Hot water recirculation pump orientation

[Niccolo]

A working PRV should hold the outlet pressure essentially constant (well, prevent it from exceeding the set value because of a supply pressure rise). It also acts like a check valve, so any thermal expansion has nowhere to go. That's why you also need an expansion tank. Setup properly, even with thermal expansion, the pressure won't rise more than a couple of pounds, if that (the water expands into the tank). Otherwise, the pressure will easily exceed 150-psi, at which point, if there are no other leaks to prevent it from getting that high, the safety valve on the water heater will open. MOst supply lines are quite rigid, so expansion immediately creates a pressure rise.

If your pressure didn't rise after hot water use and you weren't using water otherwise, then there's no check valve (the water pushed back into the supply), and likely, no ET. IF, otherwise, the pressure did spike, it was probably because the utility raised the pressure. You could get a spike if there was a high rate of flow and a quick acting valve closed as it rebounds off the the valve (a water hammer situation).

Yeah, all day long the pressure fluctuated between 53 and no more than 58 PSI (assuming my gauge is to be believed), even with the hot water heater sporadically coming on.

I'm curious to see whether it spikes to ~140 PSI again tonight. EDIT: Spike to 180 PSI overnight, if the tattletale hand is to be believed!

I do not have an expansion tank.

Sounds like there's not a check valve.

Sounds like there either isn't a PRV or if there is it isn't set up properly. Where would a PRV typically be located, i.e. where would I look for it and, assuming I don't have one, where might one get installed?

I'm inferring than an expansion tank wouldn't help with my apparent utility-imposed pressure spikes, only a PRV would help with those (but a PRV would necessitate an expansion tank).

EDIT: Found my water meter in a box near the street, photo attached. Can one infer anything about a possible check valve or PRV from that photo? I'm guessing it's just a water meter.

 

[James Henry]

It's nothing more than a water meter and 2 shut offs. if you do some digging you might find more.

 

[Sponsor]

 

[Niccolo]

So a pressure relief valve (PRV), sometimes also called a pressure reducing valve, is a device that can be set to prevent excess pressure, the level of which can be adjusted, from transmitting through the municipal water line into a house.

The valve on a water heater that vents above 150 PSI is also called a pressure relief valve (PRV), right?

And there's yet another device which dumps water if excess pressure is present (with the downside that it wastes water), is that also called a pressure relief valve (PRV)? I don't mean the ones on water heaters, I mean something for a house as a whole, and set much lower than those on water heaters.

In my situation, with excess pressure around 140-180 PSI overnight, apparently as the utility fills up the water towers, would that third kind of device, which dumps water, waste a huge amount of water? I'm guessing yes, so much so that it might not even be allowed under building code, we have scarce and expensive water here in coastal California.

 

[Jadnashua]

A pressure RELIEF valve and a pressure REDUCTION valve are two entirely different things. They may achieve similar results - limiting pressure, but do it in entirely different ways. A RELIEF valve literally opens and dumps fluid out of the system to try to keep the pressure down. A REDUCTION valve has an internal system that does what it says...reduces the pressure of the outlet.

The safety valve on the top of the WH has two separate functions and is called a temperature and pressure valve (T&P). It will open if the temperature exceeds 210-degrees, and/or, if the pressure exceeds 150-psi. Its goal is to prevent the WH from exploding from steam expansion, or just damaging the thing from excess pressure. Liquid water to steam has about a 1:1600 ratio...humongous force. Thermal expansion of liquid water is still impressive, though, but because it is essentially incompressible while liquid, can do as much damage, but the amount of expansion is almost insignificant that of steam.

THe more common location for a PRV is inside the house before the water branches off to various devices. An expansion tank should be installed at the cold water inlet to the WH so that there's nothing between it and the tank.

 

[Niccolo]

I just had a very substantive conversation with yet another licensed plumber; really appreciate the education this forum provided me on these topics!

He definitely thinks a pressure reducing valve is warranted, given the nighttime pressures (140-180 PSI) I'm seeing. My water comes into my house sheltered by a modest roof overhang, near my front door, via a pipe that runs from my water meter in a concrete box next to the street, under the sidewalk and front lawn. This seems to be the obvious location for a pressure reducing valve. We're in a mild climate where frost is very rare. The proximity to the ocean does corrode some metal stuff faster, though. I'm guessing the outdoor location is fine under the circumstances, there isn't an obvious location inside.

It would be easiest to install the pressure reducing valve after a junction that feeds automatic landscaping sprinklers, but I'm guessing it's worth a little extra plumbing to also protect the sprinklers from excess pressure?

Is there anything I need to know about what sort of pressure reducing valve gets installed?

It's interesting, the plumber wasn't all that knowledgeable about the need for an expansion tank given a pressure reducing valve that will act like a check valve, meaning that my water heater will generate pressure that can no longer be dissipated out into the supply lines. His take on expansion tanks is that they're only required in code for commercial installations, but he admitted to being fuzzy on the science. It's my impression, from you guys, that an expansion tank is basically required if I have a pressure reducing valve installed.

