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Old 25-03-2015, 19:33   #91
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Re: Running 220v water heater on 110v supply

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Originally Posted by CarinaPDX View Post
If true, there go the RWNJ sites!

I have sympathy for Dave - having contributed so much about the safe installation of mains AC on a boat it must be hard to take a thread which starts with (roughly) feeding 120VAC into a boat's 220VAC system then asking after the fact if anything could have been broken.

Making hot water with an electric water heater is simply not a critical enough need to be taking such risks IMHO.

Greg

Thanks for the input Greg. The question was actually asked before the connection and the general consensus was that so long as no equipment such as motors were run and that the battery chargers were 110/220 there could be no damage. It also seems to be the general consensus with this thread. Interesting that many members with a lot more electrical knowledge than myself can't seem to agree on the basics, and this is similar to what Ive found with the marine electricians I've contracted to do various works in the past. Simple things like installing battery monitors incorrectly without wiring all through a bus, or adding additional batteries but connecting the cables mid system instead of +ve one end and -ve the other end of the bank. Since the original installer did this I've asked several electricians to change it but their replies are always, " oh it's fine fine that,they just did that in the old days with large banks and long wire runs"
So I'm dubious of my chances of finding a qualified marine electrician that really knows his stuff, and not someone that learned the basics 20 yrs ago and has been winging it ever since, while cruising the Caribbean Islands.
Agreed making hot water isn't worth taking any risks, as is plugging into shore power in any marina, which is why I asked for conformation that there is NO risk.
The question seems to have been answered clearly so thanks again.
Monte
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Old 25-03-2015, 19:58   #92
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Re: Running 220v water heater on 110v supply

Thanks for the considered response. You are quite right to be concerned about the quality of advice from many "boat electricians". I have seen some really bad wiring on boats, done by "professionals" at silly prices. OTOH there are disagreements among the most knowledgeable people about the safest practices, which plays out in different standards (e.g. American ABYC vs. EU CE standards). For a 220VAC system you should probably use the European approach (or Aussie if that applies). Either track down the standard or find a book that explains how to do things according to that standard. For international cruising you should consider installing the isolation transformer I mentioned earlier. There are cheaper transformers available, but most are not isolated or packaged in waterproof housings. In addition to allowing use of all mains power, it will greatly increase the safety of the AC system (as well as blocking the DC which causes electrolysis).

[BTW standards are a moving target - no American boat built in the '80s would meet today's standards. It is worth it for owners of older boats to review the current standards.]

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Old 25-03-2015, 20:16   #93
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Re: Running 220v water heater on 110v supply

Thanks again, will definitely look at having a step up transformer installed if we are travelling more often in 110V areas. At this stage it looks like we will be heading back to 220V areas and as the main reason we plugged in was to equalise batteries that had been in continuos use off solar only for the last 4 months, were unlikely to be using 110V again for the foreseeable future.
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Old 25-03-2015, 21:39   #94
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Re: Running 220v water heater on 110v supply

If the water heater is wired to genuine Libyan plutonium, providing 1.21 Jigawatts and the boat speed is increased to 88 mph, then you won't need roads.
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Old 25-03-2015, 22:04   #95
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Re: Running 220v water heater on 110v supply

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If the water heater is wired to genuine Libyan plutonium, providing 1.21 Jigawatts and the boat speed is increased to 88 mph, then you won't need roads.
This clip shows the correct pronunciation for Jigawatts.


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Old 26-03-2015, 08:54   #96
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Re: Running 220v water heater on 110v supply

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Which explains 1/4 wattage output on a resistance heater.

Same power, half voltage, current is double:
However, when a load is on a motor, the motor must draw a fixed amount of power. The required power is about equal to volts times amps. Should the voltage fall below the nameplate rating, the amperage will increase accordingly.
Why "must the motor draw a fixed amount of power?". As the voltage drops the power output will decrease proportionally. A DC motor will have less power and RPMs available at lower voltage.

