Electric water heater in series with boiler

Discussion in 'Water Heater Forum, Tanks' started by Christopher Bagge, Apr 7, 2014.

  1. Christopher Bagge

    Christopher Bagge New Member

    Messages:
    3
    Location:
    PA
    Hi,

    I currently have a boiler that sends hot water through base board heaters and provides hot water for showers etc. I am installing an electric water heater and need some advice.

    Do I take the boiler out of the equation as far as my dhw is concerned, so that I would feed the cold water directly into the water heater, or should I feed the hot water from the boiler into the cold side of the water heater. I am not doing anything as far as my baseboard heat is concerned, I am only referring to the water going through the coils to get heated for showers etc.

    I do not have a valve restricting flow back into city water, so I do not think I am required to install an expansion tank but see that there can still be benefits installing one, is this correct? And should I install an expansion tank in both scenarios (if I install the water heater with water being fed from the boiler, or directly from the city?)

    Thanks,
    Chris
  2. Dana

    Dana In the trades

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    2,810
    Location:
    01609
    If the boiler is gas-fired it's probably better overall to install an "indirect" fired tank using the boiler's heating output to heat the water in the tank (which has an internal heat exchanger), and stop using the embedded hot water coil. That allows you to lower the standby temp of the boiler (or even cold-fire it), saving a double-digit percentage of the total gas bill by reducing the standby losses.

    If the boiler is oil or propane fired the operating cost of an electric hot water heater is going to be a comparable or cheaper than with an indirect-fired tank.

    Embedded hot water heating coils lime up over time, and it's more trouble than it would ever be worth plumbing it in series with an electric tank.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    21,924
    Location:
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    What some people do is use the boiler's hot water output into the electric during the heating season, and just turn the boiler off during the summer. But, to get that water hot out of the boiler means that it must constantly be run at a higher temperature, which isn't all that economical. WIth the electric WH there, you could still lower the standby temp on the boiler...the outlet water temp wouldn't be as high, but then you'd have the electric to bring it up to temp for use, and feeding the thing warm water is better than cold.

    Personally, I'd probably go with an indirect. Connected as a priority zone, the tank would get the entire output of the boiler so a smaller tank is generally sufficient than with a standalone one, and the recovery rate is faster since nearly any boiler is bigger than the burners or elements of a stand-alone WH.
  4. Dana

    Dana In the trades

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    Location:
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    Only go with an indirect if it's gas fired. Heating oil & propane are too expensive to use/waste at 50-60% efficiency, with pricing more volatile than electricity.

    Of course if heating oil were to drop below 2 bucks I'd be all wet, but don't hold your breath on that one. :)
  5. Christopher Bagge

    Christopher Bagge New Member

    Messages:
    3
    Location:
    PA
    My boiler is burning oil. The idea was to use the boiler to preheat the water but lowering the temperature, then feed the preheated water into the electric heater letting the wh bring up the water to temp for use, as Jadnashua describes. I have still not decided if I would turn of the boiler in the summertime, as it appears there are mixed opinions on whether or not this is good for the boiler. The main problem is that the boiler does not supply enough hot water for all of us, and especially in the mornings it takes a while for it to have consistent hot water. The water will be hot for 2 minutes then cold for 4 and then hot again.

    Is it advisable to bypass the coils in the boiler and just use the electric water heater? Will the boiler be fine without water going through the coil; I still need it to heat the house.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,924
    Location:
    New England
    You can disconnect the inlet and outlet of the potable water to the boiler. Using oil, it's not likely to be the best solution preheating the water with it, and using an indirect with it may not be the best bang for the buck, either. It would let you get more hot water, depending on the size and setup. A tank WH is great until you run out, so you need to specify the size for your needs (like maybe filling a large tub, or long or serial showers).
  7. Christopher Bagge

    Christopher Bagge New Member

    Messages:
    3
    Location:
    PA
    Can I do a set up where I can use the water heater most of the time, but still be able to switch to the boiler. I figure this could be accomplished using valves and just turning off the cold and hot side of the boiler while leaving on the cold and hot of the water heater. Then when my water heater needs replacing I can switch over to the boiler for the time I need to bring in a new water heater. Or if oil drops to 2$ a gallon :)

    Would it be ok to leave stagnant water in the coil for longer periods of time? I figure by switching to the boiler every so often I could flush the coil out with clean water. Let's assume I once a month turn off the valves to the water heater and turn the valves on for the coil and just let the hot water run for 5 minutes.

    Or should I just cut the pipes to the boiler and leave them open, I assume whatever water is in the coil will evaporate over time.
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