Pressure relief valve question

[Jadnashua]

While it is convenient to have an isolation valve...plumbing code (which is partially based on manufacturer's instructions) specifically disallow it. Given that the things seem to last 7-10 years (in my limited experience)...it's not a big deal to not have it isolated with a valve. When they fail, it's usually a long time before they start to leak, and you'd notice the T&P valve opening long before the ET also started to leak.

 

[Mikey]

I knew somebody would catch that ... but, while the Watts installation instructions show no such valve, nowhere does it say you can't have one. In my case, it'd be really convenient due to space constraints and the occasional need to use the space occupied by the ET for other purposes. I don't have access to the relevant plumbing codes, I don't think, but I'll look around.

 

[Sponsor]

 

[MACPLUMB]

The reason no valve is because of the very good possibility that the vale is closed thus stopping the true purpose of the
E.T. to keep the water heater tank from bulging or bursting, if it bulges it will press down on the burner and or cause the
vent to collapse eather condition causing carbon monoxide to come out into the house,
all this is reason to void the warranty per all MFG"S !

 

[Mikey]

It's an electric water heater. Also, it's been running without an ET for 25 years, so I suspect the 150psi bursts are transients of some sort (which I don't understand) which trip the P&T valve briefly but don't cause any other damage.

 

[Jadnashua]

The ET size calculators will tell you how much volume increase there will be under various conditions. The physics of it is solid...heat water, it expands. In comparison to say the copper pipes, there's no comparison...the water expansion radically overshadows the copper. So, that larger volume has to go somewhere or the pressure WILL almost instantly rise high enough to open the T&P valve-water isn't really compressible. So, where can it go? Could be lots of places. Easiest one is if you have an open system...it goes back into the supply system. Because under some unusual circumstances, you might pollute the water, water companies have been installing check valves when they upgrade their systems to prevent that possibility, making your system now closed. If you have a closed system, there are still some choices:
- if your checkvalve leaks
- if your prv leaks
- if your prv has an internal relief valve then it will only rise to the level of the supply on the other side
- if your hoses (washing machine, toilets, faucets, etc.) balloon
- if any valve leaks. Note, a common one is a toilet fill valve...some of those can leak, but you'd never probably notice as that excess water will just raise the tank level a little, or just end up going down the overflow. MOst of them are quiet about it, too. If that, it can shorten the life of the washer or gasket, though. Plumbing fixtures for homes are designed with a safety margin where the highest normal pressure is 80psi. For example, a water heater is designed with about a 100% safety margin because of the setting on the T&P valve (typically, 150psi). Testing the elasticity of hoses is risky...eventually, they can weaken and fail.

As an easy test, shut the valves to your toilets, then take a nice long shower or a load of clothes using hot. Wait a little bit and see if you get a discharge from the T&P valve of your WH...if you do, you've found your weakest point - your toilet fill valves are leaking, preventing the pressure from rising too high.

 

[Mikey]

Thanks, Jim. Local rumour (to be confirmed) is that the local utility does not install a check valve. Not sure what that test would reveal, since I AM seeing a discharge from the PT valve, and the Watts telltale gauge usually shows a peak of 150-160 psi. This would imply there IS a check valve, and things in my system don't leak.

 

[Jadnashua]

It's rare, but sometimes, the supply pressure can reach that high, so it's possible it's not from expansion. IF that's the case, you need a PRV, and that would make a closed system, which would call for an ET as well.

 

[Mikey]

I've written to the utility, who are usually responsive. The water tower is about 3.94 miles away, about 220' above my house, so absent any pumps, I'd see about 95psi. I'm hoping for a more elaborate description from the utility.

 

[Reach4]

The water tower is about 3.94 miles away, about 220' above my house, so absent any pumps, I'd see about 95psi.

220' altitude difference from the base, or the top of the tower?

 

[Mikey]

The base, estimated from a topo map. I don't know how tall the tower is, but it's really a huge tank, not the classic "tower" like the one in Sussex.

 

[Jadnashua]

FWIW, often, late at night or very early morning, when the utility is refilling the water tower...they must raise the pressure above what the tower provides in order to actually refill the thing. Now, they may have a reduction valve on the outlet, but at least somewhere in the system, it has to get higher than the 'normal' pressure, or they couldn't refill it. There might be a pump right there at the tower to get it refilled...hard to say for sure. IOW, that, along with the frictional losses from people using water, the pressure will vary during any 24-hour period.

