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Old 12-23-2015, 07:15 PM   #1
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Water Heater Pressure Relief Valve

Every time I turn the power on to the water heater (water in tank is cold) and after it heats up I get about a cup of water in the bilge from the pressure relief valve. If I leave a hot water faucet drip it does not happen. I have replaced the pressure relief valve three times (thinking maybe the second one was bad off the shelf) and still have the same problem.). They have all been 150 psi valves. Is this normal? I know water expands when heated but other boating friends say their's do not do this. If I leave the water heater circuit breaker on all the time I do not have a problem, but when we are on it when it is cold I need to turn it if so I can run electric heaters and other electrical devices without tripping the main breaker. Any thoughts?

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Old 12-23-2015, 07:25 PM   #2
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Every time I turn the power on to the water heater (water in tank is cold) and after it heats up I get about a cup of water in the bilge from the pressure relief valve. If I leave a hot water faucet drip it does not happen. I have replaced the pressure relief valve three times (thinking maybe the second one was bad off the shelf) and still have the same problem.). They have all been 150 psi valves. Is this normal? I know water expands when heated but other boating friends say their's do not do this. If I leave the water heater circuit breaker on all the time I do not have a problem, but when we are on it when it is cold I need to turn it if so I can run electric heaters and other electrical devices without tripping the main breaker. Any thoughts?

Thanks,
Jerry
I suggest contacting the manufacturer of the water heater and/or the valve.

Let me ask you this: Have you had this problem ever since you got the boat or is it something new? Were any changes made to the plumbing system just before you noticed this problem? And of course, is 150 PSI the correct rating for the valve?


And is the thermostat adjustable and set to too high a temperature or is it possibly sticking and overheating the water?
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Old 12-23-2015, 07:44 PM   #3
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Do you have a water storage / pressure tank in your water system?

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Old 12-23-2015, 07:45 PM   #4
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Put an air charged pressure tank in the system somewhere to absorb the volume from expansion. Or just let a faucet drip.
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Old 12-23-2015, 08:05 PM   #5
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It would appear that your Pressure Relief Valve is serving as both a PRV and a Thermal expansion relief. There should not be a problem with that except that over time the valve may start dripping from numerous opening and closings. You may wish to install a true thermal expansion tank. It looks somewhat like a small pressure tank attached to the water heater.
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Old 12-23-2015, 09:03 PM   #6
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This problem has been with this boat the 5 years I have owned it. I have the temperature set as low as it can go (120) degrees, but think I need to test the temperature of the water to verify, but it does not really feel all that hot. The system does have an expansion tank in it but there is a one way (check valve?) valve between the expansion tank and water heater. Maybe there should not be but suspect that is to prevent water in the tank from draining back in the system when system pressure drops.
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Old 12-23-2015, 09:30 PM   #7
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Remove the check valve. When the water heats, it expands. Has to be some place for it to go. Water won't back flow as it's not compressible and there shouldn't be any air space after the water heater.

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Old 12-23-2015, 10:48 PM   #8
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Expansion tank may be waterlogged. I've had this happen in homes, just bleed some air in.
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Old 12-23-2015, 11:02 PM   #9
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Jeff...if there is a check valve between the water heater and the pressure tank the problem would not be the pressure tank would it? Researching the internet I found a couple articles on installing water heaters on boats and two of them recommend the check valve just before the cold water input to the tank to prevent hot water migration back to the pump but not sure that is a problem.
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Old 12-23-2015, 11:34 PM   #10
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I don't see a good reason for a check valve on the inlet to the water heater. And you must have the only check valve on the planet that actually seals!! Try your system with it removed.
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Old 12-23-2015, 11:52 PM   #11
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Are there any signs of visible rust on the outside of the water heater?

