Water heater T&P valve problems

Jul 7, 2014
90
Hunter Passage 42 Jacksonville
Well, guys, I found the check valve. It's a tricky little bugger and, with most of it buried in the side of the water-heater, I mistook it for a regular 1/2" male-to-male stub pipe. It's actually one of these: https://www.rveparts.com/qest-check-valve-1-2-q2500/

In other news, I reran my pressure test and the results were terrifying.

I emptied all the hot water out of the water heater and switched it on. This time I left the pressure gauge connected to the shower and the hot valve open so I could get a continuous reading. (Rather than relieving a bit of pressure every time I read it, as with the previous method)

By ~12 minutes it was up to 100psi. I relieved it, ~10 minutes later, again 100psi. Same thing happened a second time.

The third time time I got sidetracked and let it go ~15-17 minutes. The gauge was indicating over 160psi. A very small amount, maybe 1oz, of water had come out the T&P valve discharge tube.

So, we've probably been living dangerously without fully recognizing the risk.

Not sure what to do with this information... Subjecting any part of the plumbing to such high pressures seems like a bad thing, so simply removing the check-valve and allowing the pressure to spread through the cold-water system doesn't really seem like the answer. Or maybe the accumulator would absorb it as I'd hoped...?

On the other hand, @Terry Cox doesn't seem to have this problem and has the same boat, I assume with the same or similar fixtures and overall plumbing system size, but with no accumulator.

Only remaining part of the puzzle is the part about the engine heat not causing T&P discharge. I'm thinking we maybe just didn't have any pressure at all in the system when we started motoring.
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
You have measured the pressure. What about the temperature? How hot is the water. It should not get to that pressure if it is not boiling. It should never be boiling in the water heater. The water heater temp should be about 120-130. Hotter than that and you have a thermostat problem.

Water pressure sources:
  1. Water tap to City water under pressure. Most city water is at 60 psi
  2. Water pressure pump. Should have limit switch about 30psi
  3. Water heater heater coil. Heating water to above 210 will produce seam and will pressurize the tank.
  4. Engine heated water run through water heater. Depending on engine can get to 160 degrees.
You need to control the sources of water pressure.
 

Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
You need to verify your thermostat operation per @jviss concerns above. As @jssailem states, if you make steam, it is a problem.

Barring malfunctioning controls, it's doing exactly what it's supposed to; based on what you have installed. The relief is a 150psi unit per your description. There is slop around that spec, so 160 would not surprise me at all. Plus, consider your highly calibrated test gauge accuracy. This is all good info that you need to get your problem corrected.

Every hot water system I designed started with calculating the system volume, calculating expansion due to temperature change, and finished with selecting an appropriate expansion tank and relief valve. The expansion tank and relief valve are related, by the way, as you are trying to select a tank that keeps you a margin below relief. After all, you wouldn't want a 100 gallon expansion tank on your boat... although, it would fix your problem.

You are expanding an incompressible liquid in a closed system. This is what it does.

Add an expansion tank, verify your controls and go sailing. If you want the pressure to relieve lower, get a different relief valve. Once you verify control and add the expansion tank, the relief valve will quit popping, because your pressure spike will be gone.

You're doing great! Stay the course. You have several smart guys and me hanging with you.
 
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Jul 7, 2014
90
Hunter Passage 42 Jacksonville
Bad electrical thermostat.
I heard a mic drop at the end of that...

Since your original comment . . .
I guess you have a check valve that you haven't found yet.
. . . was so spot on, I don't want to discount this.

But I did already replace it once...

These are non-adjustable thermostats fixed at the factory to 140 deg F. To test it, if the water temperature coming out of the faucet is ~140 deg F, then the thermostat is good, right?

It should not get to that pressure if it is not boiling.
Why do you say that? Water expands when it is heated. According to this, 40 gallons in a water heater expands to 40.6384 gallons when it is heated from 4 deg C to 80 deg C. In many areas, building code requires an expansion tank be fitted with a water heater for this very reason. Especially if there is a backflow device at the supply, because without that, the expanded water volume could push back into the supply lines.

Granted, this is a much smaller water heater, and a smaller temperature range, however, there is still an increase in volume and that has to go somewhere. The check-valve is preventing the water from expanding towards the supply side, so the only direction it can go is into the boat plumbing / fixtures or out the T&P valve discharge tube.
 
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Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
Depending on actual fill temp, fill pressure, and final operating temp, this scenario can happen with the components you have working perfectly.

