My wife loves long hot showers and until recently that has been no issue. however I took off the shower head and cleaned out the filter screen, found little there. Immediately after she runs out of hot water after 15 minutes. We have a 40 gallon gas water heater and as mentioned hot water had never been a problem. I have flushed the heater with little debris noted and replaced the dip tube with no change. Heater is 8 years old but had been working fine until now. I am afraid to just replace the heater to find the problem remains. Anything else i should check first?