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Thread: Tankless water heater Scenario

  1. #1
    DIY Junior Member USFishin's Avatar
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    Default Tankless water heater Scenario

    I know the general consensus here is against tankless electric water heaters. I think I’ve got a unique situation here though. What do you think…

    I live in the Tampa Bay area so it looks like the average water temp is somewhere in the mid 70’s. Currently my electric tank water heater is about 6 feet from my electrical panel. I know I’ll need to upgrade my electrical service. I am an electrical engineer and plan on asking the Siemens or Square D rep for a “sample” panel and breakers to upgrade my service. I am also planning on adding a tankless under my kitchen sink (for the sink and dishwasher) because of the distance from the water heater. I’m going to add the tankless under the kitchen sink whether I do a whole-house tankless or not. So the whole-house tankless will run 3 showers and 4 sinks. We have 2 adults in the house and a baby. My wife luckily doesn’t take very long showers like some females like to take. So the whole-house tankless won’t run very often.

    So assuming minimal installation cost (just a little wire and plumbing parts) I have two scenarios…
    1) I can just add a tankless under my kitchen sink and keep my tank heater (it doesn’t need to be replaced yet but will soon, 15 yrs old now). Replace the tank heater with another when it dies.
    2) Or I can add the tankless under the kitchen sink and replace my tank heater with a tankless to serve the 3 showers and 4 sinks.

    Thoughts?

    (EDIT: both of my options were the same thing, duh!)
    Last edited by USFishin; 10-12-2013 at 11:46 AM.

  2. #2
    DIY Senior Member dj2's Avatar
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    Go with the last option.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Having warm supply water certainly helps with a tankless, but keep in mind, your electrical panel may not be the only thing that needs upgrading...you may need an upgraded power drop from the street, too. Gas is still generally cheaper to run (not install, though). If you are planning on selling the house in the next 5-10 years, you'll want to size the WH plan as if the new family wanted to use all showers at the same time. A good sized gas tankless may be close to 200,000BTU range. The equivalent electrical power is nearly 58KwHr, and at 240vac input, over 200A (if I did my math correctly, it's been awhile, you should double-check). Electric tankless generally are not available that big, at least not residential units, so they will not achieve the same gpm flow. Your saving grace is the incoming water temp, but it may not be enough.

    A kitchen sink and dishwasher are both flow limited to maybe a little more than a typical showerhead, so smaller can work.

    Parts of FL can get cold in the winter, and your average temp then is almost certainly less than that 70-degrees, unless you're way far south.

    It can work, and work well, but you may need to learn to live with the limitations.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Test, Don't Guess! cacher_chick's Avatar
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    I don't know if there are any undersink units that will satisfy the average homeowner. I've tried a couple for hand washing sinks and won't do it again unless proven otherwise.

  5. #5
    DIY Junior Member USFishin's Avatar
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    We don't have any plans to sell the house any time soon so we would size the WH based on our usage. I don't foresee us using more than 2 showers at a time. I was looking at possibly getting a 27KW heater. It appears as though that should give us enough water to run 2 showers at the same time even in the winter here. All of the groundwater temp maps I've seen show us half way between the 72 degree and 77 degree lines. So I figured somewhere around 74 or so.

  6. #6
    DIY Junior Member USFishin's Avatar
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    I'm looking at a 13KW unit for under our kitchen sink. That should give us 2.5 GPM at a groundwater temp of 65 degrees.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Assume you have a 2.5gpm showerheads and have both of them in use, you're using about 40# of water per minute, round numbers. A 27KwHr unit is about 1500BTU/min. 1500/40 = 37.5 degree temperature rise assuming the heat exchange was perfect (doesn't happen, especially as the heat exchanger starts to get mineral deposits). So, if your water was say 70, using all hot, it would get to 108 at the shower...certainly not hot, but hotter than many people shower, but that's using all hot. I'd call that marginal and not accounting for any cooling off in the distribution system in your cooled house (a/c running much of the year). If you tried to use any other hot water in the house, say washing clothes (even just in warm), your shower performance would drop. It might work for you, and certainly would if you compromise and only run one at a time. I'm not sure what your actual ground water temps are. If that chart shows averages, it could be much higher in the summer, and lower in the winter. If it got much lower than the average, you would find the performance unsatisfactory for two showers. If that number represents the minimum, it's back to marginal.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member USFishin's Avatar
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    All the charts I've seen are average groundwater temps, not lows or highs. I guess in this scenario I'd be concerned about what the low temp is. I'm not in a big hurry to put it in or anything so I think I'll monitor our water temp this winter and see what temp it drops down to when it's "cold" for a few days down here. We don't ever wash clothes with warm or hot water, just cold, that's why I didn't include that in the number of fixtures using hot water. In the meantime I'm still going to install the tankless under my sink in the kitchen and see how that goes too.

