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).

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Temperature: If there is an issue with aquastat, the tank may over heat the water resulting in the temperature getting higher then that of the setting of the relief valve. This will cause the reloef valve to open and discharge water.

Relief valves discharge when the temperature or the pressure gets too high in the tank. When the tank heats up water, the water will expand and the pressure will increase.

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I can't find the reason why that happens. My two guesses are the usage of water or temperature in the basement. However, I have normal usage -- just laundry, or the dish washer. Nothing odd there. I checked the temperature, and it's sometimes hot, sometimes cold (I live in Pennsylvania). I can't find any pattern.

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This is basically a similar answer to all the above , but if you have high mineral content in your water (Hard water) it may have built up on the T+P discharge port. Inside is a rubber washer that has a spring keeping it closed. If enough deposits build up on the washer it may leak. the fix is to open and close the T+P valve a few times. Careful water will be hot!

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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.

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Pressure: If there is no allowance within the system to absorb the increase in pressure such as an expansion tank or a thermal expansion valve and all the taps are closed, the relieve valve will open and discharge water until the pressure drops to below the setting of the relief valve. This would most likely happen after a large amount of the tank was been used and it has to heat up the majority of the tank's volume.

I have a Gas Reliance Water Heater 1212. I found recently (in winter) there is a water dip through the relief valve into a 5 gallon bucket. Sometimes, it can fill the whole bucket. Sometimes, it has no water drip at all.

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.

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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.

Simple Test: I would drain the tank empty and then refill it with cold water, allow it to fully heat the water backup to the set temperature. Do not open any taps during this time as you don't want to release any pressure. Once the water is fully heated, check the bucket for any new water.

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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.