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Thread: Tankless WH for MB supplement???

  1. #1
    DIY Junior Member fitzyva's Avatar
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    Default Tankless WH for MB supplement???

    I have an existing oversized tub in the MB. Existing electric WH is a lowboy to fit beneath house in crawl space (about 5 ft head room at WH). Not sure capacity but thinking around 35 or 40 gallons. Never issues with demand for hot water EXCEPT when trying to fill oversized tub. WH not able to provide enough hot water and tub is useless. WH is on the same end of the house as MB, so it's not an issue of just heat lost on a long run. Have been looking into options to address issue. Considering 1) electric tankless supplement, 2) electric mini-tank near tub, or 3) others??? Thinking that the additional capacity provided by the mini-tank won't be enough help, so was leaning towards tankless supplement specifically for tub. Reading lots of the posts about issues with tankless heaters, I'm questioning whether to go in that direction. I have plenty of room in panel for electrical requirements of tankless, but would have long wiring run for connection. Thoughts and options would be great! Thanks.

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    In the trades Dana's Avatar
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    Adding a mini-tank isn't going to fill a tub- it has to be something "real". But mini-tank under the bathroom sink would waste less water (and less hot water) for short draws. With the tank currently located at the far end of the house you cold easily be drawing a gallon or two of hot water for a 1 pint hand washing even, leaving the bulk of the hot water drawn in the plumbing.

    Right-sizing the total amount of tank storage volume to the tub size is the best option, and it probably means adding a local 30+ gallon tank, plumbing it in series with the output of the remote tank. Plan-B would be an electric tankless, but even a 20kw version could be painfully slow to fill a soaking-tub, and the expense of upgrading the power to the house & wiring to support it can be daunting. (A dedicated 100A, 240V for the tankless would be needed this isn't just adding a run of romex and big ganged breaker- you may even need to upgrade the power drop from the transformer to your house, depending on your other loads and the existing situation.)

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    DIY Senior Member Runs with bison's Avatar
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    Man, that's a tough problem. Being electric it looks far tougher to me since the heat flux of electric elements is lower than gas burners, making recovery time slow--I grew up with electric water heaters and loath them (replaced too many elements and scraped sediment from too many tanks...we had VERY hard water.) A 50 gallon natural gas fired at 120-125 F can recover quickly enough to fill a big tub after only a short delay. Adding a second tank of similar capacity as Dana suggested seems the most logical choice. You can give yourself more "effective" capacity by increasing the set point for the water heater, although with a 30 gallon tank this will likely be insufficient even at 150 F. Of course high setpoints run the additional potential for scalding unless a tempering valve is included. And higher tank temps generally mean shorter element life, and probably shorter tank lifetime.

    If you haven't already done so you can get some small improvements through insulating of the lines to/from the tank. If I built a house today, every single hot water line would be insulated right up to the fixture.

    Another thing I would do if installing a large bathtub today would be to add some spray-on insulation to the tub so that it would not lose heat so rapidly. It is irritating to get a cold ass in a tub during winter while the water is hot, all because there is zero insulating value to the plastic tub wall.

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    In the trades Dana's Avatar
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    I've insulated cast-iron tubs by dense-packing cellulose under/around them, improving soaking-time noticably. The thermal mass of the iron requires a slightly higher fill temp than with plastic tubs though.

    Insulating the hot water distribution lines (and the near-tank cold feeds) improves efficiency and limits the losses of successive short-draws but doesn't appreciably affect tub filling capacity. A run of 100' of half-inch or 50' of 3/4" is still only about a gallon.

    Bumping the storage temp to the max yields a low double-digit percentage improvement in capacity, but can double the standby loss. This should never be done without first installing a tempering valve or thermostatic mixing valve on the output, as the scald risk soars at temps above 125F.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Dana View Post
    Bumping the storage temp to the max yields a low double-digit percentage improvement in capacity, but can double the standby loss.
    Depends on the season, supply water temp, and the original setpoint. I would guess about 50 F would be a good starting point for supply temp. At 150 F I think that works out to around 40% more effective capacity than with 120 F setpoint. If one was using 125 or 130 the improvement would be less of course.

    For a nat. gas/propane unit the standby losses would be high because of the uninsulated heat transfer surfaces, but with a modern electric unit the standby losses are fairly low to begin with--roughly 8%. Adding an insulation blanket could probably offset this loss.

    What would be really nice is to be able to dial in a new water heater set point from the bathroom 30 mins or so before drawing a large bath, then having it resort to the original set point at the end of a fill.

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    In the trades Dana's Avatar
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    In many places these days code requires the installer set it up at 140F storage temp, and most families living with a 30 gallon tank have it cranked to at least 130F. But you're right, if it's currently set at 115-120F taking it to 150F would be more than a low double digit improvement (but still not enough to fill a 50 gallon soaking tub.)

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Dana View Post
    In many places these days code requires the installer set it up at 140F storage temp, and most families living with a 30 gallon tank have it cranked to at least 130F.
    Interesting, I hadn't heard about 140 being a code requirement anywhere. It is probably prudent with electric water heaters as their design is more susceptible to fostering legionella growth than gas fired types.

    Wouldn't be surprised to find the temp cranked on a 30 gallon--assuming it is that small. Might be one of the 38 gallons or so based on the original post. Even a 50 gallon gas fires has trouble at 120-125 F without giving it some time to recover...however at 130 F it was adequate the last time I tried it.

    The bath temp is likely between 100-105.

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    Moderator & Master Plumber hj's Avatar
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    Most manufacturers are sending their heaters out set at 120 degrees. This person's heater is ALREADY located near the bathroom, NOT a "long way away". Adding a second heater would limit it to a 50 gallon one because it is located in the crawl space, and would need an additional power line to it. Electric tankless heaters, of ANY size, are seldom satisfactory, at least for the few I have encountered in residences.
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