120 AC?DC water heater element

twinnomadtwinnomad Registered Users Posts: 1
I have tried several DC hot water elements and so far none work as well as my test bench 120AC heater. I am using a standard 5 gallon hot water tank and wanting to use a 120AC element on top and a 48DC on the bottom. I have heard you can use 48DC directly on a 120AC but the efficiency will be very low. Is there any dual voltage operational hot water elements available that can use either AC or DC for equal performance?

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: 120 AC?DC water heater element
    twinnomad wrote: »
    I have tried several DC hot water elements and so far none work as well as my test bench 120AC heater. I am using a standard 5 gallon hot water tank and wanting to use a 120AC element on top and a 48DC on the bottom. I have heard you can use 48DC directly on a 120AC but the efficiency will be very low. Is there any dual voltage operational hot water elements available that can use either AC or DC for equal performance?
    There is no problem with using either AC or DC on the same heating element, as long as the voltage is the same. But the standard AC thermostat may not be happy controlling DC.

    The low output you are seeing is entirely because of the voltage difference.
    If a heating element is designed for 120V, then 40 volts (to make the math simpler) will produce only 1/3 x 1/3 = 1/9 the wattage of heat.

    So you really need to get a heating element designed for closer to your DC voltage or else use an inverter to step up the voltage to 120VAC.

    By the way, the efficiency is still just about 100% on a lower voltage, it is just that the power input and power output are both lower. :-)

    And if you have a 12 volt element and are trying to drive it directly from a 48 volt DC panel set, you will get very low power because the panels cannot produce enough current to drive the 12 volt element to full output. Sort of like using a 48 volt panel through a PWM controller to a 12 volt battery.
    SMA SB 3000, old BP panels.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: 120 AC?DC water heater element

    A water heater element is just a resistor and it doesn't care if you use AC or DC power to it. The two things that determines how much power the element will draw is its resistance and the amount of voltage applied to it.

    When it comes to heating water you may as well forget using low voltage (<60V) DC power. I played with many different methods of using DC power for water heating for years and could never get the efficiency high enough to make it pay off. Today we heat all our water with AC power from the inverter, which actually works and yields serious amounts of hot water. You just have to come up with the DC power to feed the inverter to do it.
    --
    Chris
  • PhotowhitPhotowhit Solar Expert Posts: 5,081 ✭✭✭✭
    Re: 120 AC?DC water heater element

    I'm happy enough with my conversion of a 240V 3600watt water heater to running on 120 at 900watt that I don't think I would consider a solar hot water heater anymore. In my climate where I would have to run a heat exchanger. Needed so I could run antifreeze in the exterior portion of the water heater.

    I may switch the 3600 watt element with a larger 4400 watt element if it will reasonably fit in my 30 gallon tank and I have considered a dedicated solar array to run 140V VMP DC into the water heater element. That was when I wasn't sure if I would sell my cabin solar array, which has since sold. I might consider it yet if boB doesn't release that battery module for the Classic Charge controllers soon... I plan to be 'over paneled' on my new home array.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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