Multiple Hot Water Heaters

Discussion in 'Water Heater Forum, Tanks' started by Offbalance, Jan 29, 2012.

  1. Offbalance

    Offbalance Advanced DIYer

    Messages:
    5
    Location:
    Snohomish, WA, USA
    Hey everyone -
    I have a question on multiple HWHs installed in my residence. When I bought the house it had two 40 gallon electric HWHs plumbed in series. It's a large house, and we have two teenage kids and relatives that visit for long periods. We have always enjoyed the voluminous amounts of hot water.

    The HWHs were Rheem, with S/Ns of 1191****** (the house was built in 1992, so that makes sense). In 2000, I did some remodeling and at that time attempted to check/replace the anodes, dip tubes, etc., but was unable to break them open. This summer, the second HWH in the series failed with rusty water leaking out the top electrode. I simply removed it from the circuit, and have been relying on the first heater. This setup seems to be working fine (provided, of course, that I get to the shower every morning before my daughter).

    During the recent winter storm we had an extended power outage, and I decided I want to put a second HWH back in the circuit. I plan to wire the second HWH to my garage electrical subpanel where my generator transfer switch is located so I can have at least some hot water during the next extended power outage. I am considering the GE 9-year electric 50 gallon from HD as it seems the best value for what you get. I would then replace the first HWH in the series within a year or so, as the budget allows, with a similar one under the assumption that it will probably fail soon as well.

    Before I jump into this, I have a few questions:

    1. Is it better to plumb two tanks in series rather than get one larger tank? It seems an 80 gallon tank costs more than two 40s or 50s.
    2. Is plumbing them in series (vs parallel) the right way to go?
    3. Is it OK to have two HWHs of different sizes (40+50) plumbed in series?
    4. Am I correct in wiring the second one to the generator transfer switch so that if I lose power and am running off backup power I don't have to run through 40 gallons of cold water before I get hot water?
    5. We have propane F/A furnaces, one of which is next to the HWHs. Should I consider propane for the second HWH? This would be nice in a power loss situation, but I have not had very good luck with propane HWHs in the past. And I have had to replace the propane valves on the furnaces, fireplaces, etc. on a fairly frequent basis around here. And I haven't heard too good of stories about the new gas HWH FVIR thingamajigs.
    6. Are there other considerations I should consider?

    Thanks all for your input.
  2. Terry

    Terry Administrator Staff Member

    Messages:
    14,943
    Location:
    Bothell, Washington
    The size of the two tanks wouldn't matter if run in series.
    The elements are the same wattage. The electrical panel won't know the difference.

    Having two tanks means you have twice the recovery. A single 80 gallon has one element working at a time.
    With two tanks, you can double that.

    Of course, you also have more exterior space with two tanks that need insulating. Most of the time we would just install a single eighty.
    Twenty years from a water heater? That's pretty darn good.
    Last edited: Jan 29, 2012
  3. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    The tanks should be identical in everyway for a parallel configuration.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,537
    Location:
    Cave Creek, Arizona
    1. With two heaters in series you get TWICE the input wattage, compared to a single heater.
    2. It is the way I ALWAYS install them
    3. In series each heater is independent. They can be any sizes and even a mix of gas and electric.
    4. it would be the smart thing to do with two electric heaters
    5. i have had a propane heater for over 12 years and have had to do ZERO maintenance to it.
  5. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    If you buy the cheap HD variety, change the anode or at least check its length. Most are 1/2 the length possible. Remove the plastic drain and put in a ball valve, I got one cross threaded that would have flooded a house.

    http://fierychill.com/catalogsearch/result/?q=anode

    I would use the .90 magnesium. You could cut off the original one and drop it into the heater.

    And you need a impact wrench to get out the old rods, but for stuck elements, a good chisel and hammer can break the rust if you have good geometry.

    Better have a BIG genset to run a water heater.
    Last edited: Jan 30, 2012
  6. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Speaking of generators running water heaters,After a hurricane we had I changed my 4500 watt element out for a 1500 w to heat my water once a day. I just direct wired it to the element and let it run until the bottom of the tank got hot. Works great.
  7. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Would it be best (greatest effective recovery rate/volume) to set the second tank a few degrees higher in set point than the first tank? If you set them the same then for the first several minutes only the elements on the first tank will run. But if the second is set incrementally higher, its thermostat should kick in much sooner. In effect you reduce the lag for the second tank starting its heating cycle.
  8. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    To get the maximum stored water out of your water heater,pipe the incoming line to the drain valve location. Install an alternate drain. Beware of thermal stacking if you pipe the cold to the bottom as this chance increases.

    This also allows you to bypass the 5/8 dip tube common to residential tank water heaters,this allows a higher flow rate. Make use of this extra top inlet for an extra anode rod.
  9. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Actually, you could have just wired the element to 120 volts and had a 2250 watt heater. I do it regularly for low use, shop type units.... and the elements last forever.
  10. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    That would have tripped my rig. I used a 1500 w
  11. Offbalance

    Offbalance Advanced DIYer

    Messages:
    5
    Location:
    Snohomish, WA, USA
    My genset will handle the 4500 Watt elements, but probably not if both fridges, the septic pump and the furnace all decide to start at the same time, so I may put the HWH on a separate transfer switch.

    And yes, Mr. Ballvalve, I had already planned to replace the cheesy plastic drain with a real ball valve as I have always done before. I'll check the anode rod tomorrow, and order one if necessary.

    Thanks everyone for your input!
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,537
    Location:
    Cave Creek, Arizona
    quote; I'll check the anode rod tomorrow, and order one if necessary.

    The only time I would check my anode rod was after the tank was taken out., My replacement for a "spent" anode rod is a 3/4" brass plug.
  13. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Is that because you have the best anode rod and or water heater in the world or is it that you have good water?
  14. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA

    Not quite. A 4500 W element that is designed for 240v, will see 1125 W when connected to 120v. For a fixed resistance, when you cut the volts in half, you also cut current by half. This results in 1/4 the power.

    If you wired the top/bottom elements in parallel and used 120v, then you would get 2250 w from the two elements.
  15. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Last edited: Jan 31, 2012
  16. nukeman

    nukeman Nuclear Engineer

    Messages:
    709
    Location:
    VA
    It all comes down to Ohm's Law. V= I x R

    Power = V x I ---> I^2 x R or V^2/R

    So, half the voltage give 1/4 the watts. 1/3 the voltage would give 1/9 the watts, etc. If you put your elements in parallel, the resistance (R) is cut in half and would double the total power. So, at 120v, you cut the power to 1/4 and would double this if put in parallel to give 1/2 the rated power (total..each element still would be 1125W).

    As you said, on 120v, your elements should last forever (probably until the tank falls apart). It is much like using 130v rated lightbulbs lasting much longer than the normal bulbs, since they are being ran at below design voltage.
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