Should I replace tank with Tankless

Discussion in 'Tankless Water Heater Forum' started by dsbeck, Sep 20, 2013.

  1. Dana

    Dana In the trades

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    Lifespeed: I definitely don't hate 'em (I even heat my house with one!) but I don't have rose colored glasses about their performance or their economics either. Properly maintained you can get 20+ years out of most of them, and you never run out of hot water. Limitations of max flow rates is their primary down side for tub-filling apps, but since you never run out of hot water you CAN fill a tub of arbitrarily large size, if you have the patience. For most of the past 25 years I've used gas fired tankless HW heaters for my hot water (both in the US and in Europe)- my current HW heating situation is an exception (unless you argued that because the heat source for my indirect happens to be a tankless HW heater it should somehow count, even though I've configured it as a boiler.)

    I don't know what hot water heaters come for free, and would not insert a zero-cost into a lifecycle cost comparison even if it were subsidized to the point where it were indeed free. Taking into account competing subsidies may be appropriate at the point of an particular buying decision though. I assume unrealistic long lifespans or zero maintenance costs tanks either. Apples-to-apples the economics aren't there at buck-a-therm gas, even if you add on a (reasonable) carbon cost to cover some of the externalities.

    Finned water tube heat exchangers have high tolerance for big delta-Ts and rapid slewing of temperature & heat rates- that's the least of my concerns regarding tankless water heaters. There is over a century of history on this type of heat exchanger- they can definitely take it.

    But the toll on efficiency of low volume draw short-cycling is real, as is the electrical power use (even in standby mode) that often doesn't reach the surface many forum type discussions of tankless HW technologies (not that it concerns me very much for this application either.)

    PG & E, the Davis Energy Group, Lawrence Berkeley National Laboratories, et al as well as the US D.O.E. have tons of actual monitoring data on comparative systems, both in lab bench testing, and in-situ measurements in occupied dwellings. This was done quite rigorously in CA in order to determine how much subsidy would be appropriately carried by the retail rate-payers, and how much the utilities could earn on subsidy-investment in customer efficiency. In the CA case the analysis even went as far as to determine how much the efficiency saved the utility & ratepayers in lower market price for fuel, and for the diminished or increased infrastructure capital that comes along with tankless water heaters. This was not a one-off simple process, and it took well over a decade to get there, but the alternatives are now well understood, way beyond marketing hype or armchair opining of either advocates or detractors.

    If you're going to count your personal perceptions as "...proven performance..." of "...great economy..." we'll need to see the test data & financial analysis on both your current & prior systems, and not just take your word for it, eh? ;-)

    For now I'll take the word of the engineers and economists who spent a chunk of their careers on it. If you want to PM me your email address I can send you the PDF of a presentation one of PG & E's engineers (Robert Davis, PE)
    delivered at the June 2008 ACEEE Water Heating Forum entitled, "Testing Water Heaters with Different Draw Profiles". It's not the slickest presenation ever, but it does a pretty good job of describing the real operational efficiency differences of at least a handful of realistic options (including a condensing tankless, as well as condensing tanks), under different scenarios. Yes, they're more efficient, than bottom-of-the-line tanks, but not by nearly as much as we all wished they were.
  2. dsbeck

    dsbeck New Member

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    4
    Location:
    NE Ohio
    Just a quick update:

    1) You guys have convinced me to keep the tank heater instead of replacing it with a tankless.
    2) Any suggestions on a good 75 gallon tank heater?


    Dave
  3. DonL

    DonL Jack of all trades

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    A GE made by Rheem is a good choice, At a nice price and good warranty.

    I think.


