Help..T&P valve leak, etc..

Bought the house in 2007, its older, built in 1959 I believe. Had to replace the hot water heater in March 2008. Has worked great since then, until a few days ago there was a good amount of standing water on the basement floor under the water heater from a constantly dripping T&P valve (I know, I need to connect a drainage pipe to outside). I replaced it and still had the same problem. That was the limit of my "vast" plumbing knowledge, so a plumber came and checked pressure (72 psi) and could find nothing else wrong so he replaced T&P valve again. Next morning, buckets full of water again. I turned off the water shutoff valve to the water heater itself, but it still leaked even with the water supply turned off (baffled!), so I finally had to shutoff the main water into the house. Plumber came back this morning (on Thanksgiving, great guy!) and suggested a thermal expansion tank as the only other option he could think of. Any suggestions? Any ideas why this problem would just now arise after a year of an old water heater and 18 months of the new one with no issues? What kind of cost should I expect for the expansion tank and labor? Thanks so much. Y'all are great.

i would say get a portable water expansion tank install reason if T&P valve letting go after been replace only thing i can think of when your water heats up it builds up pressure to much of will let ur T&P to pop as for cost all depends you can buy it home-depot make sure you ask for a portable water expansion tank.

Save some Turkey will ya!
have a safe one.

Also and a buddy same issue he had a in-line fillter install in the main line when he took it off this problem started.

Make that Potable not Portable....

The valve opens for one of two reasons: pressure too high, or temperature too high. If the tank's thermostat isn't defective and the temperature is too high, then it's pressure.

Measuring the static pressure one time will not tell you if the pressure gets too high. It is fairly common for thepressure to be (sometimes) much higher overnight while people are asleep. So, to validate this, you'd want a peak reading meter. You can buy one for about $10. Leave it hooked up overnight.

But, if you have a PRV, or a checkvalve somewhere in the system, you need an expansion tank. Water expands when you heat it, so it needs somewhere to go. In some configurations, it can just push back out into the street's supply line. If there's a checkvalve preventing that, you need an expansion tank, or it will come out the weakest point...often the T&P valve.

ANother but, a WH heating the water would not generate a couple of gallons...it just doesn't expand THAT much. So that leaves pressure (or a bad T&P, but not likely three in a row).

I agree with jadnashua, you need to put a test gauge on the system overnight. This will show the peak pressure of the system with a resettable indicator needle. If it's a thermal expansion issue it will be apparent after the water heater finishes heating back to temp after a shower or other substantial draw.

The buckets of water aspect is weird though and suggests something else if it is really that much.

Have you changed ANYTHING with respect to plumbing in the month or two before this problem appeared? Did you fix any drips? Did you replace any toilet fill valves?

My T&P started relieving shortly after I replaced the old toilets. The new toilets have tighter fill valves apparently, because when I had the test gauge on before temps would spike somewhat after showers, but not much over 100 psig max. When I replaced the final toilet the T&P started making a puddle by the heater during the firing cycle. Reinstalling the test gauge confirmed that the pressure was above 150 and relieving as a result. So I added a thermal expansion tank which eliminated the problem.

I was able to use the smaller standard expansion tank and it took about 1 hour labor to install so my cost for parts and labor was ~$150. If you really have that much water coming out you will need a larger tank than what I used though. (I had mine set up so that I can swap to a larger tank if I need to.)

When you shop for the expansion tank, just look at the boxes. There will be a chart guiding you to the right size for your home. No advantage in going larger than necessary.

tank

NO real disadvantage is going undersized either, since most only have to absorb about a cup of water to maintain the pressure within limits. Unless you have a closed system, an expansion tank will be a waste of time and money, and if you do have one, there is no way to eliminate the problem without one. The drain from the T&P valve can only go to the exterior, IF you can do it without going higher than the relief valve at ANY point in the piping. In other words, it MUST run/drain downhill all the way.

HJ makes a good point about only a closed system requiring an expansion tank. It used to be that a PRV automatically created a closed system, but some PRVs today have a means of allowing the expansion to bypass into the city water main. However, some cities are worrried that expansion into their main could cross contaminate their water supply so they are installing water meters that have a check valve to block the expansion from the main, and that would create a closed system.

A PRV with a by-pass will only by-pass if the internal pressure is greater than the street pressure...the street pressure might be so high that the PRV would still trip, since it may be lower than the street. Safest is to just install an expansion tank if you have a check valve or PRV, then you aren't subject to that possibility, or water everywhere if the valve fails or sticks.