I do have DE Hayward filter and natural gas heater.

Incident was in the in-line chlorinator and it was pretty violent (inside of it). Nobody got hurt and I learned a lesson. You are correct of being very quick to disolve and produce heat.

My today readings are:
FC - 3-5
TA - 80
PH - 7.2
Calcium Hardness 120
TDS 600
Phosphates 500

Thanks for all your help.