Electric water heater on a timer?

Discussion in 'Water Heater Forum, Tanks' started by pit monster, Jan 12, 2010.

  1. pit monster

    pit monster New Member

    Messages:
    8
    Location:
    Lancaster, PA
    Does anyone have an opinion on putting a timer onto an electric hot water heater (brand new A O Smith - 50 gallons)?

    I'm thinking it may be a help in our situation with a 2nd home which my wife is at Monday evening through Friday morning. She is normally not there during weekends and really only needs hot water Tuesday through Friday mornings for a shower before work.

    While it seems like a timer to shut it off for most of the week would be helpful there was one site that suggested that they typically only use 1.3 hours of electric in a 24 hour period.

    Thoughts?
  2. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,308
    Location:
    Yakima WA
    Modern water heater, gas or electric, are so well insulated that the savings over a weekend would be quite small. When the power was returned, it would require using a considerable amount to reheat the water in the tank. To me, the savings would be not worth the bother. Just an unscientific opinion.
  3. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    I would agree with Gary.
  4. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    Timers won't save much energy with a modern (meaning reasonably insulated) water heater because of the nature of the cycle and how much the tank will actually cool down. This is double/triple true of electric water heaters because their configuration allows much less in the way of storage losses.

    Where timers will save money is for customers of electric utilities that do peak/off peak metering and charges. There won't be much change in overall electricity use, but a potentially large change in your bill because water heating can be shifted to off peak hours.

    There is a potential concern with this though. Electric tanks are far more susceptible to legionella growth than gas--according to studies that have been. So if you only run the tank at 120 (just above the danger zone) and use a timer, the tank could spend half the day or more at a temperature ideal for the growth of nasties... Therefore, if one used a timer with an electric it might make sense to run the tank much hotter so that the cold cycle will still average out in a safe zone. Again, the storage losses with electric are less so the increased temp. won't be as detrimental as it would for a gas water heater.
  5. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    Disagree...a timer set for only those times when needed might save a substantial amount of $$$ over the long run depending on the cost per therm and the temp of the air surrounding the heater...

    They can be set so the off time is while people are at work and sleeping as well as times during the weekend when not needed...

    it is really hard to say unless a trial was done on an average family over time and as electric costs rise the savings would be even greater...
  6. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    One can work through the potential savings of storage losses to see what is reasonable:

    Typical electric water heaters have R16 insulation (2" of foam), without the flue and combustion chamber that add a lot of heat losses to a typical R8 insulated gas heater storage tank. EF's for electrics are about 0.91 at 50 gallons. This leaves storage losses of 1-0.91 = 0.09. (And I'm being generous here as the Energy Guide EF derates 0.02 for electrical resistance heating efficiency loss, actual storage losses would be near 0.07 max.) The Energy Guide rating is 4825 kwh/year for a 0.91 EF heater. So the maximum potential energy savings would be 4825*0.09 = 434 kwh/year or 14.8 therms/year. This sounds pretty good until you start analyzing it further.

    To achieve this the water heater would have to be at room temp 24/7. Nobody is going to end their shower with the water heater at room temp. Plus there would be something like 4 hours of heating time required if this happened and probably an hour or two of cushion built in. Let's call it 24 - 6 = 18 hours of day in a cooler operating mode--75% of the time, and that is probably optimistic. 75% of the time one would not have hot water. That might be okay for preset bathing times (at normal set point), but it won't be okay for laundry, hand washing, dish washing or much else.

    Let's also assume that the water heater is left at 100 F at the end of the showering cycle with a year around ambient of 75 (summer/winter averages in home) and normal hot water set point of 120 F. The delta T's are 120 - 75 = 45 and 100 - 75 = 25. So the "cold tank" case will have about 25/45 = 56% of the heat loss rate of the hot case.

    Now sum them up and take the difference fo the cases:
    Base case = 434 kwh/year
    Timer case = 434 * 0.25 + 434 * 0.75 * 0.56 = 291 kwh/year.

    So the actual savings might be = 434 - 291 = 143 kwh/year, less than 5 therms/year. The water heater timers I've seen are not cheap...roughly 2-3x what an insulating blanket costs with less than half the energy savings potential of an insulating blanket on an R8 insulated gas heater. Annual dollar savings per year should be better than that because electricity is usually considerably more expensive per therm than gas. Adding the Energy Guide resistance derate would yield 113 kwh/year savings instead.

    But I don't consider letting the tank sit at 100 F for 16 hours a day to be feasible from a health standpoint. So to compensate one would end up running the tank at a higher set point, effectively negating any energy savings from the timer.

    Adjust the numbers and assumptions to your liking and run through the calcs. When I examined water heater timers about a year ago I noted that the proponents of them said they only really made economic sense for electric utilities that did peak/off peak metering. That is no surprise as one could save a bundle by shifting nearly all of the 4800 kwh/year to a lower price rate.
  7. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    I lived in France for a year. The electricity there has a normal rate during the day (7am-11pm) and a low rate at night. Our WH had a breaker that was triggered by the electric company when it switched from normal rate to the low rate. Heated during the night and then used the stored heat during the day. It worked pretty well. We also used the delay timer on the washer, so that it would start after 11pm to use the off-peak rates.
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,292
    Location:
    Cave Creek, Arizona
    timer

    There are only two cases where a timer saves money, and they both have the same downside. They are;
    1. When you have a lower night time utility rate. OR,
    2. When you have a lot of daytime usage, but the timer is set for morning and nighttime operation.

    In both cases, the heater could run out of hot water before the timer turns it back on, and during that period you would have to exist with cold water only. Then when the timer turns on it only has to heat a complete tank of hot water, rather than the amount you would have used during the day, which WOULD save electricity but at the expense of being very inconvenient. ALL other times, the heater just operates to heat the amount of water you use, and whether it does it periodically as you use the water, or all at once when the timer activates does not change the amount of electricity you used.
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