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Thread: Which Tankless To Install

  1. #31
    DIY Senior Member kcodyjr's Avatar
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    Quote Originally Posted by jocko91 View Post
    still want to press on with electric tankless, but that is really what I want to do unless it is just downright dumb. So please tell me like it is -- keep me straight. Please!
    Silly, yes. Downright dumb, no, not quite, depending.

    If you're willing to cope with the installation hassles (pulling lots of new 240V circuits, mostly) and have the fat power service, then it can be made to work. If you don't have the power service, factor in the cost of upgrading your panel and feeder to carry several hundreds of amperes.

    At the end of the day, the energy economics are about the same between a tankless and tanked electric. Both get energy into the water at about 99% efficiency, one has standby losses, the other has maintenance overhead, both have the same distribution losses. The only real advantage to tankless is how little space they take up by comparison, and endlessness which isn't really an issue here. Tankless also have start/stop quirks and "cold water sandwich" issues, but that's less of a concern if it's only serving a bathtub.

    The cheap solution is to put in a tempering valve and crank your heater up to 140. That's become recommended practice anyway, even though it shortens the life of the tank somewhat, due to concerns about legionella bacteria.

    The next cheapest is to put in a larger tank just like the one you have.

    Tankless electric is probably the most expensive way you could possibly tackle it. I doubt that hot tub circuits is anything like the amount you'll need, so that means exposing the wiring path from the bathroom to the basement. I can't even guess how much money is involved.

    You may well need two tankless heaters in parallel if you want the really high flow rate. Which model were you looking at?

    Edit: hypothetical installation.

    I'm on a slow connection so I can't look it up myself, so I'll assume their biggest model, the Tempra 36 Plus, and assume two units in parallel to achieve the 8GPM.

    Each unit calls for 3 double pole 60A breakers on 6AWG wiring, for a peak actual draw of 150A. Two units makes for six such circuits, which would be silly. You'd install a 400A subpanel in or near the bathroom and feed it once.

    Think about that. 400A is four times the amount of power available to my entire home, and that's just your bath heat subpanel. Your total service would need to be larger still to support that without dimming the lights.

    If you've got your heart set on tankless electric, I'm not sure why, although there is a certain out-there sci-fi cool factor to it. It sounds godawful expensive up front though, past the point of being impractical.

    I think the best performance you'll get is to install a larger gas tank, with a tempering valve, and if you're really stuck on tankless, you could install one in-line before your tank to reduce recovery time. Just pick the biggest one your existing service can support comfortably.

    What might be more worthwhile as an out-there upgrade is to replumb from the tank to the tub with larger pipe, up to the size of the pipe at your main shutoff, to reduce the fill time even further. You do have to sit there inside while it fills.

    Also, since you'll be sitting in there while it flows, I'd think of it more like a tub/shower in terms of temperature safety. Running a hotter tank and then tempering it down means you need thermostatic controls at all the points-of-use. A walk-in tub or shower should be an ASSE 1016 type P or T/P valve to guard against both hot and cold swings. Faucets and step-in tubs can get away with ASSE 1070 types.

    Also, if you do go with the tempered conversion, you might think about separately plumbing your dishwasher and clothes washer to take advantage of the higher temperature water.
    Last edited by kcodyjr; 11-27-2013 at 08:08 PM. Reason: Add some numbers, more rambling.

  2. #32
    DIY Senior Member lifespeed's Avatar
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    You have to factor in the cost of upgrading your electric service from the heater all the way to the power pole. The expense will be great. There is a reason tankless electric are almost never used.

    That is surely the most expensive way to go.
    Lifespeed

  3. #33
    DIY Senior Member kcodyjr's Avatar
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    What lightspeed said.

    Which is what I said, but he said it shorter.



    * Almost never used this far north. From maybe Florida on south, they're quite viable at smaller current draws.

  4. #34
    DIY Junior Member jocko91's Avatar
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    Hoooo boooy, another duhhh moment in my life!! I had no idea of the ramifications/cost of where I was headed! :-/

    Thank you guys so much for your patience and the time you spent to keep me straight. I don't know what a tempering valve is but I'm sure my plumber will.

    One more question: Because we will not use the tub every day and I really don't want my "regular" water that hot all that time, about how much lead time would I need to set it on 140 to get that temp?

    The next thing I've got to figure out is how to stay warm while the tub is draining because it takes 5-8 minutes to drain while we are sitting there wet.

  5. #35
    DIY Senior Member kcodyjr's Avatar
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    The tempering valve mixes hot water from the tank with cold supply water before it enters your hot plumbing system. You'd still see the same water temperatures you always did, it would only be hotter while stored in the tank. You wouldn't adjust it down, and really couldn't because the tempering valve requires a minimum temperature difference to do its job. This would be a full-time modification.

  6. #36
    DIY Junior Member jocko91's Avatar
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    could the valve be installed at the tub rather than the whole system?

  7. #37
    Retired Defense Industry Engineer jadnashua's Avatar
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    If your WH is set higher than 120 (and in some places, it doesn't matter) you are required to have a tempering valve on the WH. Where I live, to pass inspection, I need a tempering valve - doesn't matter what temp I run the WH. The standby losses do go up with higher storage temps in a WH.

    The standby losses on a quality WH aren't all that big of an issue. If that electric WH is inside of the heated envelope of the house, the heat lost from it is just that much less the furnace or boiler needs to put into the house. A gas WH, with it's flue, does lose more than an electric, but in most places, that is more than made up for by the relative costs of the energy (gas is typically much less to heat with than electric). It may make a difference in the summer during cooling season. Getting better insulation on the thing can help, and on an electric WH, is easy and cheap.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #38
    DIY Junior Member richtow8's Avatar
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    Rannai is a great unit. The RISE from ground water temperature is a huge factor and it CAN be overcome. I created electrical and plumbing schematics for Rannai and available upon request at www.ACEnergySaver.com just send an email and I'll attach the MSWord document to you. This utilizes circulator pumps and also heats luxury motor homes, radient heat and remote insulated inexpensive tanks for FAST AS POSSIBLE - near instant hot water at all areas of your building. Happy to help! Rich

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