Can I replace 120V Element with 240 to achieve half power?

Discussion in 'Water Heater Forum, Tanks' started by grimdestripador, Nov 15, 2012.

  1. grimdestripador

    grimdestripador New Member

    Messages:
    2
    Location:
    Alaska
    I am to install a point of use, 13 gallon Whirlpool electric hot water heater. The hot water heater claims to use up to 1750W, using a 1600W, 120V element. Is there any consiquence to replacting 120V element with a 240V element. From my understanding, it seems these are just resistive elements, and power/current should behave accurding to ohm law (V=IR, P=VI=V^2*R). Of course warm-up times will be doubled (or perhaps even x4).

    Thanks for any insight.
  2. DonL

    DonL Out of the Trades

    Messages:
    3,902
    Location:
    Houston, TX
    Sounds like you have it correct.

    Recover time will be very slow, But the element will never burn open until it gets corroded from the outside in.
  3. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    The power will be 1/4 with 1/2 the voltage. So, a 1600 W, 240v element would become a 400W element at 120v. Depending what elements will fit, you could go to a higher wattage 240v element to still have a decent recovery at 120v.

    What is the reason for going to a 240v element? Are you trying to tie the water heater to an existing kitchen (assuming this is going in the kitchen) circuit? If so, the problem is that maybe a future buyer could stick the 120v element back in and would overload that circuit. Best to run a dedicated line for it.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,485
    Location:
    Cave Creek, Arizona
    IF you want slower hot water recovery, or less power draw, they do make 1000 and 1200 watt 120 elements, ( in fact, I have seen 850 watt ones but they are hard to find).
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    FWIW, the power to heat the water will be the same, regardless of what element you use assuming the thermostat stays the same. It's just that the higher wattage stuff will get there faster. With an electric WH, there isn't a huge standby loss, so most of the energy is used to heat it in the first place, not reheat it after a long time of disuse.
  6. grimdestripador

    grimdestripador New Member

    Messages:
    2
    Location:
    Alaska
    For those concerned. I am not intending to use this as a perminante installation, but rather a semi-portable hotwater + generator combo. I have a 55 gallon rain barrel, with a RV type- pressure regulating pump (120V). I'm not concerend about recovery time and more concerened about power requirements (of generator vs 10guage extension cord).

    I see that home depot has replacement elements in stock for reasonable prices. I have been unable to find a supplier for these rare 750-1000w, 120V elements, but have been able to find lots of 240V elements for near $10. When I look at the elements, I can make guesses why they don't advertise a 1000W, 120V element as a capable 4000W, 240 element. But why can't a 240V, 3200W element run at half voltage to achieve quarter power.

    Resistance of a 3200W, 240V; P = V^2*R; R = P/V^2 = 3200/240^2 = 3200/57600 = 0.055555 [ohms]
    Resistance of a 1600W,120V; P = V^2*R; R = P/V^2 = 1600/120^2 = 1600/14400 = 0.11111111 [ohms]

    Power of a 3200W, 240V element running at 120V: Power = V^2 * R = 120^2*0.055555 = 800 watts (PERFECT).

    Other thoughts I've had, include... (1) using a power diode to rectify the wave. (2) using an variac (variable transformer). (3) using a stove top resister dial. But these seems risky because I don't know if the thermostat circuit requires exactally 120V or can run at arbitary values.
    Last edited: Nov 16, 2012
  7. DonL

    DonL Out of the Trades

    Messages:
    3,902
    Location:
    Houston, TX

    I would be careful using a diode. It would work but the thermostat may fail.

    Sometimes the DC can cause the thermostat to stick ON. They are normally AC rated.

    If you use a variac then you would have even more loss, but the thermostat should not care if it is switching less AC power.

    Propane or solar may be more cost affective for heating water in the sticks.

    Please post the outcome and let us know how you make it work.
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