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Old 11-09-2012, 02:25 PM   #1
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Is it OK to power a 220v tankless with only 120v?


There is another thread which said you can wire a 220v water heater to a 120v line and you will just get 1/4th the power but it would still work.

So the question is this.
Will a 220v tankless that is wired to a 120v line heat water roughly the same as a 120v tankless on a 120v line??

The reason I ask is because the 220v tankless water heaters are the same price as 120v and why not keep the capability to upgrade it to 220v in the future.

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Old 11-09-2012, 02:37 PM   #2
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Is it OK to power a 220v tankless with only 120v?


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Originally Posted by wonderworm View Post
There is another thread which said you can wire a 220v water heater to a 120v line and you will just get 1/4th the power but it would still work.

So the question is this.
Will a 220v tankless that is wired to a 120v line heat water roughly the same as a 120v tankless on a 120v line??

The reason I ask is because the 220v tankless water heaters are the same price as 120v and why not keep the capability to upgrade it to 220v in the future.
If possible, why not just wire it for 240V? A tankless running at less than full power will not heat very well.

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Old 11-09-2012, 02:45 PM   #3
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Is it OK to power a 220v tankless with only 120v?


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If possible, why not just wire it for 240V?
Unfortunately, it's not possible. My options are to buy a 220v wired as a 120v or 120v wired as a 120v. Do you think the 220v at 120v power will heat equally as poorly as the 120v tankless?
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Old 11-09-2012, 02:46 PM   #4
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Is it OK to power a 220v tankless with only 120v?


If your talking about the situation with your washer and sink...

Quote:
The water heater that feeds a clothes washer and a kitchen sink went out and I'm going to replace it with a tankless.

I have both a dedicated 120v line with a 30amp breaker to use or I can have the electrician tap into the 220v line that powers the living room airhandler.

My question is this: The current breaker on the 220v line is 30amps. The 220v tankless needs up to a 60 amp breaker. If an electrician taps into that 220v wire and adds the 220v tankless on it, can he then just upgrade the total amps on the breakers to 90amps to ensure that both can run at the same time without tripping the breaker?
Basically, it sounds like your trying nickle and dime it.....

The cost of the tankless unit is much greater than the cost to do the wiring right....if your not going to do it right...then just wash your clothes in cold water until you can do it right.
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Last edited by ddawg16; 11-09-2012 at 02:52 PM.
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Old 11-09-2012, 03:24 PM   #5
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Is it OK to power a 220v tankless with only 120v?


Either way it will be pretty anemic.
Maybe 30 degrees rise on 1 gpm.
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Old 11-09-2012, 03:36 PM   #6
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Is it OK to power a 220v tankless with only 120v?


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Unfortunately, it's not possible. My options are to buy a 220v wired as a 120v or 120v wired as a 120v. Do you think the 220v at 120v power will heat equally as poorly as the 120v tankless?
For the 240v unit, it may use the same wiring as the 120v model, if it is straight 240v and not 120/240v.

Do you have space in your panel for a double pole breaker?
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Old 11-09-2012, 03:42 PM   #7
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Is it OK to power a 220v tankless with only 120v?


You really need to quit looking for ways to do this on the cheap. Do it right, or not at all. You obviously know nothing about electricity, and no reputable electrician is going to do what you're asking about.
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Old 11-09-2012, 03:55 PM   #8
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Is it OK to power a 220v tankless with only 120v?


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Originally Posted by wonderworm View Post
unfortunately, it's not possible.
Why???
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Old 11-09-2012, 04:25 PM   #9
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Is it OK to power a 220v tankless with only 120v?


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Why???
The utility room where this would go is on the opposite side of the house from the breaker box and you would have to run the new line behind a bunch of sheetrock and across 2 stories and subfloors to get it there. Not worth it just to convert from a 20g tank to tankless.
The only reason why I would put in a 240v tankless is in case any future owner wanted to run the 240line to it. He would already have the tankless to utilize it.

I find it interesting that I get attacked but noone seems to have an answer to the single question I asked.

Will an entry level 240v tankless at 120volts produce roughly the same heat as a 120v tankless at 120volts? I think I'm just too outside the box for some here.
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Old 11-09-2012, 04:31 PM   #10
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Is it OK to power a 220v tankless with only 120v?


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Originally Posted by wonderworm View Post
The utility room where this would go is on the opposite side of the house from the breaker box and you would have to run the new line behind a bunch of sheetrock and across 2 stories and subfloors to get it there. Not worth it just to convert from a 20g tank to tankless.
The only reason why I would put in a 240v tankless is in case any future owner wanted to run the 240line to it. He would already have the tankless to utilize it.

