Hi. Located in uk. The heater has as you say 2 x cutout relays, one on the live connection and one on the neutral. One is a 50 deg temp cutout to protect from hi temp damage, the other looks like a hi current trip. Before putting new heater in it measured 18 ohms across the two element terminals with no connections to it, and infinite resistance between either of the terminals and the body of the heater. So all ok resistance wise.
Fitted heater and started system. Water flowing ok and the heater getting hot. After 30 seconds system stopped. Measured element and it is now open circuit (inf