An AC induction motor will maintain speed at lower voltage but it will have less torque and will stall if the torque is less than the load.
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Old 26-03-2015, 11:04   #97
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Re: Running 220v water heater on 110v supply

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Why "must the motor draw a fixed amount of power?". As the voltage drops the power output will decrease proportionally. A DC motor will have less power and RPMs available at lower voltage.

An AC induction motor will maintain speed at lower voltage but it will have less torque and will stall if the torque is less than the load.
You are right. An AC motor will try to turn at a constant speed (i.e. constant power). If the voltage is reduced the rotor windings will slip more and this causes the current to increase thus holding power approximately constant. Actually, the input power will go up a bit because the motor losses increase. If the motor stalls then the current really goes up because there is no back EMF to reduce the current.

A DC motor will turn at a speed dependent on the shaft load and the DC voltage. If the voltage goes down the speed goes down. Assuming the motor doesn't stall the DC current will go down too. This is why DC motors are less likely to burn up with reduced voltage. Some DC motors can be burned up by applying voltage when they are stalled. All the DC input power is converted to heat in that case.
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Old 26-03-2015, 11:10   #98
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Re: Running 220v water heater on 110v supply

As a coil heats, the resistance increases. All conductors resistances increase with temperature.
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Old 26-03-2015, 11:42   #99
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Re: Running 220v water heater on 110v supply

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Ok thanks again for the feedback and link to the waterheater. Most useful. Still having trouble getting my head around how the waterheater manages to not suck air as it is used in the event of an empty tank. I imagine the pump would continue to pump air though the system and cause air locks and spluttering. But perhaps it can only do this on outlets above the pump and water tank height.
The hot water heater outlet should not be drawing from the bottom of the hot water tank. That would introduce a situation where any time the system was run dry it would cause the heating element to be exposed and overheat. That would be a horrible design.

When the water system is run dry, the hot water tank is still full of water. It should fill and draw from the top, not the bottom. Even if you rupture a water line that back drains the system dry, the hot water tank will remain full.

Drain the hot and cold water lines, then drain the hotwater tank and you will see.
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Old 26-03-2015, 11:52   #100
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Re: Running 220v water heater on 110v supply

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The hot water heater outlet should not be drawing from the bottom of the hot water tank. That would introduce a situation where any time the system was run dry it would cause the heating element to be exposed and overheat. That would be a horrible design.

When the water system is run dry, the hot water tank is still full of water. It should fill and draw from the top, not the bottom. Even if you rupture a water line that back drains the system dry, the hot water tank will remain full..
I posted that way back. Post #49. The cold water comes into the bottom and comes out the top. The tank remains full unless it is drained by the water heater drain valve.
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Old 26-03-2015, 12:00   #101
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Re: Running 220v water heater on 110v supply

Yes this makes sense, but doesn't really explain what happens when a water tank runs dry, but the pump continues to pump air into the system from the empty water tank, causing the taps to splutter. Could it not in that case pump air into the water heater? Or is this unlikely given the water heater is 1m below the bottom of the water tank.
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Old 26-03-2015, 12:06   #102
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Re: Running 220v water heater on 110v supply

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Yes this makes sense, but doesn't really explain what happens when a water tank runs dry, but the pump continues to pump air into the system from the empty water tank, causing the taps to splutter. Could it not in that case pump air into the water heater? Or is this unlikely given the water heater is 1m below the bottom of the water tank.
The air will not force the water out, it will travel through the water and exit the top into the hot water piping.
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Old 26-03-2015, 12:09   #103
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Re: Running 220v water heater on 110v supply

Makes sense
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Old 29-03-2015, 08:44   #104
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Re: Running 220v water heater on 110v supply

OK next question.

My boat has two 30 gallon water heaters (due to size constraints where they are installed). Each has its own thermostat, both set to the same temperature.

Should they be plumbed in series or parallel?
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Old 29-03-2015, 08:50   #105
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Re: Running 220v water heater on 110v supply

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OK next question.

My boat has two 30 gallon water heaters (due to size constraints where they are installed). Each has its own thermostat, both set to the same temperature.

Should they be plumbed in series or parallel?
They should both be replaced with just one 6-gallon, like most boats.
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