Why would T&P valve leak when T and P are really low?

[masterjer]

I had a leaking T&P valve that caused quite a mess in my basement. I figured the 30-yr old valve was gunked with sediment, so I replaced it, but the new one also leaks.

I turned down the house supply pressure on the Watts pressure regulator. My pressure meter shows I'm at about 40psi (which makes filling the tub take forever). I also turned down the temperature on the heater so it comes out at about 125*F.

What else can I look into to solve the T&P leak? If it makes a difference, I have a 100gal gas water heater and the house was built in 1981.

 

[masterjer]

I also have an expansion tank installed on the incoming cold water line.

 

[havasu]

Just to confirm, you aren't flipping the valve open before of after installation, right?

 

[masterjer]

Yes, that is correct, I have not flipped the valve.

 

[havasu]

I become concerned when that valve pops open. I also know many are defective from the factory. Something seems amiss. If you are at wits end, it may be time to get a professional in there to see if something is wrong.

 

[frodo]

replace the 30 year old valve, with a new one
then watch the new valve, if it does not pop off it was just an old crusty valve

the valve should be piped to a floor drain or outside so that it does not make the mess that it made

 

[masterjer]

I did replace the valve - that was step 1 for me. There was a bunch of sediment in it, and when I drained the water heater, a bunch of gunk and brown water came out the bottom of the water heater. Unfortunately, there isn't a drain anywhere near the water heater, so the valve does cause a mess. I'm not quite at my wits end yet, but I'm trying to understand what else could be the cause for this so I can continue my trouble shooting.

I'm wondering if perhaps my pressure regulating valve on the incoming water supply is bad and the city is bumping up the pressure at odd times which trips my T&P valve?

 

[frodo]

I did replace the valve - that was step 1 for me. There was a bunch of sediment in it, and when I drained the water heater, a bunch of gunk and brown water came out the bottom of the water heater. Unfortunately, there isn't a drain anywhere near the water heater, so the valve does cause a mess. I'm not quite at my wits end yet, but I'm trying to understand what else could be the cause for this so I can continue my trouble shooting.

I'm wondering if perhaps my pressure regulating valve on the incoming water supply is bad and the city is bumping up the pressure at odd times which trips my T&P valve?

very possible, pressure goes up at night because every one is asleep, non use

https://www.grainger.com/product/20JN40?cm_mmc=PPC:+MSN+PLA&s_kwcid=AL!2966!10!14921217446!1102900071574&ef_id=WcNaHAAAAY-QNw0N:20171017182148:s
get one of these gauges, it will record the highest pressure that it reads
install it, behind the clothes washing machine on the cold water valve
check it in the morning
if it has has spiked, you need a new pressure reducing valve
BTW...30 years on the water heater? that thing is cooked, get a new one

 

[CT-18]

T and P valves generally go at 150psi and 210°. I don't think you're getting that much pressure at the street.

 

[masterjer]

Thanks Frodo, I just ordered the gauge with the high pressure arm. The one I got locally doesn't record the highest pressure. At least that will allow me to diagnose the PRV. Why would you suggest installing behind washer? Would that read differently than a hose bib? Unfortunately, my PRV is behind a bathroom vanity cabinet and inside a wall. That will be fun if I need to replace it.

 

[voletl]

Check and see if your expansion tank is still good

 

[PlumberMike]

were is the temperature at on the tank u could be heating the tank up to high

 

[masterjer]

I had the temperature turned down pretty low, so much in fact that my kids were complaining that the tub doesn't get hot enough. I turned it back up a bit, but it is nowhere near a dangerous temperature, so I don't think I'm superheating the water to a point that would trip the valve.

The expansion tank is relatively new, within the past 3 years, so I don't think that is the issue. I will get the pressure gauge with the lazy arm tomorrow and check to see if my PRV is going bad.

 

[frodo]

Thanks Frodo, I just ordered the gauge with the high pressure arm. The one I got locally doesn't record the highest pressure. At least that will allow me to diagnose the PRV. Why would you suggest installing behind washer? Would that read differently than a hose bib? Unfortunately, my PRV is behind a bathroom vanity cabinet and inside a wall. That will be fun if I need to replace it.

some hose bibs are before the prv,
since i have no idea as to your piping configuration i said to install behind washer where i KNOW that piping is after the prv
IF you KNOW the piping to hose bibs is after the prv, then by all means
feel free to install outside

 

[masterjer]

I've resigned myself to the fact that I believe I need to replace the PRV. Even after adjusting it multiple times, the pressure at my hose bib is over 100psi. When I turn on a sink, it drops to below 10psi. After I turn off the sink, it slowly climbs back up to over 100psi.

 

[frodo]

me thinks you are correct

 

[masterjer]

me thinks you are correct

I spent a few hours over the weekend moving the bathroom vanity and opening up the wall to get good access to my PRV and water shut off valve. The shut off valve was a joke. It is a ball valve with no handle and no stops, so I have no idea the orientation of the ball.

I cut a section of the existing copper to make room for a new shutoff valve and the new PRV. I opted for the double union PRV to make replacement much easier in the future. I had to raise the sink water supply valve a few inches. While I used too much solder (more is better than too little I figure) I was very happy that I got no leaks and the pressure is now rock solid.

I had 110psi at my hose bib and turning on a faucet dropped it to 10psi, now I am solid at 50psi and it drops to 46psi when I turn on a faucet. Must better.

Thank you all for your comments and suggestions.