Electric water heater in newly acquired Oxygen - Fiberglass RV


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Old 04-16-2017, 01:46 PM   #1
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Electric water heater in newly acquired Oxygen

I plugged in my Oxygen egg and everything worked well except the electrical water heater. I checked for ac current going to the water heater and it does. My next step was to check the element so I turned off the breaker in the converter. To my surprise I still had current at the water heater. I unplugged the shoreline to check the element. Obviously that shut off the current. Is it ok to not have breakers in the trailer to shut off the water heater? I suspect that the previous owner bypassed the converter and fuse box. Is that ok. My next step will be to check the continuity in the element. A zero reading will make it obvious that it is broken. Anybody know what resistence reading I should be getting if it is fine?
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Old 04-16-2017, 01:59 PM   #2
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I plugged in my Oxygen egg and everything worked well except the electrical water heater. I checked for ac current going to the water heater and it does. My next step was to check the element so I turned off the breaker in the converter. To my surprise I still had current at the water heater. I unplugged the shoreline to check the element. Obviously that shut off the current. Is it ok to not have breakers in the trailer to shut off the water heater? I suspect that the previous owner bypassed the converter and fuse box. Is that ok. My next step will be to check the continuity in the element. A zero reading will make it obvious that it is broken. Anybody know what resistence reading I should be getting if it is fine?
The element should read 10 to 12 ohms depending on the wattage of the element. The water heater requires a separate circuit and a separate over current device ( Breaker)
A branch circuit shall not feed a fixed appliance which exceeds 50% of the branch circuit rating when also supplying general purpose lighting or convenience receptacles .
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Old 04-16-2017, 02:08 PM   #3
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If you are talking about a tank style water heater, 4 or 6 gallon, for instance, it should have a disconnect near it and usually that is simply a line cord and plug for a 120 volt water heater. It would just plug in.

It cannot be powered up when there is no water pressure or it will burn out the element. It will probably burn out if just full of water, but not under pressure, and absolutely will burn out if the water tank is empty. An easy disconnect allows you to have the power on with the pressure off and it allows servicing the water heater.

The combination heaters that run on propane or electric have a small snap switch that controls the power in electric mode, based on temperature. I've had a number of those go out.

If the element is burned out, there will be no continuity between the two terminals on the element and there might be a connection between the terminal and the outside of the rod itself. You can also do a visual and will see a burned out spot on the element.
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Old 04-16-2017, 02:10 PM   #4
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Thanks Steve. I will recircuit the wiring through the fuse box that has a breaker that is already identified as water heater. I take it that the air conditioner should have it's own breaker too. I'll check to see if the water heater breaker shuts off the a/c. In that case it would mean that it is the guys who installed the a/c that modified the circuit to use the water heater breaker for the a/c. Thanks again
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Old 04-16-2017, 02:30 PM   #5
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Thanks Steve. I will recircuit the wiring through the fuse box that has a breaker that is already identified as water heater. I take it that the air conditioner should have it's own breaker too. I'll check to see if the water heater breaker shuts off the a/c. In that case it would mean that it is the guys who installed the a/c that modified the circuit to use the water heater breaker for the a/c. Thanks again
The A/C and the water heater require a separate circuit . Normally you have 1 -30 amp main breaker , 2 breakers for the the convenience receptacles , a breaker for the A/C and a breaker for the water heater. The water heater element can not be run dry , it needs to have the tank full of water so the heating element is totally submerged and water is in contact with the thermostat and high limit
The water heater doesn't know or care if the water system is under pressure . The water heater tank could be full of air and still be under pressure.
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Old 04-16-2017, 04:36 PM   #6
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Your a/c should have its own 20 amp breaker.
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Old 04-17-2017, 08:53 AM   #7
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You are right there could be air in the water heater. When I filled it up I left the hot water tap open figuring that all the air would purge out. Since the hot water outlet from the tank is not quite at the top of it, it would be logical that there is still air trapped there. To purge the air I intend to open the pressure valve until water starts coming out. Is that the best method?
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Old 04-17-2017, 09:21 AM   #8
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Listing. My Boler currently has a cold water tank, I want to replace it with something to provide warm water, use 15 AMP AC only, should shut down automatically when not enough water in the tank.
Thanks for any info.
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Old 04-17-2017, 09:58 AM   #9
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Listing. My Boler currently has a cold water tank, I want to replace it with something to provide warm water, use 15 AMP AC only, should shut down automatically when not enough water in the tank.
Thanks for any info.
There is no sensor in the water heater to shut down the element if there is insufficient water in the tank . The element will be destroyed long before the high limit reaches it set point . The high limit is there to prevent the water heater from producing overly hot water or steam if the controlling thermostat fails.
The conductor size for a fixed appliance is based on the current draw . Usually you take the full load current and multiply by 1.25 to determine the conductors size IE A 12 amp water heater X 1.25 = 15 amps . A #14 AWG conductor is rated for 15 amps
( You can use #12 AWG or #10 AWG wire if you wish -- oversized conductors are allowed)
Normally circuit breakers are only loaded to 80% of their rating
A 15 amp circuit breaker X .8 = 12 amps.
The heating element in an RV water heater is normally around 1400 watts -- 1440 watts 120V AC = 12 amps.

