Water Heater Elements

[Yianie123.]

Can you use a standard 240v water heater element instead of spending $100+ on a 12v,24v or 48v element?  What would happen?

[gww]

I use a 2000watt element but I do run it through the inverter.  It works very well.  I believe you can use them for dc but you really have to derate it.  So 240 volt 4500 watt is 18 amps = 18 amps at 12 volts= 216 watts.  They have a home made divertion load on the (Thebackshed) website that goes into this and may have one on the host site here under projects.  I can't remember.
Good luck
gww

[kitestrings]

I don't think this is quite right.  You really need to look at the resistance of the element.

A 240V 2,000w element would be (2,000/240 =) 8.3A.  The resistance of this element would be (240V / 8.3A =) 28.8 ohms.  At 12 volts then the current would only be (12 / 28.8 =) .41A, or 5 watts; at 24V it would be .83A and 20 watts; at 48V it would be 1.67A and 80 watts.  A 4,500 watt element is pretty common, but still your only looking at 180 watts at 48V.

gww, perhaps you were thing a 2,000 120V element, but even then it would be 16.7A,so the element would be 7.2 ohms.  The power at 12, 24 & 48V would be 20, 80 &320 watts respectively.

This is the basic problem, so generally you either go through an inverter at the intended voltage, or buy low-voltage (DC) elements.  With DC elements you also have to go through a relay if you're using the stock thermostats.

~ks

[kitestrings]

For some reason it would not let me modify my earlier comment.  I realized that gww had, in fact said '240V 4,500 watt". Most of what I said is the same, but here are the adjusted numbers:

I don't think this is quite right.  You would not have the same 18 amps at 12V.  You really need to look at the resistance of the element.

A 240V a 4,500w element would be (4,500/240 =) 18.8A.  The resistance of this element would be (240V / 18.8A =) 12.8 ohms.  At 12 volts then the current would only be (12 / 12.8 =) .94A, or 11 watts; at 24V it would be 1.9A and 45 watts; at 48V it would be 3.8A and 180 watts.

This is the basic problem, so generally you either go through an inverter at the intended voltage, or buy low-voltage (DC) elements.  With DC elements you also have to go through a relay if you're using the stock thermostats.

~ks

[gww]

ks
Was bad posting on my part,  I thought I had put that was the principle but look in these other places to be sure.  I was just guessing from memory cause it was in my ts60 cc manual and had read a bit on the sites I had mentioned.
Thanks for keeping me honest.
gww

[kitestrings]

It depends a lot on how much of a load your looking for, and what voltage you are using.  If you only need say 300-500 watts, and you're at 48V, you can probably put a couple of stock elements in parallel and ditch the interlocking contacts found on most tanks.  Higher power especially at low voltage becomes tough because you really need lower resistance elements.

We put in a pre-heat tank a few years ago, and used 48VDC elements.  IIRC they are about 1200 watts.  It's worked really well for us, I only wish the tank were a bit larger.  Here's a post on it:

http://www.fieldlines.com/index.php/topic,130053.msg845357.html#msg845357

If you have ample inverter capacity stock elements are probably simpler, so that could be another consideration.  Times have changed and I'm admittedly old-school in that I don't like having too many things that are solely dependent on the inverter, and I don't like unnecessary conversions of energy.

~ks

[gww]

ks
I like the inverter cause I had to put the hot water heater maby 70/80 feet away.  The sense wire on the ssr doesn't have to be much, so only one big line of ac to the sub-panel. 50 gal preheat but not always gotten fully hot.
Cheers
gww

[AzSun]

I ran into a similar situation years ago. I wanted to heat water with an early 1940's D.C. wind generator. It was 2KW model rated at 40 volts/50 amps. The proper resistance to meet the power curve at that point was 0.8 ohm. I made a resistor by tightly wrapping ~ 200 ft. of #16 enameled wire around a section of 3/4 copper pipe. Heat sink compound was applied to the pipe before winding. When all finished, fiberglass insulation was wrapped around the outside.  The pipe was a little shorter than the hot water tank height. It was mounted vertically outside the tank with the top being connected to the tank hot water outlet and the bottom teed into the tank drain. The two ends of the coil were connected to the generator output. In operation, as the wire got warm, heat was transferred to the water inside the pipe. The warm water would rise and flow into the top of the tank drawing cold water from the bottom to replace it. This is known as a sidearm heater. It  worked very well !  I plan on making another one for solar panel use. The resistance will be that which will allow the panel to operate at its maximum power point. Wire size is just a guess. You want it to get very warm so obviously need to pick a size that is not rated for the current you will be putting thru it. As long as there is water in the pipe and you use thermal paste, the wire will only get a little warmer than the pipe itself. Keep the assembly away from contact with any flammable materials. I will be using the wire used to wind electric motors. Insulation varies. Choose a grade rated for very high temperatures.         

[Ungrounded Lightning Rod]

Resistive devices operate on either AC or DC.  In the early days of electrical appliances they were operated on either:  Utilites might be DC (Edison) or AC (Westinghouse), windcharger/battery systems were, of course, DC, and small water generation might have been either.  AC is rated using "RMS" (Root Mean Square) volts, amps, and watts, because that way the power dissipation and heat/light production in a resistive heater or lamp come out the same.  Resistive element construction hasn't changed significantly since that time.

An AC resistive water heating element should work just fine on DC.  Better, in fact:  The heating is proportional to the RMS current and voltage, but the peak voltage it experiences with DC is the DC voltage and with AC is the cycle peak voltage, which is higher.  A DC application does not experience AC's twice-the-frequency power cycling (and resulting vibration and mechanical stresses).  The only added problem with DC would be increased galvanic corrosion IF the conductors get wet.  But a) that should not happen and b) if it does you have other problems.

A resistive water heating element will work fine at any voltage NO HIGHER than the rated voltage.  It will run cooler and last longer at lower voltage.  (It will run SUBSTANTIALLY cooler, and dissipate substantially lower power, because the power dissipated in a resistance is the SQUARE of the current through the resistance.  Running a 120 volt heater at 48V only burns about 1/6th the rated power.)

A resistive water heating element's resistance varies somewhat with the operating temperature of the resistance wire.  So, if you assume the resistance is constant, the power you calculate for operation at lower voltages will be somewhat in error.

If you use a thermal switch to interrupt the power to a resistance water heating element, you need to use one that is rated to interrupt DC at (no less than) the operating voltage and current.  DC is harder to interrupt than AC because AC periodically goes to zero as the cycle reverses, helping to extinguish the arc.

So you should have no problems using a stock resistive water heater designed for mounting in your tank as a lower-voltage DC heater / dump load, provided your thermal cutout switch is rated for DC service and you can accept the lower power drain.

[OperaHouse]

It should be mentioned that the heater temperature controls are not rated for DC.  If you get over 30V, there could be contact welding over time.  If you install a 12V heater element, half the power will likely be lost in the wires getting to it.  As said before 120V 2000W heater elements are available and most tanks have two heaters.  Conventional wiring only allows one at a time.  Two in parallel starts to be a good load.  I monitored the energy use on my 30 gal heater at home before installing a HPWH.   Daily use was about 3.5KWH, half of that is just heat loss.  If you are dumping to hot water you run out of capacity quickly.  I heat hot water with excess PV at my camp.  36V panels is about 52V and a good match for 120V heaters.  Next year I'm going to 48V panels.  Surprised more hasn't been done here with inverters.  They could be modified as dump controllers.