My electric water heater for the last few years would drain a little out of the pressure relief valve, so I kept a bucket under the hose and emptied it every month. In the last year or so, the water seemed not as hot as it used to be when the heater was new (it's 9 yrs old with a 12 yr parts warranty), and then one day a couple of weeks ago the water got scaldingly hot, and then cold the next day. The high temp. shut off switch had been tripped, so I reset it and we had hot water again.

Suddenly, though, the TPR valve started filling a 5 gal. bucket a day in intermittent bursts or trickles. I bought a generic TPR valve from a big box store and replaced it, but that did not seem to have much effect and the high temp reset button kept popping every couple of days. Then I replaced both top and bottom thermostats, and the symptoms changed slightly: the TPR does not leak during the day, but I have an electric timer on the heater, and when the heater shuts off in the evening the TPR valve will overflow a 5 gal bucket within a few hours. I now have to shut off the cold water every night just so that it won't leak.

I understand that thermal expansion occurs when cold water is heated, but why is this happening when the hot water cools? Thanks in advance for any advice.