Again electric or gas storage,style water heaters do not have run dry protection , automatic or otherwise . You have to manually ,physically , visually make sure that the tank is full of water.
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Old 04-17-2017, 11:40 AM   #10
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A little bubble at the top that doesn't come out with the hot faucet open will not hurt anything. That would be above the element. The element must be submerged and the tank pressurized when the power is turned on. You might get away with a full tank and the pressure off, but don't count on it.0
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Old 04-17-2017, 12:15 PM   #11
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A little bubble at the top that doesn't come out with the hot faucet open will not hurt anything. That would be above the element. The element must be submerged and the tank pressurized when the power is turned on. You might get away with a full tank and the pressure off, but don't count on it.0
Why does the tank need to be pressurized ?
Pressure is needed to move water through the tank and out to the faucets . The tank can sit forever and not be under pressure
The heating element needs to be submerged but has no way of knowing whether the water in the tank is at static pressure or pressurized to pump system pressure
I fill my water heater in the Spring and with the pump shut off and the system depressurized have turned on my water heater.
The only thing that happens is the water in the tank gets warm.
If pressure is required then the water heater would have a pressure switch . IT HAS NO SUCH SWITCH.

As a side note , A tankless water heater has a flow switch
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Old 04-17-2017, 03:20 PM   #12
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You are right there could be air in the water heater. When I filled it up I left the hot water tap open figuring that all the air would purge out. Since the hot water outlet from the tank is not quite at the top of it, it would be logical that there is still air trapped there. To purge the air I intend to open the pressure valve until water starts coming out. Is that the best method?


I let water flush through the pressure valve to make sure that there was no air left. I checked to make sure there was 110-120 volts arriving at the tank. I measured the resistance at the element and got a no resistance conductivity. No resistance is no good? I checked it with the breaker at the off position. What is the right way to
check the element? Does it matter if it is plugged in with the power off?
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Old 04-17-2017, 10:28 PM   #13
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Why does the tank need to be pressurized ?
Pressure is needed to move water through the tank and out to the faucets . The tank can sit forever and not be under pressure
The heating element needs to be submerged but has no way of knowing whether the water in the tank is at static pressure or pressurized to pump system pressure
I fill my water heater in the Spring and with the pump shut off and the system depressurized have turned on my water heater.
The only thing that happens is the water in the tank gets warm.
If pressure is required then the water heater would have a pressure switch . IT HAS NO SUCH SWITCH.

As a side note , A tankless water heater has a flow switch
Electric elements in water heaters can burn out if the water is not pressurized. Not saying it will always happen, but I've had it happen more than once. Modern elements have the minimum surface area. Have you ever heard the hissing sound of an electric water heater? That is the water boiling at the surface of the element. When it boils there is only steam in contact with the element and it is likely to burn out right there. Higher static pressure reduces the boiling at the element surface and saves the element. It has nothing to do with moving the water to the faucet. And it has nothing to do with a tank pressure switch, since there is none.

Another fix is to put a 220 volt element in a 120 volt tank, if you don't mind the reduced recovery time. Then you have much less energy per area of element, so simply being submerged will not likely burn it out. Or look for ones with more surface area. They can also become coated with calcium, which insulates them and then they are more likely to burn out.

I'm only talking about tank type water heaters here. Not tankless units with flow switches.

It's just good practice to only turn on the electric water heater when the water pressure is on to reduce the chance of burning out the element. Been there, done that.
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Old 04-17-2017, 10:35 PM   #14
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I let water flush through the pressure valve to make sure that there was no air left. I checked to make sure there was 110-120 volts arriving at the tank. I measured the resistance at the element and got a no resistance conductivity. No resistance is no good? I checked it with the breaker at the off position. What is the right way to
check the element? Does it matter if it is plugged in with the power off?
There is no need to try to purge out any small remaining bubbles in the water heater. The faucet is the best place to purge the air out.

Start looking for an open circuit, which can be a burned out element, with an ohm meter across the plug terminals. Then move to the element terminals with the wires disconnected. Open circuit means it's toast. You should also not have continuity between the terminals on the element and the metal tank.
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