But, you need to verify.
 
Oct 22, 2014
20,989
CAL 35 Cruiser #21 moored EVERETT WA
The check-valve is preventing the water from expanding towards the supply side,
That is to maintain "in a small system" both hot water and cold water. If the check valve was not there the scalding hot water would heat the cold water and you would have scalding hot water from both faucets.

This is a simple system try not to over think with comparisons to house plumbing. Keep it simple. You have only 6 gallons. It will heat quickly, cold water will cool it as you use the hot water. The cold will mix with the hot and away you go again.

I know I turn on the water heater in prep when I am going to need hot water. The water heater is well insulated and maintains the water at temperature for 36 hours. Once I get the water hot about an hour or when ever I motor, I turn the water heater off.
Granted I am not a live aboard and I do not have a usable on board shower, but the method works and I have never had the issues you are describing.
 

Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
https://www.westmarine.com/buy/raritan--1700-series-water-heater-temperature-and-pressure-relief-valve-75-psi--256862?mrkgcl=481&mrkgadid=3202694708&cm_mmc=PS-_-Google-_-GSC>NonB>Product%20Type-_-256862&product_id=256862&adpos=1o1&creative=108421552684&device=t&matchtype=&network=g&gclid=EAIaIQobChMIku65q8qm4wIVGozICh12zwBYEAQYASABEgJWZ_D_BwE

You can get it cheaper likely, but just letting you know that 75 psi is available if you want to drop down a bit. You will have to match up everything to run that low, regulator, pump, expansion tank, but it can be done if you're afraid of it. I would verify control and add the expansion tank. You've proven your system can take it. I will confess that I'm sorry you tested your shower hose to 160. You should have chickened out at 100. I would have.

As @jssailem says, don't remove the check valve.
 
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Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
Please tell me where the check valve is. I can't make it out in the pic.
 

Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
Your cheapest fix is likely going to be a residential 2 gal expansion tank. Based on your gauge readings, you should charge the tank (WHEN EMPTY), to 35psi to match the supply water regulator pressure. I recommend you check your on board pump pressure and charge to that, if higher than your regulator pressure. Charge to the higher supply pressure.

If you don't have room for a 2 gallon tank, we can sharpen the pencil and try a marine accumulator of smaller size. The tank can be mounted remote from the heater as long as the connection is between the check valve and the cold water inlet to the tank.
 
Jul 7, 2014
90
Hunter Passage 42 Jacksonville
Please tell me where the check valve is. I can't make it out in the pic.
Yeah, it's not obvious at all, in the photo or from the typical angle you'd be looking into this compartment.

I had to stick my hand inside the compartment with my phone to take this photo...but you can see it better from that angle.
 

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Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
Thanks! Amazed you found it.

So, looks like you’ll have to move it to install the expansion tank, maybe back to the tee. Anyway, you’ve got it on the run now. Congrats!
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
This has been an informative and useful thread. I learned a couple of things, both directly, and indirectly.

First, if you have a heater, you should have an expansion tank.

If you have isolated segments in your water system, the hot side, at least, should have an expansion tank, even if you have an accumulator (expansion tank, really), on the cold side.

(I had started a discussion last year, maybe the year before, about putting a check valve before the heater, as I detected some hot flowing back into the cold lines. While there was a spirited discussion, the need for an expansion tank on the hot side never came up, that I recall.)

The vent nipple on the T&P valve should have a hose securely clamped to it, led to a safe place, and securely fastened along its path, for safety reasons.

Water heaters with electrical heating elements can explode. (Engine-heated will never get hot enough to generate steam.)
 

Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
The vent nipple on the T&P valve should have a hose securely clamped to it, led to a safe place, and securely fastened along its path, for safety reasons.
Amen. If you are in the wrong place when the safety valve goes off, it can be very bad.

Looked like the actual volume increase for his circumstances, was in the 12 oz range. So, fill a steel tank full of water, then try to force 12 oz more water into it. No question why the 150psi relief was popping.

The same phenomenon should occur with engine heat, given the same starting pressure, and start/finish temperature. So, my hunch is, something is different, since the relief valve didn't trip. I hope he will verify his pump pressure and thermostat operation in both modes before he calls it done.

The expansion tank is very important for systems that start with a cold tank often (like when my oldest daughter comes to visit and turns the second floor into a steam room every evening). As @jssailem pointed out, you could bleed the expansion pressure off at a faucet as it warms up. Then, once it's stable at the max temp, you're good till the next cool down/warm up cycle.