I had similar symptoms in my new-to-me tank in 2008. Changed the relief valve and the problem continued. Two months later, the whole tank started to leak. When I started paying closer attention, I saw rust strips in two corners. It was toast. When I pulled it, it had a stamped date of 1976. My boat was built in 1977...OEM tank! I think it did pretty well for 30+ years!
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Old 12-24-2015, 07:10 AM   #12
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Move the pickup location for the expansion tank to the HW line.

Pressure test the tank to be sure there is some air in it.

The best is the largest (12 20? Gal) expansion that can be fit below , and monitor its pressure at least once a year.
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Old 12-24-2015, 08:26 AM   #13
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As mentioned remove the one way valve or have the water drain into a plastic container and over time it will evaporate. Many older boats use to have bilge drain sumps where the sink shower water was drain into and the bilge pump pump the water over board, but there was always some water in the sump. Your boat probably has a sump? Besides its fresh water? The easiest is open a faucet a tad, scoshe.

We have the hot water heater turned of most of the time and on a timer as AC power is limited on the boat than heating hot water.
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Old 12-24-2015, 09:47 AM   #14
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Quote:
Originally Posted by Ski in NC View Post
I don't see a good reason for a check valve on the inlet to the water heater. And you must have the only check valve on the planet that actually seals!! Try your system with it removed.
Ski and OC Diver are absolutely correct IMHO. Had a new tank installed in 2013 to replace the POS aluminum tank. The new install TP valve kept popping off. Removed the newly installed check valve (innards) and no more popping off as pressure then regulated by expansion tank.
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Old 12-24-2015, 10:28 AM   #15
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The only reason for the check valve on the inlet side of the water heater is to allow the water heater to be replaced/serviced without having to shut down and or drain out the cold water side of the system. I agree that by removing the check valve any expansion taking place within the water heater would then be relieved through the systems existing pressure tank.
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Old 12-24-2015, 10:39 AM   #16
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Move the pickup location for the expansion tank to the HW line.

Pressure test the tank to be sure there is some air in it.

The best is the largest (12 20? Gal) expansion that can be fit below , and monitor its pressure at least once a year.
This will not work very well. With expansion tank on hot leg downstream of check valve, if you use just cold water, the check valve will be shut and the expansion tank effectively isolated from the cold leg. Pump will then short cycle.
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Old 12-24-2015, 11:09 AM   #17
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Will remove the check valve today and see what happens.

Thanks guys.
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Old 12-24-2015, 11:15 AM   #18
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I don't see a good reason for a check valve on the inlet to the water heater. And you must have the only check valve on the planet that actually seals!! Try your system with it removed.
Many water heaters have a check valve built into the intake fitting. Other manufacturers suggest an external one. There is a reason for the check valve. Removing it will allow hot water to mix with the cold.

jstauffer, My boat has a short hose on the TP valve outlet that leads to an empty one gallon container. If any water escapes, it goes into the container and eventually evaporates. I've never had to empty it and don't even know if the valve has ever opened. My water is heated by electricity and/or excess engine heat and gets dangerously hot.

Adding a hose and container (mine is an empty antifreeze jug) may solve your problem with no additional modifications.
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Old 12-24-2015, 12:17 PM   #19
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The only reason for the check valve on the inlet side of the water heater is to allow the water heater to be replaced/serviced without having to shut down and or drain out the cold water side of the system.
Doesn't work that way. Water would still flow from the cold water side through the check valve. That is the normal flow direction. The check valve keeps it from flowing the other way.

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Old 12-24-2015, 01:40 PM   #20
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Jeff...if there is a check valve between the water heater and the pressure tank the problem would not be the pressure tank would it? Researching the internet I found a couple articles on installing water heaters on boats and two of them recommend the check valve just before the cold water input to the tank to prevent hot water migration back to the pump but not sure that is a problem.
Yeah, I take back my comment. Was thinking that the expansion tank was on the HW side, and that's not the case. And if there was an issue with the expansion tank the pump would be cycling too much as someone else pointed out. Sounds like your check valve is working a bit too perfectly :-)
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