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Thread: Water heater sizing help

  1. #16
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by jadnashua View Post
    But, it still comes back to my main point...you can't size the WH until you know what you expect out of it. Someone that takes 'military' showers - maybe a minute or so in length could easily get by with a much smaller tank than someone who likes to turn into a prune before getting out.
    We take 7-10 min showers typically and have very cold water in mid-winter--below 40 F. We can run all three showers simultaneously and run washing machine or dishwasher without problems. Of course we have a front loading washer, an energy star dishwasher, and 1.5-1.6 gpm showerheads. We've not ever run out of hot water except for the whirlpool tub, even when 3 more adults were staying with our family of four. This was true even at 125 F set point in mid-winter for 50 gallon tank with company staying.

    However, I note the dual showerheads listed by the original poster for the master bath. And I assume all of their showerheads will be in the 2.2-2.5 gpm range. Then add in the slow recovery rate of electric vs. what we have for gas. So it seems best to go for some overkill in the infrastructure, even if a 2nd tank is never installed.

  2. #17
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    Quote Originally Posted by Runs with bison View Post
    Being as this will be an electric water heater set up (per the response) it might be best to plan for two water heaters, even if one large one is used instead. This way the circuits are there and the floorspace for it is available if a two tank system is needed.

    Electric is a Catch 22 because of the competing problems of legionella and retrograde solubility of minerals in the water. Electric is more susceptible to fostering an environment that legionella can still grow in, so it really needs to run above 120 F. But at some point if the water is hard enough this will result in a lot of sediment because calcium and magnesium carbonate solubilies decrease as temperature rises.




    Series installation has been suggested. To maximize the system's recovery rate the 2nd tank could be set a few degrees hotter than the first (minimizing the lag time before the 2nd heater's elements pick up the load. I'm guessing on this but something like 130 F for the first tank an 135 F for the 2nd tank might make sense.

    One problem with series or parallel electric...how will the user know when one of the tanks has reached failure/near failure and therefore unsafe temperatures (below 120 F)?
    If the last tank in the series failed they would notice immediately. If the first tank failed and they dont use very much water,they might not would realize it.

    In a parallel install with gas they would notice right away because the tanks draw equally and when one tank goes out the water temp drops alot because its mixing cold water from one tank.

    If the parallel install was electric and one of the top elements when out they would notice a drop in initial temp right away....if one of the bottom elements quit working they wouldn't know unless they made a substantial draw.

  3. #18
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Hackney plumbing View Post
    Could you imagine having 3 teenage girls and a wife that loved to take baths in the jaccuzi tub every morning at the same time.......then its your turn to get in the shower???

    You could figure each girl would take ATLEAST a 10 minute shower. At a thermosetting of 120 incoming cold at 40.........a 70 gal wouldn't be big enough for ONE girls shower.
    In the home we rented before we bought this house the WH was electric, believe it was a 50 gallon. My wife had a habit of taking 15+ minute showers with the 7+ gpm waterhog showerhead (which gave a poor spray...I've had better showers from 1.5 gpm showerheads.) There was no warm water left by the time I got to the shower.

    The kids were still little enough that they would play in the shower for 20+ minutes if we didn't drag them out. Fortunately, their shower had a regular 2.5 gpm showerhead so this wasn't a problem until the 2nd one tried to do the same thing.

  4. #19
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    Quote Originally Posted by Runs with bison View Post
    In the home we rented before we bought this house the WH was electric, believe it was a 50 gallon. My wife had a habit of taking 15+ minute showers with the 7+ gpm waterhog showerhead (which gave a poor spray...I've had better showers from 1.5 gpm showerheads.) There was no warm water left by the time I got to the shower.

    The kids were still little enough that they would play in the shower for 20+ minutes if we didn't drag them out. Fortunately, their shower had a regular 2.5 gpm showerhead so this wasn't a problem until the 2nd one tried to do the same thing.
    I know what you mean. My wife can drain a 50 gal by herself with just a shower and we have average 65 degree cold water and I have my thermostat set at 135 degrees.

    I made a mistake on the post you quoted me on......I said a 70 gal tank wouldn't be big enough for one of the girls showers but I meant to say a 70 gal wouldn't be big enought under those conditions for all three girls to shower.

  5. #20
    In the trades Dana's Avatar
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    High-flow showers and heating hot water with electricity SCREAMS for drainwater heat recovery on the showers, independently of how many HW haeters or total volume. Even at 7 cents kwh there's a good economic argument in high-volume shower uses. (IIRC In Seattle PSE's residential retail rates are running more like 12 cents/kwh, delivered.) At typical one-showerhead flows it would roughly double the effective showering time, even for an electric tank (and turns a typical small-burner 40 gallon gas fired tank into about an hour-long shower at Seattle's incoming water temps.)



    Sizing the tank volume for the anticipated tub fills is most critical, but you'll need to go higher than that if you anticipate other large draws simultaneously tub fills. Drainwater heat recovery may do wonders for shower performance, but does SQUAT for tub fills.

    From an efficiency point of view the distance from the tank to the load counts. In an average CA house something like 17% of the total water heating energy is thrown away as "abandoned" hot water in the distribution plumbing. Locating the tank nearest the most-often used taps (which aren't necessarily the highest volume use taps) makes a difference. Insulating all distribution and near-tank plumbing (even the cold side) with 5/8" wall closed cell foam pipe insulation does too.


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