T&P Valve Leak Mystery

[Doug Bert]

SUSPECT: A 2 1/2 year old Rheem 40 gal short gas water heater equipped with an expansion tank.

PROBLEM: In the last few months it will drip 2-3 ounces of water when someone takes a bath, we run the washing machine, or a shower.

First I opened the T&P valve and got some rust out. I have bled it three time now in the last month. Only the first had anything but clean water. After the second bleed it seemed to stop for 1-2 weeks but now it's back to square one.

The pressure tank was pumped to the pressure of the line at the time and I've tap tested it and there is still air in the bladder.

SOOOOO - Do I need to replace the T&P? Or is there something I'm missing.

Don't know if it is useful information but our old AO Smith (replaced at 17 years old) never had an expansion tank and only dribbled about three times in it's life. None of them being more than a few drops.

 

[Reach4]

SOOOOO - Do I need to replace the T&P? Or is there something I'm missing.

The bigger your water heater is and the hotter you heat your water, the bigger that thermal expansion tank needs to be.

You need to set the precharge air when the water pressure is zero. When the water is on, and a faucet is dribbling, the tank should be empty of water.

If you suspect the T&P valve, I suggest you buy a water pressure gauge with a garden hose thread. I expect it will show high pressures after you take a hot shower and stop using water.

 

[Sponsor]

 

[Dj2]

Replace the T&P valve, and see if the leak stops.

They go bad from time to time.

 

[Jadnashua]

The T&P valve will open for one of two reasons (well, there' a third, if you manually open it!): excessive temperature (typically, just below boiling), or excessive pressure (typically >150psi). If it DOES open for any reason (this includes you opening it) and some crud gets on the seals, it MIGHT not reseal, or, if the shaft of the thing has some crud built up on it, it may not freely move back into position to seal it.

By far, the biggest reason it opens is when the ET has either failed, isn't large enough, or has lost its precharge. As said, if you check the ET without shutting off the water supply and opening a faucet to relieve the water pressure, when you check the air pressure, you're really just checking the water pressure at that time. To check and adjust the air pressure, or precharge, you must shut the water off and relieve any water pressure in the pipes, then check and adjust the air pressure. It should be set to the typical system water pressure so it sits at its neutral position normally, ready to accept any expanding water caused during heating cycle.

If you have a PRV in the system, it may have failed, and that, in some places, may have high enough incoming water pressure to cause the T&P valve to open...In this case, the ET can't help - it only helps if the rising pressure is the result of expansion and you have a closed system.