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Gas line to tankless water heater

rjwatt616 Member Posts: 2
My tankless water heater needs up to 200k btu- current tap to main gas is 1/2".

My existing line is 1/2" and will need to run about 8' from my main. Looking at flow charts, that's obviously not going to supply 200k btu.

Can I put a reducer in reverse directly at the T and run 3/4" pipe the rest of the way?

I believe the 2 or 3" of 1/2" pipe should be able to flow 400-500k btu, and 3/4" pipe for 8' will easily do up to 200k. What are the problems with doing this? Is it acceptable to use a reducer in reverse?


  • rjwatt616
    rjwatt616 Member Posts: 2
    Main and meter can support this - I have a gas generator on opposite side of house from meter. It no longer functions and I needed it once in the last 15 years.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,696
    What other appliances on the gas line and what is the furthest run from the meter ?
    I have enough experience to know , that I dont know it all
  • Zman
    Zman Member Posts: 7,542
    The code says that the drop should be no more than .5"WC.
    If you look at the 1/2" tee, nipple and coupler as 3' equivalent, you end up with .42" WC just in that connection. https://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html

    When you put in the other pipes and fittings, you will certainly end up higher than code. That being said, whether or not it "works" will depend on how the rest is piped and how sensitive the appliance is.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein