Bit of Hot Water Tank Science

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by molo, Oct 11, 2007.

  1. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    How would you figure out if it is cost effective to turn off your water tank for a certain number of days. eg. We have an 82 gallon electric hot water tank. How many days away from home could justify turning it off, and turning it back on to reheat upon returning to the home; 4, 10, 20 days? How would you determine this?

    TIA,
    Molo
  2. markts30

    markts30 Commercial Plumber

    Messages:
    630
    Location:
    Phoenix, AZ
    It is hard to say...
    Where are you located?
    What time of year is it going to be? (outside temps etc...)
    Do you have other forms of heating that will be left on?
    Is any piping in un-heated/poorly insulated areas?
    If you are going for a while and not planning on heating, are you going to have the house winterized?

    Unless you really need to, I would just turn the temp down to the lowest setting (vacation mode etc) and leave it as is... You should not use too much power doing that....
  3. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    The space will be kept 45-50 degrees. The pipes are uninsulated. Cold New York.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    You need to do an experiment. YOu'll need an accurate thermometer. Right after the burner finishes heating things up, take a small sample of water to determine the starting temp. Turn the burner off so it can't run. Either overnight, or preferably more often, take another small sample to develop a curve of how fast the temp drops (the quality of the insulation). Knowing how many hours it takes to cool off the tank's worth of water, you can figure out how many BTU's it would take to just keep it hot. If you are really lucky, the specs are available on the manufacturer's website, but they often have the tank at a nominal 70-degrees. The bigger the delta between the room temp and the water temp, the faster the thing loses heat.

    You also might get an idea by monitoring how often the burner turns on when there is no useage. This would require knowing and believing the accuracy of the thermostat and its repeatability. Say it allows a 10-degree drop before it turns back on, and it takes 4-hours, with 40-gallons, you can figure out how many BTU's it is losing. 40 gallons * ~8 pounds, * 10-degrees = BTU. Extropolate that for the time it would take to get to the room temp.
  5. depends on how long and how old

    It really depends on how long you are going to be gone and how old the wate heater is...

    All you have to do is shut it down at the breaker
    and it only takes 30 minutes to heat back up anyway...
    when you arrive home.....

    if the heater is old , sometimes just turning it off could
    cause the elements or thermostats not to come back on
    or burn out when it is switched back on.....

    I doubt that the cost of the electricity would be more than one meal out
    while you are on vacation


  6. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    This raises an interesing question. The tank is in an area that stays around 70 during the summer, but in winter gets into the 30-40 range. I've wondered about how to determine how much heat the heater is losing during these cold months. Does anybody have an idea how I can determine this?

    TIA,
    Molo
  7. theelviscerator

    theelviscerator New Member

    Messages:
    72
    Location:
    Elkhart, IN
    Just turn it to low heat most have a vacation setting...
  8. GrumpyPlumber

    GrumpyPlumber Licensed Grump

    Messages:
    1,404
    Location:
    Licensed Grump
    Molo, in an area where the temps might get to freezing...it just might not be a good idea to leave it off.
    "Vacation" is a good idea.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    Same thing...see how long it takes in between running when there is no use. You can relate the time to the temperature change the thermostat allows to the size of the tank.
  10. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    Along the lines of MPM's "tankless experiment", Is there some way I could instal a usage meter on this electric water heater to isolate the usage of the heater?

    Molo
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,689
    Location:
    Cave Creek, Arizona
    heater

    It is immaterial. If the heater is going to be idle for any period of time, you can turn it off. If still has some hot water in it when you return, it will just use the same electicity as if it was maintaining the temperature, and if it is cold, then it only uses the energy to fill the tank, which will be less than if it were maintaining the heat.
  12. molo

    molo Member

    Messages:
    845
    Location:
    cold new york
    I'm not going to turn it off, It is allready at it's lowest setting. I'll just leave it there. I am interested in the isolated meter for monitoring month to month usage and determining how the temperature of the room effects the operation
  13. Just put one of these in.....set on low and it will heat the entire house in the winter.

    I just had to take a picture at the supply house this morning.

    No two-wheeler for this one. Maybe a fork lift.

    [​IMG]



    That heater is going into a Senior Citizen Care center.


    Gotta keep those bedpan washers piping hot!!!
  14. I read somewhere once that an electric water heater can represent up to 14% of your electric bill.
    I turn the water heater breaker and my water OFF even if I just leave for a weekend. I had an inrteresting flood experience over a weekend once with a burst washing machine hose.
    Mike
  15. TMB9862

    TMB9862 New Member

    Messages:
    206
    It doesn't hurt to shut the main when you go away. The only thing it could possibly damage is your boiler if it springs a leak and you don't have a low water cutoff.
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