mr_clean wrote:it's not always the chemical balance as I stated when I showed up and checked them they were good.
Many people say their chemicals are "in balance" when they are not. Did you calculate the Calcite (Langelier) Saturation Index? If all the parameters of pH, TA and CH are at the high end of their "recommended" ranges, then the result is scaling at higher temperatures -- namely at the heater. Also, scale takes a while to build up so the chemistry could have been out of balance for a while before you showed up, and then the pH went lower and the scale already formed in the heater started to flake off. Many of the saturation index pool calculators are wrong. The best one is the Taylor Watergram found in some Taylor combination kits (such as the K-2005 and K-2006), but you can also use the online calculator here that Jason developed based on my spreadsheet (and matches the Taylor watergram). By the way, when such a saturation index is negative and says it's "corrosive", that is inaccurate -- it's corrosive only in the sense that it will tend to dissolve plaster or grout (specifically calcium carbonate), not necessarily metal. Metal corrosion has other factors including oxidizer level (chlorine and dissolved oxygen), pH and salt level (conductivity, mostly, though chlorides are additionally corrosive to stainless steel by interfering with the healing process of forming a protective layer as I described in the link you didn't read because you apparently know all about this already).
For people with copper heat exchangers, I would recommend they think twice about getting an SWG system or get a new gas heater (with titanium heat exchanger) since there is no way around the corrosion problems at the higher salt levels of SWG pools (3000 ppm salt). They could run their pools at very high pH to minimize the problem (and keep TA and CH lower to compensate to prevent scaling) but this will be harsher on the eyes. And they can keep the lowest chlorine levels possible by using supplemental algaecide (PolyQuat 60 weekly) as that will also help. But really, if you are getting an SWG, then you most likely need to get a gas heater made to tolerate the higher salt levels. People used to use stainless steel filters until those also had problems at higher salt levels -- now they are almost all plastic (of course, cost was part of the reason for the change as well). I should point out, however, that most damage to copper heat exchangers is from improperly maintaining the pool, SWG or not -- namely, putting Trichlor pucks in the skimmer with the pump not always on, for example, or just using Trichlor as the source of chlorine and not monitoring the pH (and TA) of the pool (so it becomes more acidic which then corrodes the copper, even in low-salt pools).
As for needing Defender or other metal sequestrants to reduce calcium levels, even those people who do not properly maintain their pools won't need that since the only way to get too high a TA or CH is to add those chemicals (Alkalinity Up / Baking Soda and Calcium Chloride) to the pool so a lack of maintenance could only lead to scale if they let the pH go way up. As I mentioned before, the exception to this is if their fill water is already high in CH and/or TA (e.g. well water) in which case I would agree that a calcium sequestrant or filtering through a water softener are reasonable options. If the pool does not have a lot of dilution through splash-out or backwash, then in areas with very high evaporation the fill water will add to the CH and TA so that's another way the CH can build up, but just recommending Defender as a blanket solution is adding a band-aid to the problem without locating its source -- where is the higher CH coming from -- is it from not understanding the saturation index so adding too much initially? Or is it from fill water initially high in CH? Or is it from high evaporation and low dilution and fill water with moderate CH? Most pools do not have high CH unless you are operating in a region where the fill water is high in CH (which you can test, of course, or look at the water quality report, though they often just report Total Hardness and not necessarily Calcium Hardness).
Anyway, you seem to have a good handle on all pool problems so I'll let you take it from here for this forum. Though I value the experience you provide and information you share, I'll stay over on TroubleFreePool(dot)com and the original PoolForum(dot)com instead. You obviously feel I have nothing to contribute.