There's not many considerations but the most important is, are we talking about a boiler,tankless water heater,tank type water heater or something else.

I know in the past, our water pressure has run rather high but I found my gauge broken so don't know exactly what it is at this time.

My understanding is that 150 psi is probably higher than the city water pressure typically supplied to a house but if one were getting 150, 160 psi or slightly higher and the pressure relief valve is rated at 150 psi, is this what would occur?
Just enough pressure to push open the valve to create a continuous leak?

I don’t know our current water pressure as my gauge is broken. I plan to purchase another soon. I know that in the past, particularly over night, the pressure has gone up to 180 psi or higher. There is no pressure reducing valve installed (yet).

Installed is a (Bosch AquaStar Natural Gas #250SX-NG) Tankless Water Heater.
At the outlet of the heater is a high Pressure Relief Valve (specifically, Cash Acme FWL-2)

As mentioned, out of it recently has been a consistent flow: not high pressure but more than a drip or a trickle.
I had always thought of these type of valves as more or less, “binary”: fully open if the pressure exceeds their rating or, otherwise, totally closed. I'm beginning to think that may not the case.

I spoke with someone who suggested the possibility of crud or sediment coming through and not allowing the valve to reseat. This seemed plausible so I tried tapping it, opening it several times attempting to flush it out and reseat itself but there was no change. My assumption was that, as it is more 10 years old, for whatever reason, the valve failed. I purchased a new valve, installed it, bled the air out of the lines, powered up the tank hoping to see everything back to normal but the new valve behaves exactly the same as the previous: a continuous flow of about 16 oz. per minute.

If I power up the water heater, open its supply valve, and open a hot water tap everything performs as it should; the tank supplies plenty of hot water & everything's fine. When the hot water tap is closed a steady stream of water leaks from the pressure relief valve; hot at first, eventually running cold.

High temperature/thermal expansion can't be an issue as this low flow condition continues even when the heater is powered off, to factor out the possibility of a faulty power switch, even entirely unplugged. I've only managed to stop the leak shutting off the water supply to the tank.

I welcome your comments regarding whether or not you feel I’m on the right track. I'm thinking my next course of action is to purchase a new pressure gauge. If water pressure exceeds 150 psi, that may be the cause of the problem and installing a pressure reducing valve is the solution.

If the water pressure is below 150 psi but a steady flow from the pressure relief valve continues, then what?

As usual, Thank you for any assistance

"I'm thinking my next course of action is to purchase a new pressure gauge. If water pressure exceeds 150 psi, that may be the cause of the problem and installing a pressure reducing valve is the solution."

That's exactly what I would do.

I don't know how tight the tolerances are on valve ratings or how accurate our new pressure gauge is but I attached it today and got something right around 148 psi. The valve is rated at 150 psi. Theoretically, it should be closed but if either of these is off by only a few measly psi, is the condition I described the outcome one would expect?

This (leaking) is a new condition but no changes have been made here in the house. I called the city water dept today attempting to learn whether they've recently increased the water pressure -- the only thing I can think of to account for this-- but I was unable to speak to anyone who might know nor were my phone calls to, supposedly, the only one there able to answer my question returned. (Perhaps too many other folks calling in about the same?)

Whatever the case, if “residential water pressure should not exceed 80 psi.” it seems a pressure reducing valve is in order, regardless. Does this seem to make sense to yall or is there something else I may be overlooking?

Thanks

148psi doesn't make sense but if that's what the new guage said that must be the case.

I would think that that is 60psi higher than it should be.

I would expect a valve rated for 150psi to have a 5%-10% tolerance.

And I would expect my DW and clothes washer solenoids to be acting up at this pressure.

One of my pet peeve's has been people who post on forums seeking assistance, asking for help, but then, once they find the answers necessary to solve their problem, don't bother reporting the solution leaving the string the dangling! It’s as though they don't realize that a public forum is not solely for their use or benefit but also (arguably more so) to serve others that will be searching for the same information about the same issue, possibly, for many years to come.

I realize we’re all more motivated to write when pressured by those problems that need solving but, considering the time spent and efforts made by others kind enough to generously offer their assistance, taking a few minutes to post a solution once found is the least one can do.

I didn't get back to this as soon as I would've liked to but I’ll not be one of those people so back to my story and its exciting conclusion.

They've recently been replacing the main gas lines in our area. I didn't immediately see any connection but it seemed too much of a coincidence that these previously nonexistent leaks and drips should randomly begin to occur very near the same time that construction began. I talked to one of the guys which lead to talking to some of the other guys which eventually lead to speaking with the guy in the water department. What I learned is that the waterline in this area is, more or less, a loop supplied on both ends. In the event that, while excavating to install the gas lines, some sort of damage is done to the water line that
necessitates an emergency shut off, they shut off the supply on one in advance so that, if needed, they could immediately stop it by shutting off the other side. This apparently caused an increase in the water pressure of the loop. He (the guy from the water department) opened up the other end and our pressure here dropped from 148 down to 120, apparently still considerably higher than normal or is desirable, but low enough to make a difference. All leaks have stopped.

So, to answer my own original questions, higher than what is considered normal water pressure —but not necessarily in excess of its stated psi rating —will or can cause flow from a pressure relief valve. Also (I was previously unaware of this) other fittings, valves etc. throughout the house plumbing are only designed to withstand up to a certain pressure.

As for me, I suppose I still need to install a pressure reducing valve but I don’t need to install one today. In the meantime, I may be able to pressure wash the house without a pressure washer!

Thanks for for the update and conclusion.

Thanks for the info. Get on that PRV though, that is really high pressure on all your fixtures. I would fear a break while I was gone and water everywhere.

Like Randy247 mentioned if you add a PRV then don't forget to add a thermal expansion tank for your water heater.

I'm certainly no expert in this area so I encourage you to correct me if I'm wrong.

My understanding is that the pressure reducing valve will act as a check valve changing the system from open to a closed system so, normally, with a traditional water heater, a thermal expansion tank might be in order but since this is a tankless water heater the only time the water is heated is “on demand” when a tap is opened making an expansion tank unnecessary.

Is the fact that we’re running a tankless water heater an overlooked factor or am I missing something?

Thanks

So, if your water pressure is above 80 psi, you need a PRV. Most plumbing is rated at a maximum working pressure of 100 psi. Just because the last chance safety device isn't opening up (150 psi) doesn't mean everything is ok!

You are correct, you don't need an expansion tank with a tankless heater (providing you don't have a recirculating pump or any other situation that would activate the heater without a faucet open).