 

[Mikey]

I had thought the tower was being filled all the time, from wellfields in the area, but maybe they'll 'splain it to me when they reply. I'm seeing a 70-80psi change, which would correspond to 160-180' of water. I'm now really looking forward to my recording digital gauge project.

ET arrived today. It's going to be a tight squeeze, and it's going to look an awful lot like Reach4's picture.

 

[Mikey]

I got a phone message from the utility; they try to maintain 87psi in the system, and there is a check valve in the meter, so I think thermal expansion 'splains it all. I'm going to capture the TP overflow tonight and see if it's consistent with an estimated/calculated expansion. The expansion tank arrived very quickly, and it looks like I'll be mounting it very much like Reach4's picture shows. That picture, with walls around and AC to the right could have been taken in my house.

The water tower/tank looks to be about 50' tall, but I'm lousy at estimating heights. I'll call the utility back today. If it is 50' tall, all they need is an additional 22psi to pump water to the top of the tank, so that's the maximum differential we'd see as the tank drains and refills, but I doubt the rise/fall is anywhere near that. And there are probably pumps and regulating devices throughout the system, so we don't depend on gravity alone.

 

[Jadnashua]

In the small city where I live, they require an ET on any WH installation. It's a reasonable position, since as maintenance and upgrades are done, that should prevent everyone from experiencing their T&P valves suddenly starting to open up.

 

[Mikey]

I was a little off in my tank-height estimate - it's 100', and every night the utility tries to ensure it's at 98' by morning. I don't know how low it gets, but there's a potential 40+psi variation at the extreme. It's primarily gravity fed, so tank pressure alone could give me 95 to 135psi, but I assume the system is designed to take advantage of friction losses, and there are other smaller tanks around that could serve as break tanks. Our neighborhood also has a couple of booster pumps for some reason, but we didn't go into detail. They try to provide 73psi at the main at all times, but it varies a little; my "normal" is about 80. There is a check valve at the meter, and we agree an ET is required. He was surprised to see no one had caught the omission in 25 years. I'm bumping that up my project list.

 

[Jadnashua]

Usually, when using hot water, you don't empty the tank, so the amount of expansion you get may not be very great because not much cold was introduced (say, just washing your hands). THen, any time you open any valve, you'll bleed off any excess pressure allowed by those things that did expand. It doesn't take a very big leak to offset that volume increase. But in a very tight system with no weak flexible hoses, there's no real place for that expanding water to go, and it will relieve itself out the T&P valve.

 

[Mikey]

It's 9:22PM here, I just took a hot shower, so there's good opportunity for bringing in 5 or 6 gallons of 55° water. But it's a 66-gal tank, and if we assume the starting temp is 135°, the resulting temp is 128°, which will then be brought up to 140°, expanding to about 66.6 gallons. Now I've run out of ideas -- I know the pressure will increase, and at 150psi the TP valve will open and leak out the excess, but I dunno how to figure how much offhand.

 

[Jadnashua]

You needed to heat 6g of water from 55-135, or about 27C (80F). The scientific community often works in metric. The thermal coefficient of expansion of water is about 0.00021 per 1 °C at 20 °C. So, you started out with 6g, but in the end, it tries to become 6*27*0.00021 = .034g larger. That equals .034*128=4.34oz, or a little bit more than 1/2 of a cup. While you can calculate how much the pressure will rise, since the copper piping essentially does not expand its capacity, nearly all of that water has to go somewhere...out the T&P valve is the likely culprit. Note, any use of water in the interim while that water is heating (it could start while you're still taking a shower) or someone flushes a toilet or uses the lavatory, will bleed off that increase in volume while the valve is opened, so you'd likely have less discharge.

I know math isn't everyone's thing, but it's not black magic.

 

[Mikey]

Where did you get the "6g of water"?

 

[Reach4]

Where did you get the "6g of water"?

He means the 6 gallons (not 6 grams) of water that you estimated-- maybe 3 minutes * 2 GPM.

Most people would use more water than that for a shower I think.