Quote Originally Posted by PoolDoc View Post
Not necessarily a correct assumption.

In your water heater, scale prevents heat from escaping as easily, but the scale stays wet (unless you hear popping noises) and so the heater element is temperature limited by the boiling point of water.
This is true, and works the same in gas or electric water heaters. As stated above ours is a gas fired, and will remain so as long as gas is cheaper than electricity as applied to heating potable water, not to mention the differing inefficiencies of both.

This may not work out as well with tubing exposed to uneven fire box heating, compared to tubing exposed to very even heat element heating.
Yes, the uneven heating is a concern of mine. What I have done to reduce -- cannot eliminate -- this, is baffles to direct the hot gasses around the tubes to attempt to even out the heat each is subjected to.

BTW, you can prevent having to replace heater elements by using a low watt density Inconel sheathed element like this one:
Camco 4500W 240V Screw-in Water Heater Element -Ultra Low Watt Density
Camco 5500W 240V Screw-in Water Heater Element - Ultra Low Watt Density
Years ago, when I was a plumber doing commercial service work, these were hard to find. But thanks to Amazon, they are now available to everyone. In most installations, this is a life-of-the-heater replacement. Because of the much larger surface area, scale formation is greatly reduced. And, because of the inconel sheathing, the elements don't fail even when they do get coated with scale.
I have used these where former employers had rentals with electric tanks. I agree that they are worth the slight extra expense, at least in my area. In only one case did I ever see one of these fail, an upper element, and that was attributed to the tank losing water when the water supply to the house failed while it was unoccupied.

Would it be too much of a stretch to assume that the calcium scale would make it's presence known by a reduction of CH in the pool water? In my ignorance, I have the idea that the calcium in the water would have to precipitate out as it encountered the heat in the tubes, attaching itself to the source of that heat. I don't know that this is what happens in a water heater, but that is my suspicion. In so doing it should manifest itself with a change downward in the tested levels.