I have a few questions:
1. Is this cracked elbow any different than the other two installed?
2. Who manufactured this elbow ? Was is part of the heater supply scope? Any QA/QC records available?
3. You said that the elbow had multiple cracks. Where? Inside or outside?
4. Are there any attachments to the elbow such as a elbowlet/thermowell or perhaps a piping support?
5. How does the heater coil expand? Is it fixed on the outlet side and the crossover pipes grow towards the back of the heater ? Or is it the other way around?