The anode rod corrodes over time, and once it does, cold water is not properly delivered to the bottom of the heater which results in luke-warm water. The simple answer is that it should be replaced as-needed.
On some water heaters, this rod is part of the cold water supply, so replacing it requires disconnecting the tank which can be a fair bit of work. Some heaters have what is known as a sacrificial anode rod that is independent to the intakes and exists solely to protect the rest of the water heater from corrosion - usually this one is easier to remove and replace.
If you have plans to shortly replace the whole tank, I would not replace the rod unless you suspect it is already corroded and causing problems.
There are three reasons a T&P valve opens. Temperature, pressure, or a faulty valve.
Temperature
If the water in the heater reaches a temperature of 210 degrees F, the T&P valve will open and release water until the water temperature is reduced. As water is removed through the T&P valve, cold water enters through the inlet and mixes with the water in the tank. When enough water has been mixed to reduce the temperature, the T&P valve will close. If temperature causes the release, there will likely be a lot of water released.
Pressure
Most T&P valves are set to open at 150 psi. If the pressure in the tank reaches this level, the valve opens until the excess pressure is released. If pressure caused the release, you'll usually see about 1 cup of water per 10 gallons in the tank (per release).
Temperature or Pressure
To figure out if the trip is being caused by temperature or pressure, you'll have to monitor both.
Test water pressure?
To test water pressure, you'll need a pressure gauge. They sell threaded gauges, that can attach to a standard hose bib. This will work well if you have laundry hookups.
- Wait until the water heater is actively heating the water (you can expedite this step by using hot water, so go take a shower or something).
- Connect the gauge to any hot water outlet, and open the tap.
Monitor the pressure through the end of the heating cycle. If you get a gauge with a "lazy hand", it will stay at the highest pressure achieved. That way you won't have to stand by and watch the gauge the whole time.
Test water temperature
To test the temperature, all you need is a thermometer.
- Wait until the heater has just finished running.
- Open the nearest hot water tap, and let it run for a minute of two.
- Use the thermometer to measure the water temperature.
Causes and fixes
Without knowing more information about the installation, it's very difficult to speculate as to why this is happening. Determining why the valve is opening (temperature, pressure, or some other reason), is the first step to diagnosing the problem. Once you understand why the valve is opening, it will make it much easier to figure out the cause of the problem.
Best Answer
If your water heater has a drain port that uses a conventional garden hose type fitting, consider to attach a short piece of hose to the port. It can also take a standard washing machine hose. Ensure that you have no pressure, as "almost nonexistent" is still some pressure.
Open the valve for the drain port while the end of the washing machine hose is at the height of the top of the water heater. There should be no water visible when the end is higher. As you lower the end of the hose, you will begin to see water approaching the opening. It will match the level of the water in the tank.
If there is pressure in the system, none of this will work, as the water pressure, even if low, will force water from the tank into the hose, regardless of the level of the hose end.