I believe my hot water heater has a bad thermocouple. I went to start to replace it, and it has a sealed combustion chamber. I removed the 4 screws and tried to take off the cover. It didn't budge. Am I just not trying hard enough? I am afraid I am going to break something else in the process. I am a novice handyman, but everything I have read says this is a an easy fix. The hot water heater is a Rheem Ever Kleen.