Well, if you were in this area, I would tell you that you have a serious hot water leak under your concrete floor slab. I once knew a plumber whose customer had the same symptom and when a new heater did not cure the problem, he tried to return the heater because he said it was defective. I had to REALLY talk seriously to convince him the he DID have a water leak, and nothing would stop the problem until he found, and fixed it.