High initial pressure that tapers off after 10-20 seconds

[Danester]

I did a quick forum search and I'm not sure I have the same issues others have had with needing to install expansion tanks.

When I turn on either hot or cold (or mixed) the initial pressure is somewhere around 150psi (confirmed with a gauge at the hose outside). Then after 10-20 seconds or so the pressure tapers off and drops to around 40psi or so which isn't sufficient for this two story house. We had higher pressure before I adjusted the PRV but had the same symptoms described above (just with a higher running pressure). I thought the problem might have been with the PRV setting so I cranked it down.

Could it still be thermal expansion even though it happens with the cold side as well? What other troubleshooting can I do?

 

[Reach4]

Could it still be thermal expansion even though it happens with the cold side as well? What other troubleshooting can I do?

The pressure increase from thermal expansion affects hot and cold lines both.

However, it takes very little water use to bring down the excess pressure due to thermal expansion if you have no expansion tank, and would take under a gallon (usually way under a gallon) of use in the presence of an expansion tank.

I think you need a new PRV, and maybe a new expansion tank.

 

[Sponsor]

 

[Danester]

I don't currently have an expansion tank. House built in '78 and the last time they replaced the water heater (before I bought the house) they didn't bother to add one.

So should I try the expansion tank first then a new PRV?

 

[Reach4]

Maybe... but that would only be enough if opening a faucet to a low dribble kept the pressure normal. Your symptom of having the pressure stay high for a significant time after opening a faucet is weird. I am thinking you will need a new PRV too.

 

[Gary Swart]

If you are measuring 150 psi you're luck you have any toilet valves or washing machine valves left. 60 psi is enough pressure, up to 80 while high is still acceptable. Your PRV has a check valve that prevents the excessive pressure from escaping into the city main. I think you PRV is not working right, and you definitely must have an expansion tank. I find it strange you don't mention the T/P valve on your water heater leaking.

 

[Danester]

Well, that's what started this all. I was about to leave town and noticed the T/P valve leaking. Turned the gas off to the heater, closed the inlet valve and left the extension pipe in a bucket once it stopped leaking.

Came back home, fired up the heater, turned the inlet supply back on and sure enough it started leaking again. Ran in for a bit to clear any deposits out but inevitably it started leaking.

Long story short, I think I've got the static pressure down below 150 psi, because after installing a new T/P valve there are no more leaks. (Adjusted the pressure before installing the new T/P to be sure the leak was not pressure related.)

Before I fiddled with the PRV static pressure was measuring over 160. Unbelievable. It definitely seems to be on its way out. I just wasn't sure about the pressure variation. Seems so odd to me. It's done this since we moved into the house. Looks like I need to call the plumber and have him replace it and add a expansion tank.

 

[Flapper]

If you are measuring 150 psi you're luck you have any toilet valves or washing machine valves left. 60 psi is enough pressure, up to 80 while high is still acceptable. Your PRV has a check valve that prevents the excessive pressure from escaping into the city main. I think you PRV is not working right, and you definitely must have an expansion tank. I find it strange you don't mention the T/P valve on your water heater leaking.

PRVs don't have an integrated check valve; they function like a check valve because they prevent backflow.

 

[Flapper]

How old is the PRV? It's almost certainly leaking. The PRV has rubber parts in it that deteriorate over time, including the seal of the part of the valve that stops the water; when the flow stops, the valve closes to keep the pressure; if the valve leaks through when closed then the downstream pressure will slowly increase until it reaches near or the same as the input pressure.

You need to either rebuild the PRV, or replace it. Rebuild kits are expensive and many people prefer to replace it, but rebuilding it is good if replacement is difficult and/or risky, like if it's in galvanized pipe.

If it has unions on both sides then it should be easy to change out.


I had the same problem; the pressure relief valves (didn't know they were relief valves at first) that came with the water heaters I installed, were leaking; turns out the static pressure was 160.
Since I didn't know they were relief valves when I installed the WHs, I took them out, and it was fine for a few months until one started leaking from the canister, then I researched and (thanks to this forum) found out that it was a relief valve and that we had high static pressure. Had no idea we had high static pressure before.

