Dual Voltage (12v dc / 120v ac) Small Electric Water Heater

[Chance]

What's the main advantage to the dual voltage setup versus installing a cheap 300 watt inverter? Or whatever size one wants/needs. Other than a 15% or so loss in efficiency, that is.

This question was probably covered already somewhere, but I must have missed it. Can't batteries feed inverter and inverter power 120 Volt heating element?

[boywonder]

Originally Posted by Chance

What's the main advantage to the dual voltage setup versus installing a cheap 300 watt inverter? Or whatever size one wants/needs. Other than a 15% or so loss in efficiency, that is.

This question was probably covered already somewhere, but I must have missed it. Can't batteries feed inverter and inverter power 120 Volt heating element?

Yes, they sure can.......

To clarify, my setup will be 12V only (or 110V only right out of the box).....technoDave's dual voltage setup is brilliant but I don't really camp where shore power is available so it doesn't matter that much to me. I could pull apart the drain valve and see of I can get another 110V element in there......maybe if I get bored.

In addition to the 10-15% efficiency gain, there are other reasons, like some of the smarter solar controllers that can divert the amps to the heater element once the batteries are topped off. (my controller is not that fancy) I was also considering a dual throw switch that when in the "on" position connected the solar controller to the water heater and when "off" is connected to the house batteries, but that will probably be too primitive......and slow.

I also like the intrinsic safety of running a 12V element.....assuming my little experiment here is successful and the thing actually works. The stock 110V heating element is 1440W so a little 300 watt heater is going to be at least 4X slower in heating the water.

My use case will mostly be turn it on when heading out for a MTB or moto ride, then take a sprayer shower when I return....and turn it on when heading out of town to pre-heat the water. My present plan is to set the heater to a fairly low temp to save battery power when camped.......My reference data point is a sun shower bag, hoping this will be better....

My sink setup is planned to have a single lever "drinking water dispenser" style faucet that will only deliver cold water when the water heater is off; otherwise it's going to be hot...or just above luke warm.....

I already have the faucet and the sink and water pump arrived today. I also have a 2 gallon pressure tank on the way. If I get ambitious, I'll assemble the whole thing in the driveway and give it a test run before installing in case it doesn't work out.

[boywonder]

Originally Posted by daveb

350 you never heard of pie or a PIE chart? He just used V for E or are you just funnin?


I'd better not say how we learned the resistance color code (BBROYGBVGW) but maybe you've been there schoolin wise?

Thanks Dave for the clarification! I have ohms wheels on taped to all of my computer monitors both at home and at work.

.....and for the non-electrical types, daveb's abbreviation above is for determining the resistance value of a resistor by it's color bands.

The colors are black, brown, red, orange, yellow, green, blue, violet, grey & white.

The way us old guys remember this is "bad boys rape our young girls but violet gives willingly"....not exactly politically correct.

I have a few Australian women engineers on my design team so I do not teach the engineers this handy little helper.

[daveb]

Teachers would be fired for that these days.

[E350]

Thanks boywonder and daveb! And yes daveb, I am not funnin'. I really am that dense. So I will study the wheel or keep it for future studying.

boywonder, why not keep (i.e., install in the drain port, the 120v element) so that you could keep the van plugged in at home and when you take off you won't be using gasoline/diesel to run the 12v to heat the water. Start out with your first shower house (Edison Electric?) heated.

chance: Here's the original thread:

http://midniteforum.com/index.php?topic=2140.0

The Midnite Kid MPPT solar charge controller will top off the batteries then divert photovotaic power to another device. tecnodave converted a water heater to run on 12v/24v/120v so he uses his extra pv power to heat his water.

[JWA]

Originally Posted by boywonder

The thermostat looks quite simple as it just opens the circuit when the set temp is reached, so this should work fine on 12VDC.

just now noticed this---hoping its not too long after the fact.........

Devices originally designed for 120 VAC don't necessarily work safely when 12 VDC is applied. If this t'stat will be directly interrupting the current feeding the heating elements contacts might overload and create fire hazards.

[boywonder]

Originally Posted by JWA

just now noticed this---hoping its not too long after the fact.........

Devices originally designed for 120 VAC don't necessarily work safely when 12 VDC is applied. If this t'stat will be directly interrupting the current feeding the heating elements contacts might overload and create fire hazards.

Thanks JWA, I realize that. The original 110V heater element is 1440W, so let's use Ohm's law to do a few calcs...

The thermostat directly is wired in series with the heater element ie directly interrupts the current flow when set temp is reached. I'm assuming that the thermostat is a simple bi-metal type, although I have not verified that yet.

Power/volts=current

1440W/110V=13A

300W/12V=25A

Power ratio between the two elements 300W/1440W=.20=20%

If it's desired to reduce the current even further, there are 12V 200W elements available

200W/12V=16.7A

Power ratio between the two elements 200W/1440W=.14=14%

...so I am doubling the current through the thermostat but putting less than 1/4 of the power through it.....I'm OK with this.......again, for everyone following along, if you do not understand this stuff please don't try this at home.

...and for everyone understanding this stuff, please feel free to keep the comments coming; this is an experiment, and I'm not planning on doing anything unsafe, as water heaters can be very dangerous.

[E350]

This is news I can use (I think*):

"Power/volts=current

1440W/110V=13A

300W/12V=25A"

Maybe not for this thread, but didn't I always hear "watts is watts" regardless of 12v or 120v? Obviously, it takes more amps to make less watts in a 12v system than a 120v system.

* For sizing circuit breakers.

[boywonder]

a few comments about water heater safety.....

In the old days water heaters and boilers would routinely explode leveling city blocks causing injury and death.

For this reason, a long time ago, the ASME (American Society of Mechanical Engineers) came up with the boiler code, which contains many volumes and chapters and is extremely difficult to lift unless you get the online digital version.

Virtually all heated pressure vessels in the US must comply to this code, so modern water heaters have various levels of redundant safety features such as thermostats, element watt ratings, vessel pressure ratings, TP relief valves, etc.

Here are a couple of typical failure modes and the mitigation for each failure:

The heater element insulation fails from corrosion and applies mains (110VAC) power to the water resulting in electrocution when you touch the hot water faucet. This is mitigated by requiring the copper plumbing in your house to be tied to ground outside, usually by your electrical panel. It is also mitigated by a chassis ground in the water heater itself (redundant safety features)

The thermostat fails open, causing the water to boil, resulting in superheated steam (extremely dangerous failure). This failure can be mitigated through proper wattage sizing of heater element, which may not be capable of supplying enough energy to create superheated steam, and this failure is always mitigated by the T/P (temperature/pressure) relief valve in the top of the tank. The T/P relief valve will open/vent relieving pressure if the tank is over temp or over pressurized.

[boywonder]

Originally Posted by E350

This is news I can use (I think*):

"Power/volts=current

1440W/110V=13A

300W/12V=25A"

Maybe not for this thread, but didn't I always hear "watts is watts" regardless of 12v or 120v? Obviously, it takes more amps to make less watts in a 12v system than a 120v system.

* For sizing circuit breakers.

Correct, watts is watts.......although circuit breakers are thermal devices and rely on current to pop open. Current flowing through a wire (or a circuit breaker) causes resistive heating which causes the breaker to open. Since all wires and circuit breakers have internal resistance, they heat up when current is flowing. A fuse works the same way.

This is called "I squared R" heating........if you look on the ohm's wheel, you'll see that power also equals I2R (sorry don't know how to do superscript in HTML stuff)