New Water Heater, now T&P leaks

I just replaced the old water heater with a new bradford white gas heater. Everything went smooth until I fired it up and about 15 minutes later the relief valve started dripping. I had the heat set to 'hot' as a starting point per the manual. I searched this site and found that a few others had issues like this, so I have some direction. Now I need to isolate what is causing it, temp or pressure. There is a PRV in the line from the city but I don't think there is an expansion tank, I'll have to look closer to see if its hidden somewhere. I also need to get a pressure gauge to see what is happening.

If you have a PRV, look no farther for the problem. You need a thermal expansion tank. The TP is doing its job. If you put a pressure gauge on the hot water line and watch it when the water heater begins to heat, you will be amazed at how fast the pressure rises. It will get to 150 psi very quickly and the TP wiill trip as it is supposed to do. What is actually happening is the water heats and expands. It needs to go somewhere and since the PRV has a check valve in it, the expansion can not be absorbed by the city water main. This causes the pressure to rise and trips the valve. The expansion tank goes in the supply line between the PRV and the water heater.

That is what I suspect but would like to test before installing a tank. I also suspect that the old heater had a bad T&P valve since it didn't drip.

While I'm still betting the farm on the need for an expansion tank, testing with a gauge is always wise. In fact, I installed a gauge between my PRV and expansion tank so I can monitor the PRV. You can get a gauge at virtually any hardware store for around $15 then adapt it to screw on to a faucet like at the washing machine or utility tub.

The old tank was leaking, why would it need a pressure relief? It was already losing pressure.

Did you recently repair a toilet, like replacing a flapper or fill valve?

Ok I got a test gauge and it peaked at 155 psi with water heater off, T&P valve started dripping at 150 like its supposed to do. Now that I think of it the heater started leaking after I replaced a toilet that had a leaking flapper, that was probably keeping the pressure below 150. So the PRV isn't working, its a Watts Type 5 that looks like it is adjustable but I didn't even try adjusting. That probably explains the intermittant leaking water filter also, its max psi is listed as 100. And I should install an expansion tank since I don't see one anywhere in the system. I like the idea of gauges mounted before and after the PRV, I think I'll try that too. Are the PRVs and expansion tanks sold at box stores OK or should I get them at a plumbing supply house? They are probably the same brands but I wonder if the ones in the box store are made cheaper. Thanks.

Rumor is that (on some stuff) the manufacturer passes stuff to the big box store right off the production line, without a QA/QC check. Most are fine, but some aren't. Those that go to a plumbing supply chain get the whole QA/QC, so the chances of a defective product are lower. On others, they are made less expensive than the 'commercial' version.

Replaced the pressure reducing valve last night, it was preset to 50 psi. Took my chances and got it from home depot. I did open the box and look it over first, the first one I looked at had one of the screws sticking up about 1/4" so I picked a different one. Now I measure 53 at the water heater without it turned on, when I open a faucet the pressure only drops to 50. Sounds good to me. I need to install an expansion tank now (tonight), when i turned the heater on the pressure rose to 100 within 10 minutes or so. Thanks for all the help, this is a great forum and full of information.

Expansion tank now installed. Still 53 psi before water heater turned on. Turned heater on to hottest setting and 30 min later pressure never went above 60. Everything looking good now.