Anything else I need to know?

 

[Niccolo]

On a separate note, recirculation pump (which is located between the hot water return pipe and the water heater drain outlet (serving as an inlet for the returning less-hot water) is being replaced tomorrow under warranty. The broken one is currently unplugged, and plumber turned off the valve just before it. That's fine, right? Basically, hot water can exit the top of the hot water heater and flow into the house, but can't return to the water heater because that valve is now closed. Plumber said this was fine and even desirable, to keep water from getting pulled the wrong direction into the hot water return piping.

 

[wwhitney]

While elevated incoming water pressure is a possibility for the pressure spikes you are seeing, I'm wondering if you have enough data to conclude that is the reason and act on it.

If you have a couple battery web cams, you could set up one to record the pressure gauge and another to record the flame on your gas water heater (if you can find a place with the right vantage). Then if the two videos are time stamped, you can check whether the pressure spike occurs as the water heater fires, with the pressure rising until the water heater shuts off. If it does, that is a clear sign that you have a check valve you don't know about and you need an expansion tank.

Or, since either way you need an expansion tank, you could just install the expansion tank now and see if it helps. If it does, that's again a sign that there is a check valve somewhere between the water main and your water heater that you don't know about. If it doesn't, go ahead and put the PRV in.

Cheers, Wayne

 

[Niccolo]

While elevated incoming water pressure is a possibility for the pressure spikes you are seeing, I'm wondering if you have enough data to conclude that is the reason and act on it.

If you have a couple battery web cams, you could set up one to record the pressure gauge and another to record the flame on your gas water heater (if you can find a place with the right vantage). Then if the two videos are time stamped, you can check whether the pressure spike occurs as the water heater fires, with the pressure rising until the water heater shuts off. If it does, that is a clear sign that you have a check valve you don't know about and you need an expansion tank.

Or, since either way you need an expansion tank, you could just install the expansion tank now and see if it helps. If it does, that's again a sign that there is a check valve somewhere between the water main and your water heater that you don't know about. If it doesn't, go ahead and put the PRV in.

Cheers, Wayne

Interesting info.

I don't have webcams, so this isn't easy to implement.

I've noticed over several days that the pressure never goes outside the 53-58 PSI range during the day, and so far has gone to 140 and 180 during the past two nights, when I've been monitoring. That's a bit hard for me to square with the notion that the excess pressure might be caused by the water heater. I get the theory: during the day, sporadic use of hot water reduces the pressure, and at night it incrementally ratchets up. But if that were true, I'd expect it to incrementally ratched up at least a little during the day.

And if there is a check valve, it's well hidden, either underground or inside a wall.

 

[wwhitney]

Hmm, now that I think about it, you can just leave one faucet open a crack (like a drop a minute) and see if you still get a pressure spike. Because if it's expansion related, that should be enough to provide a release; and if it is from your city water supply, that level of flow is not going to cause any pressure drop at your outside hose bibb.

Cheers, Wayne

 

[Jadnashua]

Look at thermal expansion this way. Most all of the piping in the house is likely rigid. IOW, except at VERY high pressures (over 2K) copper doesn't really burst, or expand much. Now, the toilet, washing machine, and faucet supply hoses will balloon slightly, but not really all that much. When you get thermal expansion in a closed system, the pressure spikes immediately...it's not a slow ratchet up. It's going to happen in the first couple of minutes while anything that can stretch, does, then they hold tight and pressure rapidly rises. Depending on how big the temperature differential is, and how much water you're heating, in a typical home, that volume is less than a pint, but try to put a pint of water into something that's already full...doesn't work very well! You can think of it like pushing against a wall. The wall generally doesn't move, but you can put a lot of pressure on it. Expanding water is similar...the pressure just goes up as you try to expand into the wall.

Some PRVs can be purchased with a built-in bypass valve. SOme swear by them, but they totally defeat the purpose of having one in the first place IMHO. They cannot let water pass back into the supply until the home's water pressure exceeds the street pressure, so thermal expansion is still going to peak until it reaches the outside pressure which is what you're trying to defeat, especially if it's getting well over 100-psi. None of the hoses and valves, or the WH really like those radical pressure swings. The solution to that is a properly sized and setup ET. The ET must have the precharge set to your normal street pressure. You can pump it up with a small compressor (easy to overinflate the small tank), or probably better, a bicycle tire pump. Check it with a tire gauge. This should be done BEFORE you attach it to the system, otherwise, you'll normally be measuring water pressure rather than the precharge.

FWIW, a licensed plumber should know this! Where I live, it's been city code amendment to have an ET to pass a plumbing inspection since the late 1980's. Now, lots of people don't get a permit when they replace their WH, but on new construction or if they got a permit like it is called for, they'd fail if there wasn't one.

 

[Niccolo]

Hmm, now that I think about it, you can just leave one faucet open a crack (like a drop a minute) and see if you still get a pressure spike. Because if it's expansion related, that should be enough to provide a release; and if it is from your city water supply, that level of flow is not going to cause any pressure drop at your outside hose bibb.

