What causes Calcium to rise?

Thanks guys. Just worried about the SWG and the heater (pentair MiniMax NT). Draining 1/2 of the pool and rebalancing frankly sounds cheaper and easier than replacing the heater and cheaper than replacing the salt cell. Admittedly, I have NO idea about how long it would take the elevated level of calcium to render either unusable. Haven't made up my mind yet, so just reviewing my 'options'.

As for rain water, I live in san diego, so not expecting much help there.

SoCalBoo,

I am in a similar situation and just replaced some of my water when my CH got above 400. I started to see some scaling on my SWCG when my PH got above 7.6 so I did a partial refill ~25% and keeping my PH between 7.2-7.4 has so far eliminated the scaling.