Tankless electric, Supply water temp

Discussion in 'Tankless Water Heater Forum' started by JaPe, Jul 7, 2019.

  1. JaPe

    JaPe New Member

    Joined:
    Jul 7, 2019
    Location:
    Albany, NY
    If the supply water temperature is say 40 degrees and I set the output to 120 degrees, what happens if the supply water temperature is raised to 80 degrees?

    In example Atmor heaters directly states in their ads the output temperature will be dependent on supply water temperature.

    I am not interested in having to adjust the dials to counter different supply temperature, if anyone knows a brand that uses sensors on the supply side to adjust accordingly please list.

    Thanks!

    Sorry, should have mentioned electric tankless
     
    Last edited: Jul 7, 2019
  2. Terry

    Terry Administrator Staff Member

    Joined:
    Aug 17, 2004
    Occupation:
    Plumber
    Location:
    Bothell, Washington
    Normally setting the output to 120 degrees and having different water temperatures passing through means that the gallons per minute changes but not the temperature.
     
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  4. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    In Albany NY you're not going to get satisfactory hot water performance out of any tankless electric water heater, unless it's for a low-flow sink or something.

    There is nothing to adjust on the tankless other than it's output temperature. As long as it has enough raw power to keep up with the flow at the input to output temperature rise it will deliver whatever you set it to, which in your case is 120F (a bit high for most purposes). Raising the set point temperature will not give it any more capacity to handle lower input temperatures if it's not keeping up. When the incoming temperature is 80F it still delivers 120F water, but would be able to handle higher flows, as Terry indicated.

    How much flow do you need?

    A typical low flow shower will run about 2 gpm, or 1000 lbs/hour of water. The desired temperature at the shower head is about 105F, give or take a couple. With 40F incoming water that would be a (105F - 40F =) 65F temperature rise, which is a heat rate of 65F x 1000lbs= 65,000 BTU/hr. Converting units to watts would be 65,000/3.412= 19,000 watts, just to run one low flow shower, with NO margin for other simultaneous uses. There are electric tankless units that big or even a bit bigger, but you don't really want it.

    That's 80 amps of 240VAC power, which takes wire fat enough to pull the F150 out of the ditch with, and adding a single circuit with that much capacity often requires upgrading your house panel (or even the service drop to your house.) While it's possible to get away with it in Florida or south Texas where the incoming water temps are much higher than Albany NY, it's really less than ideal even in those locations.
     
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