Scale decreases electric hot water heater efficiency?

[semipro]

I worked on my electrical resistance hot water heater last weekend (post link below) 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.

Post: https://www.hearth.com/talk/threads...ut-while-showering.167062/page-2#post-2247022

Edit: "hot water heater" seems a bit redundant doesn't it. Mods feel free to modify title to remove "hot" if you like.

 

[heat seeker]

The scale inhibits the heat transfer from the elements to the water, and the elements run hotter, probably reducing their lifespan and wasting electricity.

 

[semipro]

The scale inhibits the heat transfer from the elements to the water, and the elements run hotter, probably reducing their lifespan and wasting electricity.

A certain amount of power will produce a specific amount of heat. How is the heat wasted?

 

[moey]

Depends how you define efficiency. Watts in will be converted to btus and you will get the same amount of heat being put in the tank. What changes with all the scale is the resistance of the element. When the resistance changes the amount of current drawn will change this will result in a reduction in the rate at which it heats water. The heat is not mysteriously disappearing your not putting as much in anymore because the resistance increased.

I suppose if its really bad the element could not get hot enough to satisfy demand and stay on forever with a low current draw. Like trying to boil water with something that only gets to 160F. I suspect the element would crumble before that but what do I know.

 

[Highbeam]

It’s a trick. The heat transfer efficiency is reduced due to the scale acting as insulation. Element gets hotter, resistance goes up, current goes down. It takes longer to heat the water but at less amps.

Conservation of energy means efficiency of the appliance remains at 100%.

 

[Highbeam]

Moey and I were typing at the same time! What he said!

 

[semipro]

Moey's point is well taken though. Although the element is still 100% efficient at converting electricity to heat, the water heater is no longer heating water as quickly, so its less effective.

You and I were also typing at the same time. It seems what has really decreased is effectiveness not efficiency.

 

[RobbieB]

It does affect efficiency of gas heaters. Less heat transfer to the water means more heat goes up the flue.

 

[peakbagger]

Kind if like the old oil filled radiators they used to sell, take longer to heat up but take longer to cool down.