This is real problem that affects several towns. New unknown gas started to appear from water heated by "boiler" (storage water heater)
Storage water heater
contains some air pocket over water,
the temperature is supposedly between 50 and 60 degrees Celsius,
water is there motionless for at least 8 hours at night,
there is magnesium anode there.
Input cold water contains
"total chlorine" below 0.5 ppm,
zero "free chlorine",
zero NH3, zero NH4,
has acrid smell ( like chloramine ).
The water comes from some "water treatment plant" ( technology unknown ). Water treatment plant takes water from the long river that goes through agricultural area and the "tight sand gas" field, gas extraction started recently.
At the output from "boiler" the water recently started to contain the highly volatile gas with strange smell that difficult to describe, the gas immediately causes the sensation of load and small pain in the forehead ( frontal sinus ? ) I notice almost nothing when I take shower but I immediately notice this gas when I wash my hands ( when I am above the tap ).
I tried to read the articles about the storage water heater and see there the information about trihalomethanes but they need the free chlorine as I understood but my tests show zero free chlorine so now I don't know where to look for.