Stray current in HTP water heater. Now what?

Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
I touched the multi-meter red lead to the water heater's relief valve and black to ground (engine). There's 16.4 mv of dc. Still there with all breakers off, even main 120v. Then disconnected shorepower, no stray current.

Had issue with dark gray water on the hot side, so contacted HTP who said to test in the above fashion. Also later had a rust-colored spurt when flushing. Stainless tank seems should be no rust - does stray current cause? BTW copper plumbed to fixtures.

How is DC getting into the heater? Must be through the ground or common but how is the internal tank itself connected to anything electrical? Seems that only the element would be connected, and it's isolated from the rest of the tank (?)

No issues currently; water is clear after flushing.

I guess I could just stay unplugged from shorepower when I'm away, but I'm hoping for a better resolution, including ideas re should I be looking for correcting a problem with the boat's wiring.
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
What else is connected to the water path? Possibly a water pump? Sounds like you may have a ground loop. To test for that you would connect you meter to two ground points and see if there is any difference of potential. If you see voltage between two ground points then you have a ground loop. Check to see if there are separate ground connections for you heater and water pump, or anything electrical that is in the path of the water.

As for the rust I have no clue.
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
16.4 mV is 0.0164 volts! this is pretty common when you mix metals and add something like water. I suspect that the relief valve is not made of the same metal as the SS heater. Any time you mix metals you get a small voltage, it is normal. A good installation would have a insulating gasket/washer between the two to break the circuit. quite frankly I would not be at all concerned.
 
Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Bill Roosa said:
16.4 mV is 0.0164 volts! this is pretty common when you mix metals and add something like water. I suspect that the relief valve is not made of the same metal as the SS heater. Any time you mix metals you get a small voltage, it is normal. A good installation would have a insulating gasket/washer between the two to break the circuit. quite frankly I would not be at all concerned.
Click to expand
Thanks, Bill.
Yeah the relief valve is bronze.
Odd that the voltage is not there with shorepower unplugged though. Which makes me wonder how the voltage from shorepower (probably common or ground) finds it's way to the relief valve. This means it could find its way to the fixtures via the copper tubes. Hopefully it stays at .0164 volts.
 
weinie

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
What happens if you leave the shore power plugged in but just turn off the power to the water heater?

ETA: nevermind... you did say with all breakers off in your original post.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,701
Canadian Sailcraft 36T Casco Bay, ME
Your water heater ground (green wire) is connected to every other boat in the marina when you "plug in"... You do have a galvanic isolator right..??
 
jibes138

jibes138

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Jan 27, 2008
3,086
ODay 35 Beaufort, NC
There may be an electrical potential delta between the fresh water and the salt water. The relief valve is connected to fresh water and the engine to salt water? http://www.rdmag.com/news/2011/03/researchers-use-freshwater-and-seawater-generate-electricity

This doesn't explain why it goes away unplugging the AC though. The dissimilar metal idea also should still exist when unplugged. Since the AC ground and the Water heater ground should both connect to the engine I'm wondering if you are measuring the neutral wire instead of the ground wire? Maybe the water heater wasn't wired correctly? When you shut off the breaker you disconnect the hot wire but the neutral wire is still live and will have some potential? When you unplug at the dock all the wires are disconnected. See what the difference is between the neutral wire and the ground wire on the water heater to see if you are getting the same or similar voltage. You probably should consider adding a galvanic isolator to your system, it connects between the shore power ground and the boat panel ground. Are you going through your zincs faster than normal?
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
I think MS is (again) on to something with the "all the boats in the marina...."
Try this, unplug shore power and test between the relief valve and the water. If you get the voltage then it is surely the shore power ground attaching you to every other boat in the marina and the water completing the circuit.
 
jibes138

jibes138

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Jan 27, 2008
3,086
ODay 35 Beaufort, NC
Another thought. Disconnect the wires from the water heater panel. Make sure the AC is off at the main panel. Now measure the potential of each of the wires using the engine as a ground the same way you did originally. This will eliminate any question of where the voltage is coming from. If you are getting a voltage between the AC ground wire and the engine then it sure sounds like stray current and again a galvanic isolator is in order. You should probably disconnect the AC ground wire at the main panel and check the potential between the disconnected wire and the engine ground. This will tell you for sure the voltage coming into the boat through the ground wire if any. If you have a long piece of insulated wire you could stick it into the ground socket on the plug at the dock and then measure voltage to your engine. This way you won't have to disconnect anything. If any device on any boat in your neighborhood is wired using the ground as the neutral you will be getting current through the ground wire.
 
Skipper

Skipper

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
Maine Sail said:
... You do have a galvanic isolator right..??
Click to expand
No, but it sounds like I need one. I used to go through shaft anodes 1 about every 3-4 months. So, I hung a giant grouper zinc off the back stay, and now shaft zinc stays all year.

There's very little visible corrosion on the boat, even in the most notorious of locations. Hadn't thought about an isolator, until my heater started with the gray water.

I'll be shopping for one this weekend.
 
weinie

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
I would be very curious if the problem instantly disappeared if you were to temporarily disconnect just the ground wire coming from the shore power at your electric panel, while leaving the hot and neutral attached.
Not saying you shouldn't have an isolator, but it would confirm if that indeed may be your problem.
 
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