My hot water heater is older. Its a 40 gallon. I recently noticed it doesnt seem to stay hot for as long as it used too. I can recall taking 40+ minute showers. Now im down to 20-25min before it starts to get cold. I checked the heater elements and both ohmed out right at 13. Everything looks fine on it. I know the answer is probably to just replace it but im curious why if its functioning properly i wouldnt get heat for longer?