    Good Luck.
  4. MikeQ

    MikeQ New Member

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    87
    Location:
    Washington
    In my application (second home, half-time use) an electric tankless is saving me a considerable amount on utilities. I was able to relocate the heater from under the stairs to under the kitchen sink. Previously the hot water had to travel out of the top of the water heater down to joist level and across a considerable run to the far end of the kitchen (the kitchen sink is almost always on an outside wall) then up to the sink. Because this was a 3/4" hot water pipe and the climate is cool (summer) to cold (winter) the water in the pipe was always cold or at best, slightly warm (even after three minutes). No amount of pipe insulation could fix the problem. Because the kitchen faucet had an aerator on it, it would take more than a minute for the water to get warm and another minute for the water to reach full hot. When cooking, washing pans, etc. a considerable amount of water was wasted down the drain. Now, the water is hot in less than 1/4 the time. Cooking and the eventual clean-up is much more fun. And we never run out of hot water, even after multiple consecutive showers, laundry or no laundry.

    I would switch to tankless again even if it were just for the convenience and extra capacity but the savings are very real, not only on the electricity but also the water bill. This house is on a septic system but many municipalities base the sewer portion of the utility bill on water usage so there is another potential savings that may not have been analyzed in the study you mention. On demand water heaters are so small they can be located throughout the house at points corresponding to where the hot water is actually needed, greatly reducing the heat wasted heating long runs of piping. All houses larger than 1000 square feet should probably be built with multiple small on-demand water heaters. That would eliminate almost half of the plumbing in a typical house.

    One of the unexpected bonuses is that I can fill my 385 gallon hot tub in about an hour with hot water rather than waiting 16 hours for it to come up to temperature using it's own built in heater.
  5. Dana

    Dana In the trades

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    You're responding to my comment about a document outlining the differences in labeled vs. operational efficiencies under different use scenarios between different GAS-fired water heaters, with a testimonial about an ELECTRIC tankless? I'm glad you're happy with your decision, but left wondering what your real point is? Was it about the water-waste issue?


    An electric tankless is a very different sort of animal. Considerably more water is wasted with gas-fired tankless than with gas-fired tanks or electric tankless due to the ignition delay, where water flows unheated through the heat exchanger for a few seconds as the flue gets purged and the flame gets lit & tested. Recirculation systems reduce the water waste, but at the cost of an uptick in fuel use.
  6. lifespeed

    lifespeed Member

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    Location:
    California
    More water waste with a gas tankless? I sure don't see this. A few seconds of ignition delay cold water in my system get's mixed with hot water in the 1" hot water main and is completely unnoticeable. And if the hot water pipe is cold from lack of use I have the recirculation button in each bathroom and the kitchen to avoid water waste. The fuel would have been used regardless whether it was cold water down the drain or recirculated, tank or tankless.

    It's all about how you implement it. Not all installations are done equally well. I know my summer gas bill (mostly hot water) has been cut in half compared to a 10-year-old tank.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    How is comparing using less gas related to switching to an electric unit? It would be a miracle if your gas use didn't drop!
  8. lifespeed

    lifespeed Member

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    California
    I use a gas tankless and have greatly reduced my fuel consumption as a result. No doubt the old tank heater wasn't the best, but still quite an improvement.
  9. MikeQ

    MikeQ New Member

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    87
    Location:
    Washington
    It's not that I'm happy with my decision - It's more that I'm relieved the bad things I was warned about before I switched to an electric on-demand heater never materialized. I'm happy with the real world performance of the smaller 29 kW tankless and am glad I did not buy into the perspective that even a 36 kW tankless system would be marginal with my cold supply water.

    Even though my incoming water is only 41-43 degrees (year round) I can take multiple showers and do clothes and dishes without having to schedule them around water heater capacity. I ALWAYS have plenty of hot water even after multiple showers and loads of laundry. I cannot say the same about tank style heaters I've lived with over the years.
  10. Dana

    Dana In the trades

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    lifespeed: The utilities in CA were all over the water wasted in ignition delays this in their investigations of the past decade- it's been measured & documented in several places. Have you even tried to measure yours?

    While installation and potable distribution layout practices can make a large difference in water waste, the ignition delay occurs even in an ideal installation. A 1/2 second delay comprises water flow that wasn't heated, and usually dumped. At 25 draws/day typ usage that adds up to a few gallons even for the comparative quick-starters like Noritz. Some others run 2-3 seconds long, with a correspondingly lbigger daily/annual water waste.

