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Thread: Install tankless water heater using coil in boiler

  1. #1

    Default Install tankless water heater using coil in boiler

    I plan on installing an electrical tankless water heater in my house. I currently have hot water supplied from a coil in my furnace boiler. Can I connect the input water supply to the tankless unit from the output water supply from the coil in the boiler? During the summer the boiler would be turned off and the water would just pass through the boiler coil but during the winter months when I'm heating my house, I would like to take advantage of the hot water coil, and delivery hotter water to the tankless heater. Will I need to limit the input temperature to the tankless unit and if so, how would I do that?

  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    The output of the boiler coil is probably already plumbed with a tempering valve to keep output temps reasonable. (If it isn't, it SHOULD be- 180F water is dangerous!).

    Electric tankless heaters modulate to maintain a constant output temp, so there shouldn't be any issues with putting it in series with the boiler coil's output.

    But they DO take a whole lot of electrical current. Many/most retrofit installations of electric tankless units require a power service upgrade. Talk to your electrician first. It may be cheaper to install an indirect-fired tank off the boiler, which will be far more efficient than using the internal coil and more efficient than a standalone gas/oil fired HW heater. Proof: http://www.nora-oilheat.org/site20/u...ciencyTest.pdf

    In most markets an indirect-fired HW heater running at 50-70% summertime efficiency will be cheaper to operate than a 99% efficient electric tankless during the summer, and will improve the annual numbers for the heating system as well. (If it's a reverse-indirect like a TurboMax or ErgoMax plumbed as a buffer for the heating system you typically get double-digit fuel savings out of the boiler in heating mode too.)
    Last edited by Dana; 06-16-2009 at 08:12 AM.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    It will be less expensive to add an indirect WH to the boiler, and at least around here, that is also eligible for a rebate...maybe not as much, but your cost should be less. Electric tankless can have a HUGE impact on your required peak load, and a good indirect will match big loads better.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014


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