Most water heaters come with a new relief valve.
What is the incomming water pressure?
Do you have an expansion tank on the system?
Was the system flushed out when the new system was installed?

The queations can make a difference; i.e. a new relief valve can be defective, [rare]

If you have excessive pressure comming into the house [approx. 65 lbs.] thermal expansion will cause the relief valve to open.

If you have an expansion tank it sounds like it is not working

New installations should be flushed out so no debris gets into the system,i.e. blow off the relief valve until it runs clean.

Bottom line: if you just had this installed it is the responsibility of the one that put it in. Hope I've helped.

"If you don't have the time to do it right, where will you find the time to do it over?"

Last year I began getting a sporadic release from the T&P valve on my gas water heater. Like most DIYers I pursued the easiest solution and replaced the T&P valve with an exact replacement (there are different T&P valves and different specs).. The problem continued.

I called Watts (the manufacturer of the OEM and replacement T&P valve) and spoke to an engineer there. I asked what the incidence of defective T&P valves is. His reply was that since T&P valves are a safety device there is immense product liability and they are throughly tested and the DOA rate is EXTREMELY low ... almost none he said.

That's when I discovered "thermal expansion". Sure enough, I did have a PRV out at the water meter that prevented the water in the house plumbing system from equalizing presuure by backflowing into the water system. With a closed plumbing system every time my water heater fired the water would expand and if that excess pressure was not relieved by coinincidentally opening a faucet or flushing a toilet at the right moment the T&P valve would "burp" a cup full of water.

I found a nifty pressure gauge with a "tattletale" needle at ... http://www.wattspremier.com/watts/showdetl.cfm?&DID=15&Product_ID=93&CATID=1

Low and behold the tattletale needle showed enough pressure to pop the T&P valve.

So, instead of treating the symptom I cured the disease ... installed a thermal expansion tank and all is well.

Funny thing is, now that I don't have a thermal expansion problem I also don't have those dripping faucets anymore. Maybe because the faucets don't have to deal with water pressure spikes up to 125lbs???

I bet an expansion tank will solve this problem.

Thanks guys...I'm planning on installing an expansion tank to see if that'll help. Is that something I can do on my own with no prior experience? I've snaked drains and installed toilets, but never sweated copper pipes.

Here's the installation for an expansion tank ...
http://wattsreg.com/pdf/1915356.pdf

Learn to solder or maybe best to call a plumber.

So I installed a Thermal Expansion tank, and it definitely leaks a lot less now, but still leaks after a couple hrs of not using the water. Could there be something else I could be doing? Perhaps I didn't precharge the Thermal Expansion Tank correctly? What else could be wrong? Incoming pressure is right at about 40 psi.