Water Supply Lines

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by slodog, Mar 4, 2008.

  1. slodog

    slodog New Member

    Messages:
    7
    Another dumb question. I am installing a tankless electric water heater. The old tank unit ( in a kitchen cabinet) has a 3/4 inlet but 1/2 copper pipe for the outlet line. I get plenty of hot water and great pressure with this set-up but my water lines are in concrete and I can only get access to 1/2 inch copper hot and cold pipes (clothes washer pipes) for the new tankless heater. My entire hot water run will be less than 10 feet. (very small house). will 1/2 inch copper pipe for the inlet and outlet be large enough or will I lose too much pressure? This is a Bosch Powerstar AE115 and Bosch support will not give a straight answer. All they say is check local codes. Can anyone help?
    Tony
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,023
    Location:
    New England
    The size of the pipe should be okay. The problem you may have is not because of the pipe, but the restrictor in the tankless heater. Try it and see. THe alternative is to abandon the pipe in the slab and run new through the walls or attic. I wouldn't do that until I tried it.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,647
    Location:
    Cave Creek, Arizona
    heater

    If your current setup is adequate then the new one should be also, but you do know that you are going to need immensely larger wiring for it than with the old heater, don't you? I have only seen one electric tankless installed, and they were extremely unhappy with it.
  4. slodog

    slodog New Member

    Messages:
    7
    water heater supply lines

    Thanks for the response. I have 150 amp service and my panel has plenty open space for the 2 double pole 40 amp breakers. My whole concern is the difference of gpm between the 2 pipe sizes but I guess ( I may be wrong) with the outlet line being 1/2 the smaller inlet size will not make a difference.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,023
    Location:
    New England
    Say that thing draws 60A (75% of 80A available), that equates to about 50K BTU. Depending on your incoming cold water, you may be very dissapointed with a shower in the winter...

    Gas-fired tankless systems are more often in the 120K BTU and over 200K is not uncommon. Except for hand washing, etc, or if you live somewhere where the incoming cold water is tepid, you may not get much hot flow for a high volume thing like filling the tub or washing machine (if using hot) and even a shower head may be marginal. You can run the numbers. It takes one BTU to raise one pound of water 1 degree; you've got about 14 BTU/second available.

    I had a moment and ran some numbers...at 2.5 gallons/minute for a shower, the thing should raise the temp of the water all of around 38-degrees. So, unless you have very warm water coming in, it may be much less than satisfactory. Hopefully, I've done the math right.
    Last edited: Mar 5, 2008
  6. slodog

    slodog New Member

    Messages:
    7

    I'm in St. petersburg, Florida. Around 75 to 78 degrees today. Not bad for the beginning of March. My cold water temp seems to average around 65 to 68 degrees from the tap. Tony
  7. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The 1/2" pipe will deliver more flow that the water heater will heat.

    The web site says it will give you 45 degree temperature rise at 2.6 GPM.

    I think you may eventually want to get a very low flow shower head.
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