    In most places that water loss doesn't add up to any big deal. In parts of CA (particularly soCA) the energy cost of getting that water to the house in the first place is measurable, and not entirely trivial on the larger scale.

    While I personally don't consider this amount of water waste a big deal, it's silly to deny the obvious- a gas fired tankless can never come up to full power as quickly as an electric tankless, and in an apples-to-apples installation it would be pretty easy to measure the difference in dumped tepid-water volumes.
  11. Dana

    Dana In the trades

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    But what does that have to do with the price of ganja in Jamaica, weedhopper?

    You quoted my characterization about the bench testing the efficiency different gas heaters at different use profiles as your entry way onto this thread, but I couldn't figure out what any aspect of your electric tankless had to do with it, and I'm still a bit lost as to how your response related to it.
  12. lifespeed

    lifespeed Member

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    Location:
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    Yes, ignition delay on my Noritz is about 2 seconds. Call it 3 seconds and a 2.5 GPM shower head. That would add up to 6 ounces of water extra, compared to how much water in the hot water pipe that was going to be dumped? Except I don't dump the water in my hot water pipe, I recirculate it if needed.

    Sounds like worst-case installation practices. This doesn't describe my setup so I don't see any reason to theorize about my results based on less-than-ideal practices. Would a super-efficient condensing tank heater be slightly more efficient? Enough to be of any significance? Perhaps if one factors in a lot of short-cycles on the tankless. But I have measured gas useage data showing a dramatic decrease in energy use compared to my old tank. I suppose it would be entertaining to plumb in a condensing tank for comparison, but that isn't going to happen ;)
  13. lifespeed

    lifespeed Member

    Messages:
    320
    Location:
    California
    Once the pipe is hot (and it has to be heated by water flow regardless of the type of heater) a short ignition delay has no effect. That 4 -6 ounces of cold water mixes in the hot water pipe and is never felt at the point of use. The mythical cold water sandwich. It never happens to me. Perhaps if one were to use a 1/2" soda straw for their hot supply it might be more noticeable, but not really sure how this could happen even then.

    The only "cold water sandwich" I have ever observed is when the hot pipe was drained by trickling a faucet below the threshold that would fire the heater. So some understanding by the user of the heater behavior is needed to avoid this. These events have been very rare, never a big deal.
  14. Dana

    Dana In the trades

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    You have asserted that multiple times, but you have shared neither the data nor the measurement methodology of your "measured" fuel use. ;-) I'll bet you didn't even separately meter the power used by the hot water equipment to factor that into the operating cost too, eh?

    It's hard to have a "dramatic decrease" on what should be a fairly small fuel use number in the first place, with many factors other than the raw combustion efficiency of the equipment having a large effect.

    The entire standby & combustion efficiency losses of a typical 80% steady state efficiency tank installed inside of conditioned space is well under 100 therms/year. Without the center-flue convection losses the standby & combustion efficiency losses of a condensing tank amount to less than half that, making it comparable to the flue-purge losses of typical condensing tankless units.

    It's rarely the case that a condensing tank is more efficient than a condensing tankless- depends on your use patterns, but at typical 2-4 person household patterns they are pretty similar, even if the tankless holds a modest edge in fuel + electricity costs (excluding maintenance costs.) Under the aforementioned PG & E testing performed in Robert Davis' lab in 2008, under all use profiles the condensing tankless edged out the Vertex on total fuel + electricity cost, but by less than 5% in all but the very lowest profile use (28 gallons/day), where it edged it out the Vertex by about 20-25%. But even the crummiest dirt-bag standing-pilot tank came in less than 100x higher fuel use at that profile. (If the .pdf were small enough I'd attach it, but if you want me to email it to you, PM me.)

    If you are that 28 gallon/day user, and your prior tank was an atmospheric drafted tank, and your total use might have dropped from 140 therms/year to 75 therms/year, which could arguably be considered "dramatic", but it's still only 65 therms/year. (Less than a Franklin at my gas-grid rates, but if you are on propane it'll be considerably more than that.) If using more hot water than that, the differences shrink pretty rapidly.
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