The flex line comes in a package that has the capacity rating printed on the label.For the flex line, we checked the manual and it says: "The use of Flexible Connectors (FLEX) is NOT recommended unless they are properly sized according to the supplier recommendations for the heater rating."
He assured me it was properly sized for the heater.
Can you show a picture of the label?
If you don't have it, ask the installer to send you a picture of the label.
Don't accept any excuses for them not providing a label.