Shutting off one of two water heaters?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DocD, Nov 20, 2006.

  1. DocD

    DocD New Member

    Nov 20, 2006
    We have 2 - 50 gallon electric hot water heaters. They are set up (not sure what it is called) where cold water comes into one....then a pipe connects to the second one....and then the hot water exits the second one.

    My question it possible to turn off one of these hot water heaters and leave the other one save money? The sticker (both almost brand-new) suggests the yearly cost for each is $415 which would be a decent savings if it is possible to at least temporarily shut one of them down.
  2. augie

    augie New Member

    Nov 20, 2006
    I will preface this by saying that I am not a plumber, just a handy guy interested in operating a home on as little energy as possible. If you only ever need 50 gallons of hot water at a time, I don't see why you would have to run both. I would make sure that the second one in line is the one you leave powered up. Otherwise you will get luke warm water forever. Does your home have a large Jacuzzi type tub that would empty a single hot water heater if you filled it up?
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  4. Gencon

    Gencon Renovator

    Nov 16, 2006
    Etobicoke, Canada
    You could turn off the power to the first one as long as its outlet goes into the inlet of the second one.
    But really, they are not connected properly anyway. How much is the second one actually working if the incoming water is already at 140*(or so)?
    Though you are paying to maintain the heat of 100 gallons of water.
    The correct way to install these would be to connect them in parrallel.The inlet pipes would be the exact same length with the same fitting set up and tee together at the midway point as it connects to the main.The outlet pipes would also need to be the exact same length and fitting configuration before connecting to the rest of the distribution system.
    With the correct valving, you could just run one at a time or use them both when required.
  5. Cass

    Cass Plumber

    Nov 12, 2005
    Turn off the one that the cold water enters into first.
  6. TedL

    TedL New Member

    Sep 17, 2006
    NY Capital District
    50 gallons is small capacity for electric, due to the slow recovery rate. But depending on family size and usage patterns, it may meet your needs. Fortunately, you should be able to run the test by merely powering off the first WH in the series.
    I believe you effectively get more first hour capacity by reason of the series connection, compared to parallel, so please don't rush to change the plumbing. If you wind up leaving the first powered down, it will still serve as a pre-heat tank, though not a very effective one if it has the insulation I would expect it to have.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    Those energy tags apply if the heater is used normally. Your installation is not using them normally. You only pay to heat the water you use, (except for minor standby losses), so if you do not use a lot of water the lonly heating that will occur is in the first heater. The second one will just store the water. So, your electric bill will change very little, if at all, because you turn off the first heater. BUT, if you usage is such that you are really operating both heaters, then the bill will go down, but you will have periods when you are out of hot water.
  8. geniescience

    geniescience Homeowner

    Nov 27, 2005
    humid summers hot, humid winters cold
    no big gain to shutting down power

    What hj said. :) .

    It's fine to have two in series. It's a good thing. :) .

    The previous posts by the others, are all right, to an extent, but they are not seeing things like the way hj pointed them out, and that is what was missing in the big picture, in my opinion. Not that they said anything wrong. That is my friendly view, which i offer to you for consideration.

    If you turn off power on the first one, your cost savings gain is minimal. Why?

    1.) Heat energy needed to warm up incoming cold water has to come from somewhere. Most of that energy comes from the heating coils of the HW heaters.

    2.) Some heat energy also comes from the surroundings (i.e. your basement, the subsoil, the entire house, etc.) since heat flows to colder places. The incoming cold water is colder than anything else. A cold tank next to a hot tank will "grab" most of its pre-warming heat from the hot tank. Why? Heat loss is higher the greater the temperature gradient between hot and cold places. A hot and a cold tank side by side will engage in a nice little heat transfer process, and the cold one always wins. Heat never flows the other way.

    3.) If you turn off power on the first tank, and if you place a big (high and wide) insulator that includes a radiant heat barrier, between the two tanks, your basement "area" will provide some more heat to the cold tank. This is a slow process, especially when your powered-off HW tank itself is insulated.

    4.) Heat loss is higher, faster and greater where the temperature gradient between the hot and cold places is greater. (Sounds like a mantra, repeated). So, some people set their first tank to a lukewarm termperature level, so it only "pre-warms" water in the first tank. When only one tank is hot, heat loss to the surroundings is lessened, to some extent. This is what hj mentioned already, in few words ("standby losses"). Some people lower the temperature on the second tank too, since heat loss is greatest where the temperature gradient is highest. No amount of insulation will stop heat loss; it only slows down the heat transfer process.

    What I might do to save money:
    Lower both tanks' temperature and make sure that the first tank is not hotter than the second. Keep lowering the first tank's temperature until you find you are not satisfied with the volume of hot water you can still get after hot water has already been used in greatest demand. Keep lowering the second tank's temperature until you find that you are not satisfied with the amount of hot water you get in general.

    Executive Summary:

    * Avoid really-hot hot water when you want to save money. Standby losses are greatest when your tank(s) are hottest.

    * Having two tanks in series is a good feature, giving you advantages you could not acquire through any other means. This set-up prevents diluting the hot water in the tank with (perhaps extremely cold in winter) water after using the tank only a bit.

    * To pre-warm cold water it is possible to use the ambient heat available from the ground and surroundings by installing a low-cost uninsulated tank, in series.

    Last edited: Nov 21, 2006
  9. DocD

    DocD New Member

    Nov 20, 2006
    Thanks for all the help!!
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