Is there a downside to wiring a water heater for simultaneous operation?

Discussion in 'Water Heater Forum, Tanks' started by STyler, Nov 2, 2009.

  1. STyler

    STyler Member

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    I'm planning on trying a 50 gallon water heater wired for simultaneous operation in lieu of a tankless unit for my barn. This will have a separate cutoff switch so it can stay off until needed, but I need to reduce the heatup time - and the recovery time during use if it will work in this application.

    Currently considering a Bradford White unit # M-2-HE50S6DS. It will be fed with 2 wire - 50 Amp breaker.

    Are there any issues to consider? Is there a better model for this application?

    Thanks.
     
  2. Cass

    Cass Plumber

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    What size wire are you going to use, the top thermostat is not made for that kind of amperage...

    I also think your nuts...
     
    Last edited: Nov 2, 2009
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  4. STyler

    STyler Member

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    #6.

    Always the option to run 2 separate 30 amp circuits, but not sure of the point.

    ... and please stop thinking about my nuts. :eek:
     
  5. Cass

    Cass Plumber

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    The upper thermostat wont care that 2 circuits are run...it won't handle the amperage
     
  6. STyler

    STyler Member

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    These units are designed for simultaneous operation.

    The amp rating of the breaker won't put any demand on the thermostat. How do you think you can plug a night light into a 20 amp circuit in your house?
     
  7. jimbo

    jimbo Plumber

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    I question whether the wiring from the top of the heater down to the thermostat area is rated for the simultaneous load. Commercial water heaters, and possibly residential models, can be ORDERED for simultaneous operation, but I believe it would be a violation of the UL listing to modify it in the field.
     
    Last edited: Nov 2, 2009
  8. Runs with bison

    Runs with bison Member

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    I think this is a bad idea, but not because of the electrical requirements. Electric water heaters are relatively efficient from a storage standpoint because of the way they can be insulated. So you are looking at saving a relatively small amount of energy by keeping it off most of the time. With a 0.93 EF the losses would amount to about $35 year if left at temperature all of the time. When you need hot water you will have to wait several hours for it to reach temp depending on how long it has been off and the time of year. (The first hour rating assumes the tank is hot to begin with.)

    But the major concern I have is that the water in the tank is going to be stagnant and lukewarm for days or weeks (rather than just cooling for a few hours as timer might do.) It will take a long time for the tank to cool and that will put it in the danger zone for however many days/weeks it takes to cool. If there is any sort of bacteria in the water supply it will have an ideal environment to grow. While I'm skeptical of the legionella concerns at a true 120 F water heater setting for gas, in electric service I would be inclined to run 5 to 10 F hotter, simply because electric recovery times are slow and the tank arrangement appears less ideal for minimizing stagnant low temp zones. What you are planning to do should be nearly perfect for growing legionella if there is any in the supply water.
     
  9. Cass

    Cass Plumber

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    I guess I misunderstood...I thought you were going to try and wire a standard water heater to operate that way...

    If you are buying one that operates that way then you will need the electrical requirements the Mfg. states is necessary for its operation...I might install 1 wire size above it...
     
  10. STyler

    STyler Member

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    This is info I searched all over for when trying to decide if a tank heater was feasible (vs tankless). I'd spend $5 - $10 a month for hot water at the ready - but not $30-$40 a month. I couldn't find any info on estimated annual electricity usage with zero water demand. Any idea where I can find more details? I'd love to see a guesstimate of energy usage of 50 Gal. vs. 65 vs 80 with zero demand - and at various temperatures.

    Very good point. With possibly 3-4 weeks of non-use at a time, I'd prefer not to take the chance. That will save me the money on a separate cutoff switch.

    Simultaneous operation will virtually double the recovery time and cut the initial heat up period in half. Keeping the unit on at all times will eliminate the need for simultaneous operation to shorten the wait time - but I still would need the quicker recovery when washing horses or I'll run out of water.

    The only chance I have of making a tank heater work, with traditional non-simultaneous operation, is if I increase the size to at least 80 gallons and run it at 145 F - 150 F. But I don't want to spend a fortune for heating hot water during those periods when there is no demand.
     
  11. Cass

    Cass Plumber

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    You could run 2 - 30 or 2 - 40 Gal. heaters set up so one feeds the other...also the element wattage rating is a factor and 5500W elements are avaliable...
     
  12. STyler

    STyler Member

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    Unfortunately, I only have room for 1 unit. Plus, I don't see how 2 - 40 gallon water heaters would yield more volume than an 80 gallon unit with simultaneous operation. Especially enough to justify the additional cost.
     
  13. Cass

    Cass Plumber

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    How much is the 80 gal. with simultaneous operation going to cost?
     
  14. STyler

    STyler Member

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    Same cost as the non-simultaneous.
     
  15. hj

    hj Moderator & Master Plumber Staff Member

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    wiring

    You CANNOT wire the lower element through the upper thermostat if you are going for simultaneous operation. You MUST have its own supply wires from the circuit breaker to the lower thermostat, and it MUST be replaced with one which has an ECO safety switch. You need two 30 amp circuits because the individual wiring in the heater CANNOT handle 50 amps, and since the circuit breaker is to protect the WIRES, they are what determines the breaker size. You can also replace the 4500 watt elements with 5500 watt elements, if you upgrade the internal wiring to #10.
     
  16. Runs with bison

    Runs with bison Member

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    I went poking back through the Federal Register trying to figure out the standby test criteria, but only found pieces.

    For electric heaters because the electrical resistance heating efficiency 98%. You need the Energy Guide figure and the Efficiency Factor for the tank you are examining. 0.98 - EF = fraction that is storage losses. Multiply this by the Energy Guide kwh estimate and you have the storage loss per year. You can do this for various tank sizes, you just need the EF for the model of tank you are considering.

    I am not certain what the assumptions are in the efficiency factor calcs. though. I'm guessing it is some indoor ambient temperature average (perhaps 70 F based on some things I'm seeing), perhaps applied against a 120 F set point. I'm not sure why I have used this basis in the past, might have seen it while perusing the Federal Register info pertaining to water heater testing...or not. Anyway, once you assume a delta T, you can calculate others by using the ratio of the Delta T's as an adjustment...such as unheated outbuilding midwinter or mid summer. You can do the same adjustment for various tank setpoint temps.
     
  17. jimbo

    jimbo Plumber

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    Looking at your original question.....you mention keeping it OFF until needed. Will this be for hours at a time? days? weeks? There are issues about keeping unheated water stagnant in a tank.. Search for the debate on legionella.

    Tankless MAY be a better option if your need is very infrequent, but what is your GALLON PER MINUTE requirement? A modest tankless @ 4 to 6 gpm typically would need 120 Amps @ 240 volts, so the install cost may be prohibitive compared to just some large tankers.
     
  18. STyler

    STyler Member

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    Runs with bison mentioned that earlier so I'm scrapping the idea of a cutoff switch.


    I up-sized my service in order to handle the 3 - 40 amp breakers for a tankless unit, but after spending some time here, the reliability issue has backed me off of a tankless unit.
     
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