Instant hot water dispenser

In a hot water dispenser having a plastic upper tank with a lower end and a lower metal tank with an upper end, the plastic tank having a greater coefficient of expansion, the improvement wherein:

• a. the lower end of the upper tank is formed with a plurality of outturned flanges and external laterally outwardly facing circumferential groove for receiving an O-ring, an O-ring supported in said groove;
• b. the upper end of the lower tank is formed with a shoulder for receiving and supporting the lower end of the upper tank and having a shoulder sealing surface located outwardly of said groove and O-ring to provide a sealing fit with said O-ring, said sealing fit being enhanced by radial outward compression forces caused by radial expansion of said upper tank when said tank is at operating temperature for forming a seal with the O-ring, a plurality of gaps for receiving the outturned flanges of the upper tank, and a plurality of outturned flanges between the gaps, one of the outturned flanges having at least one small notch therein;
• and
• c. a retaining ring inserted between the outturned flanges of the upper and lower tanks to hold the tanks in assembly, the retaining ring having at least one upturned locking portion that protrudes into the small notch in the notched flange of the lower tank,
• so that the notched flange will deform under internal pressure in the dispenser to release the retaining ring and release the pressure.