An incident in my house - in geyser

Today, while our bathroom electrical geyser (G) (instant) (3 lit, 3 KW) was ON  After some time,  bubbling sound was heard from the overhead tank (T) containing cold water,  located at about 0.5 m height. The outlet from geyser was opened and initially there was no flow but within 15-20 seconds, steam started coming out from this bottom outlet as well as thro' vent. The steam along with few water droplets spew from the vent (V). After 1-2 minutes, I switched off power to the geyser sensing, it may explode. 

The piping is like this: Bottom outlet from cold water tank supplies water to the geyser and hot water can be collected in two bath rooms located side by side.A parallel connection from the outlet of the overhead tank is also connected to one tap in each of these bath rooms. A vent is let above tank height. The tank is rectangular, 300 lit.

The geyser got thermo stat to cut of power supply once water is heated up. This is functioning and was checked after the incident by trial run. And, it started heating the water normally as before the incident.