 

[WJcandee]

the pressure relief valves (didn't know they were relief valves at first) that came with the water heaters I installed, were leaking; turns out the static pressure was 160.
Since I didn't know they were relief valves when I installed the WHs, I took them out,

 

[CountryBumkin]

Long story short, I think I've got the static pressure down below 150 psi, because after installing a new T/P valve there are no more leaks. (Adjusted the pressure before installing the new T/P to be sure the leak was not pressure related.)

You really need to "be sure" the pressure is now under 80psi. Otherwise your going to be chasing problems for a while.

 

[Danester]

I'm certain it's not under 80 which is why I've got to get the plumber out to install a new PRV and T/P valve. I think the PRV swap out is beyond my skill set. That said the old one and new one I plan to use are the exact same model and are FPTxFPT so even for me it wouldn't be that hard.

Old one is at least 20 yrs old if I had to guess. It's a Watts unit and they make the exact Sam one still.

 

[Reach4]

There are rebuild kits for some.

Your new PRV may be a different model number that starts with LF, which stands for Lead Free. But otherwise it may look the same.

 

[Danester]

Good to know that's what the "lf" means. This is the model I would use. I guess the original is old enough to not carry the "lf" in the model number.

https://terrylove.com/watts.htm

 

[Flapper]

Your new PRV may be a different model number that starts with LF, which stands for Lead Free. But otherwise it may look the same.

I noticed that too; our old PRV is 25AUB which is discontinued but replaced by LF25AUB... I recently realized LF meant lead free and that's the difference. I also realized that all our old brass stuff is probably not lead free... is that bad?

 

[Flapper]

Good to know that's what the "lf" means. This is the model I would use. I guess the original is old enough to not carry the "lf" in the model number.

https://terrylove.com/watts.htm

Oh cool terry has an article about our model of PRV with a nice diagram; thanks.

 

[Gary Swart]

Sorry about the major error when I said the PRV has a check valve instead of saying the PRV acts as a check valve. Wow. Anyway, let's start over. You need a PRV to reduce that insanely high pressure to at least below 80 psi. Next, you need thermal expansion tank on the supply line following the PRV, and the expansion tank should be pressurized to the same pressure as the PRV. There was nothing wrong the the T/P. When it opened, it was functioning as is should. Basically what happens is this. When water heats, it expands. It does not compress like air. That expansion has to go somewhere. A PRV prevents it from being absorbed by the city water main, so it seeks a weak spot in the system. Sometimes there maybe a weak toilet valve and that will provide an escape for the expansion, and since the volume of water is quite small, this leak would go unnoticed. If the expansion can't find a weak spot, the pressure in the water heater quickly rises to the T/P limit and the T/P opens until the pressure lowers and the valve closes. The actual volume of water loss is quite small, but it can damage floors, but if there was no T/P, the water heater would explode as shown in the previous video. Replace the PRV, add an expansion tank, and you should be good.

 

[Reach4]

I also realized that all our old brass stuff is probably not lead free... is that bad?

No.

https://terrylove.com/forums/index.php?threads/pitless-adapter-stainless-or-brass.66455/#post-493976

The lead is locked up in the brass, unless you have acid water.

I would expect you could use the LF rebuild kit, but I am not sure.

 

[Flapper]

No.

https://terrylove.com/forums/index.php?threads/pitless-adapter-stainless-or-brass.66455/#post-493976

The lead is locked up in the brass, unless you have acid water.

I would expect you could use the LF rebuild kit, but I am not sure.

I used parts for an LF25AUB in a 25AUB; the parts are the same and they all fit the same; there were only some minor design differences like the shape of the top of the caps.

If I were to rebuild the loop, can I reuse the old not-lead-free RPZ? The loop is galvanized pipe and the main valve it connects to, leaks, etc, so I plan to redo it with copper in the future.

 

[Jadnashua]

Personally, if I were going to rework the old install, I'd probably opt for new parts.