Quote Originally Posted by Charlie_R View Post
For our domestic hot water, I've found that I need to clean our gas fired water heater only every 2nd year, so I think I can assume that the heat exchanger in the pool heater won't have too much of a problem with this before corrosion failure.
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 may not work out as well with tubing exposed to uneven fire box heating, compared to tubing exposed to very even heat element heating.

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.