In systems without expansion tanks, the T&P valve basically becomes an operating control, bleeding off the expansion pressure at every cycle, unless usage, or leakage takes care of it. As the intent of the T&P is safety, from an impending catastrophe, I'm sure that's why code officials decided to mandate expansion tanks on residential systems.

Note also, that without the check valve, the accumulator and cold water piping (and dock hose, in his configuration), would have probably absorbed the expansion, preventing a T&P trip.

Thank you for your patience and participation @jviss. You have a good handle on the situation. Enjoyed the discussion. I hope he can get his system up and running to his satisfaction soon.
 
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Jul 7, 2014
90
Hunter Passage 42 Jacksonville
Figured I'd update those of you who have been following along with the latest...

Finally got a temperature measurement: 143 deg F. Seems within the margin of error for the factory setting of 140, so I think that means the thermostat is ok.

I relieved the pressure while heating, as mentioned by @jssailem, by letting the galley tap drip on hot (1 drop per second or two) into a bucket. Used the pressure gauge on the shower hose for continuous monitoring and pressure never rose above normal and about 1 quart of water was wasted.

I figured this would be a workable solution, temporarily anyway. But I wanted to test the pressure through a re-heat cycle. I left the gauge on the shower hose and waited for the water to cool enough to turn the thermostat back on. About 3 hours later, the cycle ran again. This time the galley sink was not dripping and there was nowhere for the pressure to go.

Unfortunately, when I checked the gauge after it had finished, about 10 minutes later, the pressure was just over 140 PSI and the shower hose had torqued/twisted itself because of the pressure. This tells me that 1) the shower hose absorbed some of the expansion, 2) there is still a bit more expansion in a re-heat cycle than the hot-side plumbing can take and stay below 100 PSI.

As far as I can tell, @Jim26m was spot-on when he said that, even with properly working parts, this could happen. Each individual component appears to be working, but in the current configuration, will have this problem.

So, what I'm planning on doing is just removing the check-valve and seeing how it changes things. I'm hoping that the accumulator will be able to absorb most/all of the increased volume and keep the pressure from getting above the 100 PSI rating of the water heater and plumbing.

My biggest concern with doing this is the dangerous possibility of hot water coming out of the cold tap. I need to purchase some parts in order to reconfigure the pipes without the check valve, but I'll post an update once I do that.

Thanks again for all your input and help troubleshooting this problem. Frankly, I'm a little surprised this isn't a more common / more frequently talked about issue.
 
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jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
So, what I'm planning on doing is just removing the check-valve and seeing how it changes things. I'm hoping that the accumulator will be able to absorb most/all of the increased volume and keep the pressure from getting above the 100 PSI rating of the water heater and plumbing.

My biggest concern with doing this is the dangerous possibility of hot water coming out of the cold tap. I need to purchase some parts in order to reconfigure the pipes without the check valve, but I'll post an update once I do that.
Good idea. That's what I have. A little hot seeps back into the cold side, which should be expected, since that's where the pressure is relieved. But no way dangerous, not even hot, just a little warm.

The ultimate solution would be to add an expansion tank to the hot side, but removing the check valve is far easier and more economical.
 
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Jim26m

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Apr 3, 2019
579
Macgregor 26M Mobile AL
@Wanderwind, thanks for the update. Really appreciate you verifying your thermostat operation. Not to put words in the other guys' mouths, but I think we were all a little concerned that there might be something else going on - since we weren't 100% sure of the final temperature.

The way your system is configured, removing the check valve may work fine. The accumulator may be just big enough to accept the expansion, without dripping a faucet. The pressure may run up a bit, but I don't expect it to trip the T&P relief. Since your shore water comes from the other end of the system, you may not have an issue of hot water at the cold taps. We'll see. Glad you have a workable solution in sight!

And, thanks for sharing the hunt with us!

It probably is a common issue. However, if the relief line is piped to the bilge, who's going to notice it? It will only give a short burst of higher pressure then settle into normal operation at the faucets. Most folks probably would ignore it.
 

jviss

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Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
However, if the relief line is piped to the bilge, who's going to notice it? It will only give a short burst of higher pressure then settle into normal operation at the faucets. Most folks probably would ignore it.
The T&P valve is to prevent the water heater from blowing up. You are likely to blow some plumbing befor ethat happens, like the OP's shower hose.

BTW, a tempering valve is an important safety feature, and there's one in my future. My engine-heated water is hot enough to scald.