I was googling for a new cold water storage tank for the loft, and started reading about the tragic consequences of faulty immersion stats in hot water cylinders filling cold water tanks with boiling water via the vent pipe. Which made me think why feed the safety vent back into the tank in the first place? In most systems, designers strive to eliminate runaway feedback loops like this. The safety vents on hot water cylinders and boilers are only needed when fault conditions produce boiling water, and surely you do not want it recirculated, you want it out of the system and replaced with cold water. Is there a good reason why, with precautions against scalding, the safety vents should not discharge externally?