    I just checked the water temp and it was 78. It's started cooling off a little bit. We've started to get lows in the upper 60's now. I'll have to see what it is when we have a few days with highs in the 60's.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    Out of curiosity, if that chart shows the whole US, see what it says for NH around Nashua (on the border with MA). I've personally measured incoming water temps here at 33-degrees after a cold spell. It certainly is MUCH warmer in the summer. I'd guess the average is probably around 50, but a tankless system here either needs to be REALLY big, or you limit your use to a trickle in the wintertime.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    DIY Junior Member USFishin's Avatar
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    Looks like it is somewhere around 50 or upper 40's. http://www.epa.gov/athens/learn2mode...enrys_map.html
    I think you'd have a much larger swing between high and low up there though.

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, that answers my question...that's the average. Yes, the difference between our wintertime and summertime temperatures is larger than FL, but it's still important when sizing a tankless system if you don't want seasonal restrictions.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    DIY Junior Member USFishin's Avatar
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    I thought of an idea... since I am going to be getting the 13kw heater for under the kitchen sink, what if after I get it I hook it up in place of my tank heater for a few days or a week? I can then use that as a gauge of 1/2 what the 27kw heater will output. I'll hook it up when it's really cold here and if at least 1 shower works good then 2 should work with the 27kw heater.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    Should work. The coldest ground temps are usually later in the winter, or at the end of a longer cold spell, so you may have to wait awhile for the worst case test. Then, it may not get that cold this winter in FL, so who knows! If you don't mind the hassle of moving things around and running wires to power it, why not?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member USFishin's Avatar
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    Yeah I'll probably have to wait until Feb. or so. The electrical panel is only 6 feet or so to the water heater so I can just rig it up temporarily for a test.

  15. #15
    DIY Member MikeQ's Avatar
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    Quote Originally Posted by USFishin View Post
    Yeah I'll probably have to wait until Feb. or so. The electrical panel is only 6 feet or so to the water heater so I can just rig it up temporarily for a test.
    I've found the performance of my Stiebel Eltron to exactly match the predicted performance here:

    http://www.stiebel-eltron-usa.com/te...empraplus.html

    That includes testing of temperature rise as well as minimum flow to activate. Spot on.

    Also, keep in mind that not all shower heads flow 2.5 gpm. I've had unsatisfactory shower heads that did consume that much water (unsatisfactory in that hair rinsing performance and shower comfort was not pleasing) but the 2.0 gpm shower head that my local utility company mailed me provides very pleasing performance and I've measured it's consumption at only 1.8 gpm (58 psi). In fact, I always prefer to turn the volume down and at typical settings it only uses 1.5 to 1.6 gpm for a very pleasing and effective shower.

    I also have two showers fitted with heads rated at 1.5 gpm: http://www.highsierrashowerheads.com/

    The High Sierra heads provide a very "wet" feeling shower, that's the best way I can describe it. And they do rinse soap from hair very quickly. But they make a bit more water noise than I am accustomed to and I was not happy when I measured their flow rate (again at 58 psi) and found they flow slightly more water than the one mailed to me from the local utility. The both measured about 1.8-1.9 gpm (which is more than the 10% out of the specification flow curves published on their website). I definitely prefer to turn down the volume when showering with these too. At that point their consumption is only 1.3 to 1.4 gpm.

    Avoid any low flow shower head that aerates the water. While this does reduce the flow rate it also cools the water considerably and requires hotter water to produce the same warming effect. I have never had a pleasing shower from a low flow head with aeration.

    The bottom line is the right shower head can increase showering satisfaction while dramatically reducing hot water demand (and thus the size of water heater needed).

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