Which is very frequent in these, the thermostat rupture (it is usually plastic and leaves ...) when this happens, such a pressure is accumulated that begins to relieve by a side of the spherical bottle (relieves by the cap in truth). But the explosion, as fellow Kikemec tells you, is usually a burst sleeve, you say that they are fine, then to review thermostat and as advice, you do not walk like this with the or you will have the breakdown of the Board of Board as little. Greetings!!