Cheers, Wayne

Hot water faucet or cold water faucet left open a crack? I'm guessing hot, because that's where we're looking for pressure that's generated within the home (by the hot water heater)

 

[wwhitney]

Hot water faucet or cold water faucet left open a crack? I'm guessing hot, because that's where we're looking for pressure that's generated within the home (by the hot water heater)

Basically either.

Hot will definitely work for this test, since you know that water can flow from the hot water tank to your faucet.

Under this hypothesis of thermal expansion, you know there are no check valves between the outdoor spigot where you are seeing the pressure rise and your hot water heater. So cold would also work for the test, unless the faucet is fed from upstream of the outdoor spigot, and there is a check valve in between that feed point and your outdoor spigot.

Cheers, Wayne

 

[Niccolo]

Basically either.

Hot will definitely work for this test, since you know that water can flow from the hot water tank to your faucet.

Under this hypothesis of thermal expansion, you know there are no check valves between the outdoor spigot where you are seeing the pressure rise and your hot water heater. So cold would also work for the test, unless the faucet is fed from upstream of the outdoor spigot, and there is a check valve in between that feed point and your outdoor spigot.

Cheers, Wayne

Okay, hot water dripped about once every 45 seconds all night long. And my the tattletale hand on my gauge was at exactly 140 PSI this morning. It's interesting that so far, the overnight readings for the last three nights have been 140, 180, 140...maybe it's a coincidence that I'm getting such round numbers, but almost seems to me like the utility might have target numbers it hits to full up the water tanks overnight.

 

[Reach4]

Tap your gauge. See the needle jump. A sudden pressure pulse from 80 to 90 could also knock the lazy hand up due to momentum.

The 180 number is suspect, because the T+P valve on the WH should have opened about 150 PSI. Put a container under the output of that to see if anything comes out.

 

[wwhitney]

Okay, hot water dripped about once every 45 seconds all night long. And my the tattletale hand on my gauge was at exactly 140 PSI this morning.

OK, that's proof it's not an expansion issue. An outdoor PRV in a box is a bit of trouble, though, so you might want to get a second opinion (check with another brand of gauge) before committing. Or maybe I'm just having trouble understanding that your local water company almost triples the pressure overnight.

Cheers, Wayne

 

[Niccolo]

Tap your gauge. See the needle jump. A sudden pressure pulse from 80 to 90 could also knock the lazy hand up due to momentum.

The 180 number is suspect, because the T+P valve on the WH should have opened about 150 PSI. Put a container under the output of that to see if anything comes out.

The water heater overflow vents outside, so it might well have released when/if the pressure went that high. Good call to put a bucket under it!

My big question now is whether there's anything I need to be aware of in terms of selecting a pressure reducing valve and an expansion tank. My plumbers bring some relevant knowledge to the table, but so far, it's been crucial that I've been well--and some regards better--informed.

 

[Reach4]

My big question now is whether there's anything I need to be aware of in terms of selecting a pressure reducing valve and an expansion tank.

Some tanks specify that they hang down vertically. Some say vertically either way. Some also allow other positions if the tank is supported by other than the pipe. Normally the tanks are empty of water, but if they fail they will be full of water and heavy.

There are tables and calculators that can tell you the minimum tank size. Bigger WH, higher temp setting on the WH, and lower incoming water temperature all contribute to the need for a bigger tank.

The air precharge should be set to the normal water pressure, or a little more.

 

[Niccolo]

OK, that's proof it's not an expansion issue. An outdoor PRV in a box is a bit of trouble, though, so you might want to get a second opinion (check with another brand of gauge) before committing. Or maybe I'm just having trouble understanding that your local water company almost triples the pressure overnight.

Cheers, Wayne

Why is an outdoor PRV (in a box or not) a bit of trouble?

Yeah, the pressure spike is pretty crazy, if it's accurate. I'm tempted to contact the water company and my local HOA. It also makes sense to check the pressure with another and/or a higher-end gauge.

 

[wwhitney]

Why is an outdoor PRV (in a box or not) a bit of trouble?

I just meant it's a lot more trouble than buying a second (better) gauge and finding out that the pressure spikes not as high as indicated. Which is probably wishful thinking . . .

Cheers, Wayne

 

[Niccolo]

I just meant it's a lot more trouble than buying a second (better) gauge and finding out that the pressure spikes not as high as indicated. Which is probably wishful thinking . . .

Cheers, Wayne

Oh, I'm definitely going to verify, and I'm curious both to get another gauge on there and also to see how the pressure shows up elsewhere in the house. For now, there are a couple hose spigots I can try. But it might also be interesting to test pressure somewhere inside the house...e.g. at a toilet supply line? Maybe at the drain valve for the hot water heater (assuming the gauge could handle the heat)?

I thought you meant that the PRV being outside--either inside or outside a box--was somehow more trouble than it being inside the house.