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Thread: Tankless water heater selection for 3 bath home

  1. #1
    DIY Junior Member vestaviascott's Avatar
    Join Date
    Jul 2012
    Birmingham, Alabama

    Default Tankless water heater selection for 3 bath home

    We have a very small laundry room which currently houses our washer and dryer and 50 gallon natural gas water heater.

    We are planning for a remodel of this tiny room and want to include replacing the hot water tank with a tankless unit (mostly due to size considerations and wanting to get back that space).

    I'm looking for some suggestions on how to spec out the job and which tankless unit might be appopriate in our case.

    Two of our three bathrooms which receive daily use for showers. The third bath is a guest bath that's never used for showers and only light duty for hot water.

    The master bedroom/bath is at the extreme far end of the home from the water heater location. This is where most usage occurs so we want to insure that the unit can adequately supply hot water to this location which is appx 80-90 feet away.
    Last edited by vestaviascott; 07-23-2012 at 09:24 AM.

  2. #2
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Yakima WA


    Your biggest problem will the the gas supply. Tankless water heater need a much larger gas line than what is standard for a home. Large supply line and larger meter.

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    Any tankless with a 199KBTU/hr burner should be able to handle two simultaneous shower loads with enough margin for washer/dishwasher/etc draws unless your showers are the multi-side-spray-massive-flow varieties. Depending on how easy it is to route the venting it may be cheaper to go with a (more expensive) 0.95EF condensing version that can use (much cheaper) PVC vent pipe than an ~0.82EF unit with stainless venting.

    If you have a large furnace/boiler or pool heater you may or may not need to upgrade the meter/regulator to handle the extra load. A 199K burner is about 6x the size of the burner on a typical atmospheric-drafted tank heater, and 3x that of a "fast recovery" 50 gallon tank, and will likely be the largest burner on your gas distribution system (unless you have a big pool heater). A tankless needs a stable gas pressure even at high flow to work properly, so the routing of the gas plumbing works best if teed off fairly close to the regulator rather than sharing a long branch with other substantial loads like furnaces or pool heaters. Most tankless installations will require 1-1/4" gas plumbing if it's any distance away from the regulator, even though the gas connection to the unit itself is usually 3/4".

    It's a competitive marketplace, but some brands have better design and local & regional support/training than others. For most of the US, sticking with Rinnai, Noritz, or Takagi would be safe from a reliability & support point of view.

    The experience and training of the installer makes it or breaks it. Going with someone who has taken the factory course and/or who has 5+ years of tankless installations with dozens of installations under their belt is preferable than going with somebody on their first or third installation. Tankless water heaters are fully modulating sophisticated systems- as complex as a modulating-condensing boiler, and have more aspects that need to be right than tank heaters or on/off furnaces, etc. for getting optimal and reliable performance out of them. A common rookie mistake when replacing a tank with a tankless is to use the same small-bore gas plumbing that the tank unit used and ignoring the max-capacity of the regulator. It will often work-kinda most of the time but becomes a nuisance of flame-outs and error codes requiring manual reset, etc. Better pro installers will test the gas pressure dips under various modes of operation with a manometer, and perform full combustion-efficiency tests as part of commissioning the unit. You'll never have to call them back- it'll just work.

    If it's possible to install the tankless closer to the master bath rather than the laundry room would improve as-used efficiency and provide more satisfactory performance. The wait for hot water will be a couple seconds longer due to the ignition delay, but clearly the plumbing delay will still dominate when calling water from 80-90 feet away.

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