Which is more efficient--one large WH or two smaller ones?

Discussion in 'Water Heater Forum, Tanks' started by queen50, Mar 30, 2014.

  1. queen50

    queen50 Member

    Washington state
    We need to replace our 40-gal electric WH. It shares a closet in the main floor bath with a washer & dryer. It was fine for two of us, but we added a basement bath last summer, and now have 7 people living here for the next two years or so.

    I can put a larger water heater in the space, but I don't want to pay to heat extra water after they leave.

    Would it be possible/advisable to put a second WH downstairs? The idea would be that the second WH would service the basement bath, but could be shut off when not needed. When off, the bath would be serviced by the main WH.

    Any suggestions?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    The standby losses of any water heater are dependent on several things: surface area, the differential temperature (water in the tank to the surrounding air), and the quality and quantity of insulation. Electric WH have typically lower standby losses than those with burners, since the burner and flue must be uninsulated. So, from a standby loss, a bigger single tank would be more efficient. Keep in mind that you only heat the water you use, so a bigger tank by itself won't have much more cost unless you draw more water from it and therefore have to heat more - IOW, keeping it hot isn't a big thing, and the standby losses of one big one would be less than two smaller ones.

    Shutting off and bypassing a WH can be done, but you'd want to probably drain the tank that was going to be unused for a long period otherwise, you'd end up with stagnant water that would likely grow some nasties in it. ANd, you'd probably want a lock-out on the circuit breaker to prevent someone from turning it on accidentally, since you'd burn out the elements in minutes if there were no water surrounding them.

    Having a second WH closer to the point of use will mean hot water faster, so that's a good thing, but the addition of a hot water recirculation system would make it faster still, even from one point.

    There are probably other pros and cons, and certainly opinions. See what others have to say. Basically, you can do it either way. You can also make any WH have a larger apparent volume if you raise the storage temperature and use a tempering valve on it's outlet to limit the outlet temperature to a safe value (generally considered to be 120-degrees max for a residence). Say, run the WH at 180 and after mixing, you'd have lots more water at 120 than you would if the tank started out at 120.
  3. if you have the room

    if you have the room, why not just install a 65 or
    80 gallon electric water heater and blanket the unit.....

    there is no reason to install another heater next to the 40
    they will just be trying to heat the area where they sit..including .
    the plumbing and electrical work that would have to be done.....

    go big with 7 people,,, install a blanket on the unit, install it in a
    drain pan... do the thermal expansion tank and live happly ever after.

  4. JerryR

    JerryR Member

    Last edited: Apr 3, 2014
  5. Caduceus

    Caduceus Master Plumber

    Pittsburgh, PA
    That link has some of the the most amazingly manipulative propaganda.
  6. Wallijonn

    Wallijonn Member

    I take it that you also installed the appropriate 20, 30 or 40 amp 240V breaker & wiring to the new WH?

    Consider the cost of electricity. Wouldn't two WHs cost more to heat?

    Most of the stickers on the larger WHs I've seen say that they will use more electricity (cost per year) than smaller units and have a lower efficiency rating. Wouldn't setting the thermostat to 140°F cause it to increase the cost of electricity exponentially? Let's say that you install a 80g WH. 2 years from now they move out. What then? Will you remove it and install a smaller WH? If you didn't live in WA. state I'd say to install a solar powered WH.

    Your best bet may be to install the mixing valve and raise the temperature. I will be installing fire brick under my WH to prevent heat losses drained out by the cold concrete at night. If you could apply fire brick to two sides (walls, and fire brick to the bottom, and install a thermal blanket, and also the mixing valve and expansion tank, your heat losses may be minimal and two years later when the basement occupants move out you can turn down the WH thermostat to reduce electricity costs.

    Last edited: Apr 24, 2014
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    A bigger tank lists for a higher annual energy use partly because it is expected to source more hot water! Especially on electric tanks without a flue up the middle and uninsulated bottoms (for the burner), raising them off the floor isn't a big deal. Also, keep in mind that the volume of a tank goes up faster than the surface area, and surface area is where the heat loss occurs.
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