Gas line size for tankless water heater?

Discussion in 'Tankless Water Heater Forum' started by DIY_Steve, Feb 19, 2014.

  1. DIY_Steve

    DIY_Steve New Member

    Nov 26, 2012
    Portland, OR
    Hi All,
    I am getting a difference of opinion on my gas line size needed for a new tankless gas water heater.
    Model is a Navien NPE-210A
    At the location it will be installed I have a 3/4" line which comes off of a 1" line. Further down the 1" line is my furnace. On the 3/4" line is a gas dryer and gas cooktop.
    One quote is taking 1/2" line off the 3/4" to the new water heater.
    Other quote is to replace the 3/4" with a 1".
    So which contractor is getting this right?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    It could be that none are getting it right, but some are clearly ded-rong.

    That sucker takes 180,000 BTU in, and it's sensitive to pressure fluctuations from other devices turning on/off on the same branch. Ideally it would have it's own "home run" line teeing off very near the regulator/meter, and have sufficient BTU capacity for the length of the run (the length includes adding "equivalent length" of about 1.5 feet for every 90 degree fitting, whether tee or ell, along the path):


    A gas dryer & cooktop together probably add up to 50-70,000BTU/hr, the water heater is 180,000 so you need a minimum of ~250K of capacity on a line that's shared, and any tees off to the other load is best located within a couple feet of the meter/regulator.

    Half inch line won't cut it even at 2' of distance from the regulator, even though the connection to the unit is half-inch.

    There no way 3/4" will cut it, even as a non-shared home-run to the meter unless the hot water heater is VERY close to the regulator.

    Even 1" might not cut it if it's shared by other appliances and more than 20 equivalent feet of gas line from the regulator.

    Most installations call for 1-1/4" gas lines, but as a non-branched home-run it's often possible to get there with 1" line.

    The closest is the quote for 1", but measure it up yourself, and if it's anywhere close, have them re-quote it for 1-1/4" plumbed as a home run, so you don't have to worry about it mis-behaving when the furnace kicks on while you're in the shower.

    Also, look at the BTU ratings of the furnace, dryer, and cooktop. Add it all up, including the 180K for the Navien, and compare it to the BTU rating of the meter & regulator. If you have a ridiculously oversized furnace (very common) there's some chance you'd be over the capacity of the regulator, which can also lead to some mis-behaviors if every thing is running at once.
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  4. saphman

    saphman New Member

    Jan 18, 2012
    Northwest Florida
    I will Share my Experience with you on this issue. I Recently replaced my standard 40 Gal Nat Gas water heater with a Rheem Tankless 199,000 BTU Unit. Everything in my house is Nat Gas in terms of heating. We have (2) natural gas furnaces, (1) 6 burner gas stove, (1) gas grill, (1) gas standby whole house generator, (1) gas lamp, (1) gas dryer and now the new Rheem .

    Everything runs PERECT even when all are in use together. The reason? We have a 2lb gas system coming into our home and each appliance has its own regulator connected in series to their respective gas line. I used 3/4 " Flexible metallic gas line off the gas manifold as a dedicated run just to my Rheem. Length on that line is 75 feet with a regulator in series.
    I say all of this because Dana is right...If you have insufficient inlet pressure to that heater you are definitely going to have error codes and nothing but problems. Do some investigative research and know what type of pressure your gas system runs BEFORE you spend the time and money to go tankless. Call the gas utility and get the specs. JMHO
  5. DIY_Steve

    DIY_Steve New Member

    Nov 26, 2012
    Portland, OR
    Hi There,
    So I went back to the contractor and asked them some questions and if they were going to guarantee performance. This prompted a more detailed analysis and their plan is now to have the gas company put in a 2 psi meter instead of the current one. Then they would put a 2psi regulator at the first appliance (outdoor BBQ) and then where the line tees (all 1") they would put a regulator to the furnace (all on its own branch) and then to the new instant gas water heater (water heater, gas cooktop, and gas dryer). Are they getting it "right" now?
  6. lifespeed

    lifespeed Member

    Apr 27, 2007
    More likely, yes. But you don't include any details like distance from the meter on the 1" line, etc, so impossible to say.
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