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.

What's your house PSI and is there a PRV,( pressure reducing valve) installed? I would certainly consider adding a expansion tank on the heater if it were me. Back to you, n Tom

Check your main pressure regulator. You may be having too much tank pressure even with an expansion tank.

Thanks for the help - I finally got around to buying and putting a pressure gauge on the outside bib, and apparently I'm getting street pressure at the house (145 psi). I found a small PRV under the house, bought a new one and will replace it Tues. Hopefully that will take care of it although I am scratching my head as to why it leaks at night more than during the day. Maybe through normal daily use the pressure is relieved just enough but at night there is no water use to relieve the pressure
Thanks again!

I am scratching my head as to why it leaks at night more than during the day. Maybe through normal daily use the pressure is relieved just enough but at night there is no water use to relieve the pressure BINGO! Dead on! Get the PSI down to at least, 60 PSI. Too high a PSI puts a strain on both fittings and faucets. Good luck, Toim

Pressure will increase at night because of reduced demand on the system. The pump still pumps but it doesn't have to supply the daytime user volume. We always did hydrant flow tests during the day because it would give you a better idea of system demand.