I find it interesting that I get attacked but noone seems to have an answer to the single question I asked.

Will an entry level 240v tankless at 120volts produce roughly the same heat as a 120v tankless at 120volts? I think I'm just too outside the box for some here.
A 240vac unit has x resistance.

If you connect 120Vac to it....the wattage will be half of what it would be at 240Vac....so it's only going to heat up half as much water for the same flow rate.

If you slow down the flow rate, it 'might' get it hot enough.
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Old 11-09-2012, 04:50 PM   #11
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Is it OK to power a 220v tankless with only 120v?


Your wasting time and money even trying to make it work.
Even the unit you picked out is junk.
You can try and prove everone here wrong and do it anyway but we all know who's going to be drawing the short straw on that one.

Got an electric hot water heater now? Disconnect one of the wires off of the double breaker and cap it off and wait a few days and see how that works for you.
It's going to be the same thing with the other unit.
You have full time plumbers and electritions that all say it will not work.
May want to try asking the nice young man at Home Depot or Lowes what they think.
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Old 11-09-2012, 05:25 PM   #12
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Is it OK to power a 220v tankless with only 120v?


A tankless water heater will use lots of amps so I doubt you can install one at either voltage to replace your tank type heater.
The smallest breaker requirement I have found for a 120 volt tankless is 30 amps. Do you currently have #10 cable installed?
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Old 11-10-2012, 12:54 AM   #13
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Is it OK to power a 220v tankless with only 120v?


Quote:
Originally Posted by wonderworm View Post
I find it interesting that I get attacked but noone seems to have an answer to the single question I asked.

Will an entry level 240v tankless at 120volts produce roughly the same heat as a 120v tankless at 120volts? I think I'm just too outside the box for some here.
It's not outside the box, it's nonsensical and unanswerable without much more specific information. Calling a tankless heater "entry level" provides no useful information about it's electrical specifications. You already provided the most specific answer possible in your own original post: A 240V heater on 120V produces 1/4 of it's rated power. So read the datasheets for the two heaters you are comparing. If you have a 2kW 120V heater and an 8kW 240V heater, then they will produce the same output when both run on 120V. If their rated powers are not a 4:1 ratio, then they won't.

You don't seem to get just how big of a deal the power supply is for a tankless heater. They require huge amounts of power. There is no way to cheat on this - the relationship between power draw and heating effectiveness is absolutely linear. Even the smallest and least effective 120V heaters use the full capacity of a dedicated 20A circuit. Achieving a good temperature rise on a hefty flow rate requires massive amounts of power - almost always more than the rest of the house combined, and sometimes more than the existing electrical service can provide. If the electrical constraints make this impractical, then a tankless heater is not suitable for your application.
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Last edited by mpoulton; 11-10-2012 at 12:59 AM.
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Old 11-10-2012, 12:58 AM   #14
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Is it OK to power a 220v tankless with only 120v?


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A 240vac unit has x resistance.

If you connect 120Vac to it....the wattage will be half of what it would be at 240Vac....so it's only going to heat up half as much water for the same flow rate..
A quarter, not half. P=I*V. I=V/R. So P=V^2/R. So dividing voltage in half divides power by four, since the current is halved and so is the voltage. An 8kW heater at 240V (a usable but small tankless heater) requires 34A. At 120V, the same heater would draw 17A and produce 2kW, which is hardly worth having.
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Old 11-10-2012, 06:46 AM   #15
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Is it OK to power a 220v tankless with only 120v?


Why not convert the line that is going to feed the tankless heater into a 240 volt line?

Install a double wide 240 volt double breaker for it, remove the neutral from the neutral bus at the panel, and connect that white wire to the other half of the double breaker. Disconnect all other receptacles, lights, etc. from the line, wire nutting the wire ends inside each box daisy chained along the way as a pass through only. (The tankless draws enough amps that the circuit should be dedicated to it.)

Or is the one half amperage one quarter wattage delivered by the 240 volt heater operating on 120 volts a blessing in disguise? Where the line can't handle the amperes the heater would draw at 240 volts but will handle the amperes the heater would draw at 120 volts?

If you put a flow restrictor in the hot water line, say, where you screw on the washing machine hose, the tankless heater will raise the water to a higher temperature. This last part will not work on inferior washing machines that control the level of (quantity of) water using their timers.

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Last edited by AllanJ; 11-10-2012 at 06:53 AM.
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