Water heater question

[TPG]

So I have a question about water heater elements. Our boat has a 220V element installed and we run off 120V on shore power as well as our inverter.

Builder made comment that the water heater will work on 120V but will take longer to heat the water. At this time I don't see that the element will heat the water at all on 120V so it got me thinking about the physical differences between the two.

Anyone have expertise they'd like to weigh in with?

[GordMay]

According to Ohm’s Law, the current that flows through a conductor is directly proportional to the voltage applied to it.

Voltage is = Interesting Reading
V = IR,can be transposed to I =V ÷ R, and R = V ÷ I

The greater the voltage, the greater the current. A twofold increase [or decrease] in the voltage will lead to a twofold increase [decrease] in the current, if all other factors are kept equal.
The water heater element is a fixed resistance, producing the desired wattage, at the design voltage.
Reducing the applied voltage, to one-half the design voltage, will result in a halving of the design wattage.

Thus: A 1,000 Watt, 240 Volt element will only produce 500 Watts, of heating, with 120 Volts applied to it.


To better understand the relationship between various electrical parameters, we can take all the equations used to find the voltage, current, resistance and power, and condense them into a simple Ohm’s Law pie chart as shown below.

[Bill O]

Should be fairly simple to change the 220V to a 120V heating element.
Your main panel is probably wired to deliver 220V to the heating element and will need to change that also if you change the element to a 120V.

[JimsCAL]

Actually, the watts will be reduced by a factor of 4 if you run a 240 volt heating element on 120 volts. Current gets cut in half as well as the volts. Wattage varies by square of the voltage. As noted, best solution is to change the heating element, if that is possible with your heater.

[GordMay]

Originally Posted by JimsCAL

Actually, the watts will be reduced by a factor of 4 if you run a 240 volt heating element on 120 volts. Current gets cut in half as well as the volts. Wattage varies by square of the voltage. As noted, best solution is to change the heating element, if that is possible with your heater.

OOPS - Jim's right.

[HeywoodJ]

Originally Posted by GordMay

... A twofold increase [or decrease] in the voltage will lead to a twofold increase [decrease] in the current, if all other factors are kept equal....

Gord has the theory, but dropped a factor, twofold voltage leads to twofold current (true/correct) which then leads to one-quarter the heating power.

This is because you have 0.5VX0.5I = 0.25P.

A 10A@240V element= 2400W. The resistance = 240V/10A = 24Ω.

120V/24Ω= 5A.

5A@120V = 600W.

I've lived this life on boat for years. If you plan to keep the boat in 120V land then probably worth changing out the element. However, if you plan to travel to both 120 and 240V lands probably best to keep in the 240V element. You can always run the 240V element on 120V, but going the opposite direction results in 4x the power and overloads the element (and probably trips the breaker - we can hope). In 120-land we have had no problem with two people using hot water/showers, don't know what it would be like with more people. We also don't use a lot of water generally (limited tankage anyway) so slow heating is not a big issue.

[Jammer]

Originally Posted by TPG

Builder made comment that the water heater will work on 120V but will take longer to heat the water. At this time I don't see that the element will heat the water at all on 120V so it got me thinking about the physical differences between the two.

Anyone have expertise they'd like to weigh in with?

The posters upthread are correct but are missing the forest for the trees.


Sure, if you have a water heater element rated for 240 volts and apply 120 volts to it, you'll get a quarter the wattage.


But if you have a boat wired for U.S. standard 120/240v 50A shore power, and you use an adapter to connect to a 120v-only shore power pedestal, then the water heater won't see any voltage at all, because the two "hot" legs are just connected together instead of having 240 volts across them.


If you're using one of those Y adapters that connects to two 30A shore power outlets, well, you take pot luck, the water heater might see 240v depending on how the marina happens to be wired.



A qualified marine electrician (maybe that's you), can reconnect the water heater at the breaker panel, so that one leg is connected to neutral rather than connected to hot. There are also ways of doing this on a switched or automatic basis so that you retain the more rapid water heating performance when the proper shore power (120/240v 50A) is in fact available.


A fact to consider is that larger boats like yours are usually designed around a larger power budget than a 30a shore power pedestal (or even two of them) can deliver, multiple air conditioners, etc. Your best course of action may be to seek out 120v/240v shore power during your travels.

[boating4ever]

did you confirm its only getting 120v with a volt meter? if so, then switch the heating element. on my Quick water heater its super easy.