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?

Thanks,
Jerry

jstauffer said:

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

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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?

Do you have a water storage / pressure tank in your water system?

Ted

Put an air charged pressure tank in the system somewhere to absorb the volume from expansion. Or just let a faucet drip.

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.

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.

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.

Ted

Expansion tank may be waterlogged. I've had this happen in homes, just bleed some air in.

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.

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.

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!

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.

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.

Ski in NC said:

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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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.

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.

FF said:

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.

Click to expand...

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.

Will remove the check valve today and see what happens.

Thanks guys.

Ski in NC said:

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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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.

ulysses said:

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.

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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.

Ted

jstauffer said:

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.

Click to expand...

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

Ok. Will wait for further discussion on this topic before removing check valve. A couple days ago I started doing what WesK says he is doing. Not sure mine will evaporate as I get about a cup of water each time I turn the water heater on. Keep in mind though, it has been cold enough here recently that I have to keep the water heater turned off most of the day to run electric heaters, stoves, etc. so when I do turn it on it is going from pretty cold to hot. Thinking the real fix would be to install a small expansion tank (smallest I have seen is 2 gl which is more than one would need) on the water heater side of the check valve. However, I don't know that anyone else is having to do this. I suspect the water heater is fairly old, but I can't imagine anything in the tank that would cause this other than the pressure relief valve, which has been replaced.

Thanks
Jerry

I think, from your original post, you have diagnosed the problem. When you turn on the WH, the water heats and expands forcing the excess out the over temp / pressure valve. If you leave a faucet dripping, it doesn't happen as the water has a place to go. The check valve prevents the expanding water from going back into the expansion tank. Removing the check valves would appear to solve the problem. Why not try it? If your not happy with the results, you can always put it back. Think you will find that most of us don't have that check valve, I know I don't.

Ted

The other reason for keeping the check valve is that if your tank runs dry the heater doesn't. This minimizes the possibility of the heater element burning in a dry tank.

I was curious enough about this to check my new-to-me boat that has a newer heater. On my boat there is a check valve and a hose running from the relief valve to the shower sump, which has a float switch controlling a small pump. The a/c drain also runs into this sump. That appears to be a simple way to keep a dry bilge.

Jeff...I don't have a shower sump as it goes directly to a pump with a manual switch to dump the shower basin. In the past I have just let the water heater pressure relief valve dump into the engine room bilge, but tiring to find a way to keep the bilge dry. Sounds like a gallon jug might be the answer, although still open for suggestions.

Here's how west marine suggests plumbing a potable water system on a boat:


Note the check valve installed before the water heater. In this case it also keeps city water pressure out of the potable water tank and pump.

Jeff F said:

The other reason for keeping the check valve is that if your tank runs dry the heater doesn't. This minimizes the possibility of the heater element burning in a dry tank.

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Ok, please explain how you think that works.

Ted

O C Diver said:

Ok, please explain how you think that works.

Ted

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It would but I'm not sure that's the only reason for a check valve.

Looking at the diagram above, if the potable water tank runs dry there is no pressure to empty the water heater through the faucets or shower. Water will remain in the tank.

WesK said:

Here's how west marine suggests plumbing a potable water system on a boat:

Note the check valve installed before the water heater. In this case it also keeps city water pressure out of the potable water tank and pump.

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The check valve is unnecessary as the pump has a check valve in the form of a discharge valve built into the pump. Now if you have an accumulator tank that can't handle municipal water pressure, that may be an issue.

Ted

WesK said:

It would but I'm not sure that's the only reason for a check valve.

Looking at the diagram above, if the potable water tank runs dry there is no pressure to empty the water heater through the faucets or shower. Water will remain in the tank.

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For the water which is in compressible, to drain out of the WH, you need to replace the water with air. How does the air get into the WH? If the check valve isn't there and you think the water is coming out of that inlet, where is it going (through the plumbing) and what's forcing it to flow?

Ted

O C Diver said:

The check valve is unnecessary as the pump has a check valve in the form of a discharge valve built into the pump. Now if you have an accumulator tank that can't handle municipal water pressure, that may be an issue.

Ted

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A quick check of accumulator tanks by Jabsco, Shurflo, Groco, and Whale showed maximum operating pressures from 100 to 125 psi.

Ted