Electrical requirements for tankless water heater

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I currently have a standard 40 gallon electric water heater on a 30 amp breaker. The panel is 100A service which also services an electric dryer. I was thinking about getting a tankless water heater to save space, but after reading some electrical specs on these units, it seems I would have some issues with the electric.
I have 1 bathroom and 1 kitchen and a washer. From the calculations I would need at least a 5 gpm unit. If you look at the specs for the Rheem units, the RT18 needs 2 seperate 40A circuits, with maximum power being 75 amps.
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Obviously my panel cannot support that. I could maybe drop to the RT9 unit that requires 1- 40A breaker, but I'm concerned it might be undersized at 3 GPM, and also concerned that it uses 38 amps. If my dryer is on, thats another 26 amps, and my 220V water pump using 5 amps, totaling about 70 amps. That leaves me with about 30 amps to spare. So I could either upgrade my panel, or just stick with what I have. This is for a seasonal home in the summer. Just wondering what everyone else thinks I should do.
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heater - for SO MANY reasons. Particularly if you have ANY water issues. They are high maintenance items at the best of times.
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On 8/3/2011 8:05 PM, Mikepier wrote:

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You can easily "do the math" and determine EXACTLY what the temperature rise is when you run 40 amps at 240 volts through a water heater.
All that really matters is the temperature of the cold water coming in and the flow rate.
If you are "marginal" you can help by reducing the flow rate and if your cold water coming from outside is "too cold" you might want to help it along by running it around the basement. It's important to "temper" the cold water to the shower mixing valve since the warmer that water is, the less "hot" water you will need for a comfortable shower.
I can give you a "data" point. We have a conventional electric water heater. Even when I "run out" of hot water, if I run 100% hot water into the shower, it's tolerable. Bottom line is that it depends on just how hot you want your shower.
You first task is to: 1) measure the temperature of the cold water; and 2) "do the math" and determine the temperature rise at various flow rates; & 3) decide whether it's "gud enuf."
The only "justification" for a demand type hot water heater is to "save energy" by not keeping 30/40 gallons of hot water sitting around & heating up the basement. You can "save" much of that energy by adding insulation to the heater, installing that gadget that prevent circulation of the hot water above the heater (I think it's just a "leaky" check valve), and if your circumstances permit, just shutting down the water heater when you don't need it.
Frankly, the cost of a new panel or the equipment and wiring costs of a "demand" WH will take years to recover.
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There's another justification and that is availibility of an unlimited amount of hot water, ie there is no tank to run out.
You can "save" much of that energy by adding

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heaters in a "tank" would provide the same "unending hot water".
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Thanks for all your inputs.
I have well water, so it is fairly cold. I think I'm just going to stick with what I have for now.
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For what it's worth, on our vacation home I installed a 30amp double pole wall switch in the hall and put the hw tank on it. When we are not there it gets turned off.
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Yes, I aslo turn off the breaker to the water heater when I go away for a week or 2. I also turn off the well pump.
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I think standard breakers really are not meant to be used as switches. You can get breakers that are specifically rated for it. I found it more convenient to install a real switch next to our thermostat which we change when we leave too.
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*I would look into a solar water heater or something to boost the temperature of the water going into your current water heater.
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GPM tells how large the water pipe that goes through the water heater is, n ot the actually heating capacity. You have to pay attention to watts. An el ectric tankless water heater has perfect efficiency so there is a strict ma ths that tells you how hot it can make the water. What you need to know is how much water flow you are going to require, how hot is good enough for yo u and you incoming tap water temperature. This explains it much better: http://toastyreviews.hubpages.com/hub/best-electric-tankless-water-heater-r eviews If you decided to do tankless, you definitely need to get something bigger than you think you need. There is going to be some heat loss in the pipes e tc. so you can never get a too powerful electric tankless water heater. Tha t's why many people say tankless sucks. They get a small device and it does n't heat enough. I doubt Rheem RT9 will do any good even for a single person, unless you are in a really hot region. 15 watts is the minimum for tankless electric.
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On Thursday, September 18, 2014 12:20:48 AM UTC-4, sol wrote:

electric tankless water heater has perfect efficiency so there is a strict maths that tells you how hot it can make the water. What you need to know i s how much water flow you are going to require, how hot is good enough for you and you incoming tap water temperature. This explains it much better:

etc. so you can never get a too powerful electric tankless water heater. T hat's why many people say tankless sucks. They get a small device and it do esn't heat enough.
And the flip side is that a big one takes so many amps that it may require upgrading the panel/service. Factor that in with the much lower cost of an electric tank type unit and the tankless doesn;t look so good. Also, many places have time of day metering now. If you can run the electric tank WH during off peak, at night, etc, it can save a lot in operating costs.

You left out the K in Kwatts. I can see smaller ones used for special applications, like if you had just a sink somewhere in a building that's rarely used. That could be a good application for tankless and a small one would fit. But I agree that you need a decent size one to do even one bathroom with shower. Whole house gets worse. If you want tankless, gas tankless looks much better, but I'm not a fan of those either, for well known reasons, high initial cost being the main one.
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On 9/17/2014 11:20 PM, sol wrote:

Can we agree to make that 15 AMPS, Sol?
Ah, I knew we could<g>
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for electric tankless figure at minimum 200 amps just to heat water, plus a different panel of 200 amps for everything else. at least a 100 amps but you might as well go 200 amps.
this will require 400 amps total, need new service drop and the power company may need to do upgrades on their side of the meter....
the payback is never...
a friend looked into this, he called it going green.
all in over 15 grand.:(
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On 09/17/2014 11:20 PM, sol wrote:

Now I'm wondering how long it would take that 15 watt water heater to heat enough water for a bath.
Probably forever, since it cools off faster than it heats. 0% efficiency.
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sol wrote:

1 BTU/hr is ~0.3W
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On Thu, 18 Sep 2014 07:20:11 -0500, Unquestionably Confused

Not unless you are saying a tankless water heater is 3.6kw
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On Thursday, September 18, 2014 10:15:34 AM UTC-4, snipped-for-privacy@aol.com wrote:

Agree. I took that 15 W as a typo where he meant 15 KW. That seems more likely, IMO.
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On Thu, 18 Sep 2014 07:32:53 -0700 (PDT), trader_4

A 15kw heater (62 amps at 240v) will give you about 45 degrees above the input water temperature at 3 gallons a minute. Even with the most efficient water saver shower head (jump around to get wet), you are probably getting a cold shower in the winter and if someone turns on the hot water in the kitchen you will be freezing. My brother in law needed 3 big propane fired units to have satisfactory hot water in his house.
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