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Thread: POU for bathtub

  1. #1
    DIY Junior Member gary_'s Avatar
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    Default POU for bathtub

    Hi all,

    I have read numerous discussions here regarding the merits and problems with tankless systems. I am thinking about installing an electric point of use unit in the master bathroom I am building.

    I live with my wife and we enjoy plenty of hot water from the 40 gallon tank we currently use. We are not big hot water users and would not need to enhance our current heater were it not for the 85 gallon bathtub I am putting in. (I believe this is total capacity, not how much water will be needed to fill it.)

    I predict that this tub will get used at most twice a week during the coldest months of the year and sometimes it will go weeks without being used. Because of this I am hesitant to get rid of our relatively new 40 gallon tank and put in a larger model just for the few dozen times a year the tub will be used.

    Understanding that tankless units have their own inherent issues, might this be a reasonable case for installing one? I'm in the Bay area and I believe the water is a relatively consistent 65-70 F temperature. Could a tankless unit be used to enhance or "warm up" warm water from my tank? My thinking is that 40 gallons of hot water could be mixed with cold on the way into an electric heater to stretch the volume further. Or perhaps there are units that can handle 3-4 gallons of cold water per minute to produce hot bath water?

    I have over 60 amps of 220 and enough space. I'm hoping to use electric instead of gas for ease of installation and I don't think the energy savings of a gas versus electric in my situation would be a significant factor.

    Thanks for any advice on this or other ideas to solve this problem.

    gary

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you found a tankless that required a 60A circuit, it would not draw that, and figure around 75% of that which equals around 11K BTU. If you divide that by 60 to get a minute's worth of energy use, that's about 180 BTU. One BTU will heat one pound one degree. 4G of water/min is about 32#, so divide the available 180BT/32 and you'll get all of about 5.6-degrees rise. Now, if I did my math right, you can see why electric tankless systems need LOTS of power. A commonly sized gas tankless would have almost 20x more energy to put into warming the water. Now, I'm not sure of the sizes of electric tankless systems out there, but one that would work on a 60A circuit might work to make warm water for a sink, not to fill a tub at any decent rate. Now, if you wanted to wait, lower flow means greater temperature rise, but then you have to consider how much what's already in the tub would cool off while you were filling it so slowly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member gary_'s Avatar
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    A compelling argument for gas over electric. I'll look into them. Installing the gas fittings, hood and exhaust now seems less work than upgrading the entire electrical service to my house.

    thanks for the reply. Do I gather from your calculated 5.6 degree figure that tankless units are used to enhance the heating capabilities of an existing system or are they are generally used independently for hot water.

    thanks again,

    gary

  4. #4
    In the trades Dana's Avatar
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    Checking Jim's math:

    60A x 220V = 13,300 watts, which is 3.412 x 13,300kw= ~45,380 BTU/hr. Assuming his 75% of max draw on the breaker that gets cut to ~ 34,000BTU/hr. A flow of 4gpm is about (4g x 8.34lb/g x 60 minutes= ) ~2000 lbs/hr, so the temp rise if you're running the breaker at capacity is 34,000/2000= 17F temperature rise, or about 3x the 5.6F number he arrived at. Better, but still woefully short of what's needed.

    For tub fills you want about 110F water at the tap, and your wintertime incoming water temps in the Bay area are probably around 50F, so you need a (110-50=) 60F temp rise @ 4gpm at a minimum. That's 2000 lbs/hr x 60F = 120,000BTU/hr, which can be delivered by an 80% efficiency 150,000 BTU/hr-in gas fired tankless. A 199,000BTU condensing tankless is probably called for if you plan on filling the tub in any reasonable amount of time. Even 5-6 gpm is a pretty tedious fill for a large tub, but 4gpm is pathetic.

    A 199K burner is usually going to require 1-1/4" gas plumbing and no other loads branching off midway between the regulator and the tankless, and depending what other burners you have on the system you may need to up-size the regulator too.

    A 75 gallon or larger tank is the right solution, since it's flow rate will only be limited by the plumbing diameters & lengths along with the available water pressure.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Ya, I didn't convert Watts to BTU...my bad. Happens when it's late at night... Regardless, electric POU devices require significant power inputs to do much other than maybe wash hands.
    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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    In the trades Dana's Avatar
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    Been there, done that. (I just corrected an order-of-magnitude error I committed on another thread.)

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, one watt-hour = 3.412142BTU, so the numbers jive if you use that multiplier (with some rounding).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    An electric tankless, just like all tankless systems, requires a clean heat exchanger. Think of the buildup in a teakettle. Since you are applying quite high temperatures to a small stream of water, it WILL get mineral deposits which act like an insulator on the walls. As a result, depending on just how hard your water is, how much you use it, and how high you set the temperature, it may require cleaning more often than the nominal annual service. If it is setup for this, and you have the proper hardware (requires a pump and special isolation valves on the thing so you can pump the cleaner through the heat exchanger while isolating it from the rest of the pipes), it's pretty simple. If you have enough amps to drive the thing, and it is sized for your flow, they can work. It is an energy hog while it is running, though, and the house needs adequate supply.
    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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