Scale decreases electric water heater efficiency?
 

I worked on my electrical resistance hot water heater last weekend in a constant battle with hard water. As I inspected the mineral-crusted heating element I recalled that its widely held that scale reduces the efficiency of electric water heaters.

My question is how? Electrical resistance heating is about 100% efficient and any inefficiencies that do occur would do so as heat (which is the end goal) so how does the coating of minerals on the element decrease efficiency?

Getting a little deeper into the weeds I can see how the efficiency of my GeoSpring heat pump water heater might decrease with mineral buildup on the inside of the tank since the HP condenser coils wrap around the outside of the tank. The mineral build up would act as insulation and force more of the heat out through the outer tank insulation.

When it takes longer to heat the same water or the element burns out and a new one has to be put in I would say the efficiency of the system has decreased.
Since the water temperature set point takes longer or costs more money to achieve.

If it takes longer to heat then it's only because less energy is entering the system,(i.e., electrical wattage), right. The element gets too hot, the resistance increases, the wattage goes down.
I guess I would say that the WH effectiveness or capacity goes down rather than its efficiency. E.g., my truck won't haul as much as it used to because the springs are worn - that's not a loss of 'efficiency'. Semantics I guess.

..maybe not in an electric water heater but in a gas water heater, the heat that isn't absorbed into the tank ends up leaving through the central tube on the top. Usually along with some popping, hissing, and banging sounds if things get really bad.

For an electric water heater, it's likely more of a maintenance concern, if the elements are running hotter because they are covered in scale, they may fail prematurely. Might have an impact on tank life too, although usually it's an issue of an anode rod reaching the end of its life where the tank rusts out.

Agreed on the decrease in efficiency of a gas heater - more heat up the stack. I can tell you from experience that electrical resistance heaters make a lot of noise too when elements get coated with limescale.

From what I've read the limescale that forms on the inside of the tank is actually protective of corrosion, this may explain why our tank has lasted 30+ years with only cleaning and occasional lower element replacement.

I believe the scale will reduce the heat transfer and therefore the efficiency of the electric heater coil and that in turn will increase the recovery time of the HW tank. You'll use less electricity per hour but it will run it longer. Still 100% efficient.

Over 35 years ago, I read that for every 1/64th inch of buildup, a boiler loses 3% efficiency. That's going from memory but the scale buildup acts as an insulator to inhibit the heat getting to the water.

Scale v. efficiency
 

The only way it could affect efficiency is through heat being conducted out to the terminals and external wires. That will be negligible from the efficiency POV, but as has already been said, it will likely affect longevity of the heater and the wires feeding it.

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.

water heater scale pdf
 

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 5°C 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 :)