Harold Kestenholz
I can worry less now. This is neither a comfort heating system or an ordinary US domestic water heating system. It is a well-pump with a domestic water heater that has an unknown pressure-setting on the relief valve (but is probably at 70 psig, an unusual pressure setting.) So probably the well-pumpset tank is coated on the inside against corrosion.
If the relief valve is also temperature-activated, the release can be from temperature. If the water in the tank stays at approximately 1/4 full and the pressure changes, then the gas pressure increase is due to heating the water and the air in the tank. The water is not forced back into the aquafer because of a check valve in the pumpset, so the heated water and gases expand to raise the pressure.
If there is a check valve between the tank and water heater, the release of water is understandable. If the water in the heater expands or is overheated, the relief valve releases water. At times it is possible for all the air in the upstairs heater to be purged by extended use, so without trapped air, the expanding water could lift the relief. The aljiber in Mexico is exposed to high ground and outdoor temperatures, so excess air normally found in ground water is bubbled out. Well pumpsets in Mexico are not necessarily inside or sheltered in low-rain areas, so entrained-air content can be lowered in a warm pressure tank. Night-cooled water in the tank would be heated by the sun in the day. All of this is conjecture until confirmed by observation.
So a temporary answer is yes, a tank can become water-logged. Expanding water or air can lift a relief valve. Further answers come from definite additional details.
