This argument is a whole lot like the one about suction pressure versus temperature in a heat pump. The argument there is that cold refrigerant is more dense than warm refrigerant, so it should move more energy per unit of volume...

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Old 03-15-18, 03:15 PM   #14
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My kettle has made zero deposits in the last 10 years if I would of checked the elements on the last water heater Im pretty sure they would of been clean. If anyone here has a kettle that has scale your water heaters elements would likely benefit from being removed and given a descaling dip.

0.5mm of scale is apparently a 10% efficiency hit 24-7 365 days a year.
A 2mm deposit = 40% increase in running cost

The elements probably burn out shortly after that point
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Old 03-15-18, 09:59 PM   #15
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Quote:
Originally Posted by Semipro View Post
Heating time is a measure of performance (e.g. capability) not efficiency. I'd argue here that efficiency is energy in/versus energy out to accomplish a task.
https://en.wikipedia.org/wiki/Electrical_efficiency
Since the end goal is heating, and all the electrical input energy is converted into heat, and the system is essentially closed with respect to external heat loss, I challenge the assertion that scaling results in decreased efficiency.
The overall heat transfer coefficient of the system takes a hit with a scale build up The pdf I linked to earlier has a list of papers / books to back up the info.

Scale sucks more heat out faster and takes longer to put the energy in to accomplish the same temperature rise. A two fold hit then there's lifespan to add to the scales effects
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Old 03-08-19, 11:41 PM   #16
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Quote:
Originally Posted by ecomodded View Post
I had a look to see what info was out there on scales energy costs and found this pdf file https://www.cwt-international.com/cw...Efficiency.pdf

Highlights below

Reduced efficiency

Over time scale deposits act as an insulator,
causing the water heater to work harder and
harder to heat the water.

Scale reduces
energy efficiency of the water
heater by up to 50%

-
Each five grains per gallon of water hardness
will cause a 4% loss in efficiency

-
Water temperature decreases 5C with a
limescale thickness of 2mm after 480 seconds

- A water heater‘s useful life can be reduced by
as much as 50% through scale build-up
-
Scale deposits will cause a 4% increase in cost
for gas storage tank water heaters when using
50 gallons of hot water per day.
-
0.5 mm of hard scale increases fuel costs by
9.4%


I have seen well water make a kettle 1/2 inch thick in calcium deposits and they do burn out water heaters there was one in the yard last time I visited
These numbers seem to refer to gas water heaters where it would make sense that heat transfer rates would result in more waste up the flue. Not so with an electric resistance heater. Although an electric resistance water heater with mineral coated elements may take longer to heat water, I remain unconvinced that the overall energy efficiency of the water heater itself is diminished. Inefficiency in electrical heating systems manifests as "heat". Longer heating times do not result in lost efficiency (energy in/energy out), only diminished performance with respect to heating time.

I was unable to find the Battelle report reference but found this one: https://www.wqa.org/Portals/0/WQRF/R...xecSummary.pdf

With this chart showing test results for the efficiency change of water heaters over time. Gas, electrical resistance storage, and instantaneous gas resistance heaters were evaluated. Check out their results table below. Efficiencies for electric resistance storage WHs did not change during the test although efficiencies for gas both types of gas units decreased.

I just found a summary of Battelle findings here: https://www.kineticonaz.com/battelle-study/
Notice although they talk about loss of efficiency in gas water heaters they don't for electrical resistance units. They discuss instead for electrical units mineral buildup and heating element failure - a problem I've witnessed first hand for 18 years - not decreasing efficiency.
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Old 03-11-19, 01:52 AM   #17
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Quote:
Originally Posted by ChetT View Post
Todd - That's certainly true for a fuel powered boiler as the heat would go out with the exhaust but not so with electric. The conversion from electricity to heat will still be 100% but the coated resistance heat element will just convert less electricity.
the only way a resistance heater can convert less electricity to heat is by lowering the supply voltage. otherwise, the element just runs hotter until a new equilibrium is reached, with the surface temp of the scale on the element being roughly the same as the surface temp of a clean element.

eventually it runs hot enough where the element fails.
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Old 03-20-19, 01:37 AM   #18
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Ok so there are free energy geniuses here. I have a simple one:

You know that when electric water heaters are new, they're almost perfectly silent. As they age, they start popping, fizzling, and making all sorts of strange sounds. Also, they don't seem to deliver as much hot water and take longer to heat up what they do produce. Why?

Well I hate to wreck the white board with all the algebra on it, but water is one of those pesky materials that doesn't quite follow the rules. It does things like superheat, flash boil, dissolve all kinds of other materials, and tends to catalyze corrosion. Flash boiling and condensing wastes all kinds of energy compared to zero. All of these factors mentioned consume energy and change all the algebra a little bit each.

As mentioned before, the element itself is not 100 percent efficient. In a test lab, the unit is connected to instruments at the wires sticking out. In a home, there is always more wire running to the source. If you are lucky, there is just one set of wire nuts on one end of one wire run. Most homes have more terminals and breaks between the unit and the source. Vampire losses all add up.

Most heating elements are designed to shed the lime that builds up on them and this fills the bottom of the tank with lime sand. If the sand isn't purged, it conglomerates and forms rocks and slabs of sediment. Needless to say this reduces the capacity and efficiency of the tank significantly. Heating rocks doesn't count as heating water. Yadda yadda yadda yadda yadda yadda yadda eventually the poor element burns out, increasing cost of ownership.

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