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Thread: Should I replace tank with Tankless

  1. #31
    In the trades Dana's Avatar
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    Quote Originally Posted by MikeQ View Post
    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.
    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.

  2. #32
    DIY Senior Member lifespeed's Avatar
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    Quote Originally Posted by Dana View Post
    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.
    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
    Lifespeed

  3. #33
    DIY Senior Member lifespeed's Avatar
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    Quote Originally Posted by Dana View Post
    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.
    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.
    Lifespeed

  4. #34
    In the trades Dana's Avatar
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    Quote Originally Posted by lifespeed View Post
    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
    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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