Which Tankless To Install

Discussion in 'Tankless Water Heater Forum' started by dtherrien, Oct 7, 2013.

  1. Dana

    Dana In the trades

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    The fact that you've been heating with a couple of Rinnai space heaters means it can probably be heated reasonably with just two mini-splits. The output of the mini-split heads probably doesn't need to be as big as the output of the Rinnai heaters though. You could just install a couple with the same output of your space heaters, but if oversized they would be more expensive, and would cycle on/off rather than modulate, running at slightly lower efficiency, more noise, and greater swings in room temps.

    It's possible to do 90% of a mini-split installation as a DIY, then have a qualified tech do the final pump-down, refrigerant fill, and system testing, keeping the whole thing under warranty. The hardware itself isn't outrageously expensive. A MSZ-FE18NA with brackets for mounting the exterior unit on the wall (above the snow line, protected from roof avalanches by the rake or eaves of the roof) plus refrigerant lines adds up to and puts out about 22,000BTU/hr @ +5F. The same unit installed by a high-end installer who backs it up runs about $4400. The 1-ton version runs about $2K with the associated hardware, and puts out about 15,000 BTU/hr @ +5F. The 3/4 ton would be under $2K, and puts out better than 10,000BTU/hr @ +5F. A 2-head single compressor 2-ton would run about $3500 for the all-in hardware costs.

    It's worth reading up on how to install them before going the DIY route, but if you have it all set up reasonably it would probably cost less than $500 for the tech time to pump down charge & test two separate systems.
     
    Last edited by a moderator: Dec 15, 2013
  2. jocko91

    jocko91 New Member

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    I need some quick help. I am 91 and am giving up my hot tub because I have difficulty getting out of it. So I am replacing our MBR jacuzzi tub with a walk in tub 32" high. But I want to be able to fill the tub quickly (60 gal) and with 102 degree water.

    So I plan to get an electric tankless heater just for the tub. The questions are:
    1) which one?
    2) hook it to the hot water line or "regular" line?
    3) if hot water line, mount upstairs next to water heater (50 gal gas) or downstairs next to tub?
    4) I plan to use the existing hot tub 220v for the heater and tub.

    The tub arrives next Wed so I need to make a decision soon so I can be ready to install it when it arrives. Thanks
     
  3. lifespeed

    lifespeed Member

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    You need to use gas if you want tankless, or get a larger tank. Electric can't do what you want.
     
  4. jocko91

    jocko91 New Member

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    Could you please explain that to me?
     
  5. kcodyjr

    kcodyjr Member

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    Well, actually yes an electric tankless can do it, but it would be a big unit, and would need two or three 8ga or 6ga 240V circuits.

    If you went that route, you'd want it right there next to the tub if possible.

    Unless the tub is being used multiple times per day, I'd go with a larger tank, and/or run the tank hotter with a tempering valve.

    Edit: In order to determine exactly how big a unit, we'd need to know the flow rate of filling a tub (more than a shower) and the cold water temperature at the dead of winter, to determine how much energy needs to be dumped into the water and how fast. Off the cuff, I think you might need the big one, a 29 or 36 kW 3-element unit. That means a 200 or 300 amp electric service into the home. Check out Stiebel Eltron, they seem to be the go-to brand and have sizing charts.
     
    Last edited: Nov 27, 2013
  6. jocko91

    jocko91 New Member

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    Tub holds 60 gal but wouldn't need that much because of body displacement. So what would you figure I would displace at 6' 2" and 220 pounds? Maybe 15 or so? Then I wouldn't need but 45 gal? Would that do it? My water heater is less than a year old so I hate to replace it. But if I did, would it get temp up to 102?
     
  7. lifespeed

    lifespeed Member

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    The electric power required to heat water at a rate that would fill a tub quickly is fantastic, probably more than your circuit panel can provide. Despite silly comments to the contrary claiming it is theoretically possible, I assume you live in the practical world with the rest of us. So forget electric tankless for your application.

    The most practical thing to do is either gas tankless (this requires gas and vent plumbing) or increase the size and/or temperature of your tank heater and use a tempering valve to increase capacity via higher temperature.
     
  8. kcodyjr

    kcodyjr Member

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    What's silly about it? Presumably you also saw me recommend against it. Showing exactly why it's impractical is a nice, straightforward, honest, nonconfrontational way to talk someone out of doing something silly, and generally more effective than just trying to present oneself as the Voice of Unquestionable Wisdom.

    Hence, I directed him to the wattage charts.

    Hope you're having a pleasant evening.
     
  9. kcodyjr

    kcodyjr Member

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    The concentric vents do preheat the incoming air, and precool the exhaust, but not nearly to the degree that the condensing units do internally.
     
  10. jocko91

    jocko91 New Member

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    OK, I checked out Stieble. http://www.e-tankless.com/stiebel-eltron-tempra-29-tankless-water-heater.php

    My data: 1) winter ground water temp here in Northern Va is 55-57; 2) desired heater output temp 102; 3) fill rate 4+ gpm. (BTW, I considered getting a hot tub heater to replace the inline new tub heat exchanger but it would void the warranty.)

    Maximum Flow Rate in Gallons Per Minute at 105°F Output temp
    Incoming Water Temp: Flow Rate (GPM)
    40°F 3.0 GPM
    45°F 3.3 GPM
    50°F 3.6 GPM
    55°F 3.9 GPM
    60°F 4.4 GPM
    65°F 4.9 GPM
    70°F 5.6 GPM
    75°F 6.5 GPM
    * this chart is based on 240v input. Please review the Stiebel Eltron Tempra Brochure for temp rise data at 208v input.

    If I read this correctly, I get output of 3.9 gpm at 105 degrees so at 102 flow would exceed 4.0. And if we pulled the water from the heater at 100(?) degrees, it would probably get to 8 or 9 gpm. And the $800 is less than a new gas water heater would cost

    I feel bad for having asked for expert advice, got it and 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!
     
  11. kcodyjr

    kcodyjr Member

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    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: Nov 27, 2013
  12. lifespeed

    lifespeed Member

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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.
     
  13. kcodyjr

    kcodyjr Member

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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.
     
  14. jocko91

    jocko91 New Member

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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.
     
  15. kcodyjr

    kcodyjr Member

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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.
     
  16. jocko91

    jocko91 New Member

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    could the valve be installed at the tub rather than the whole system?
     
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
     
  18. richtow8

    richtow8 New Member

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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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