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3/4 gas line reduced at T to 1/2?

pwspws Posts: 1Member
I have 3/4 gas line that branches with a T (3/4x3/4x1/2 to the appliance). I am installing a tankless water heater that specifies a 3/4 in line. Can I use a 1/2 to 3/4 adapter at the T to increase the line size or must I replace the T to 3/4x3/4x3/4? Thanks


  • adambnycadambnyc Posts: 252Member
    What are are the BTUs of the tankless? I’m almost positive that your existing line isn’t going to be able to handle it.
  • j aj a Posts: 1,795Member
    What size incoming line do you have?
    What size meter do you have?
    What is the total load of all your appliances?
    Are you high or low pressure ?

    Reducing and increasing a gas line is basically a no no..due your homework prior to moving forward...
  • adambnycadambnyc Posts: 252Member
    And lastly, you need to pull a permit for this work. The town must inspect and approve. If you’ve never worked with Gas pipe before you might want to get some help. New piping should be pressure tested to ensure there are no leaks. 15 PSI for 4 hours is the rule in my town in north Jersey. The inspector will want to see this test.

    I self installed my own NG tankless. I needed to get my entire gas line to my house upgraded. With the load of all my appliances and the new tankless, I needed. 1.5” gas main that branches off to 3/4” and also 1/2” depending on distance and demand. I had to repipe every gas pipe in my house and hand cut and thread all the pipe.
  • ratioratio Posts: 1,563Member
    Tankless water heaters use anywhere from Lots to ICANTBELIEVEHOWMUCH gas depending on the size of the unit. Most existing gas piping, unless it was planned for, will be undersized for correct operation. Symptoms range from unhappy customers to mad customers—The Wall is full of their stories.

    There's no shortcut, you must do the load calcs & size the gas line. Use the old tables, they're sized for ½" drop in 100', you'll be so much happier than if you use the greatest-pressure-drop method. (<- That's just me being snarky, but you can't go wrong being generous on the sizing.)

  • LeonardLeonard Posts: 421Member
    edited January 9
    As said, There's more to it than just looking at the Tee. Need to look at all of gas pipe lines and loads. If have long lines likely have to upsize from 3/4 inch. Goal is to have min acceptable pressure at appliance as it's drawing gas , other wise you'll have problems.

    As said, a tankless water heater draws HUGE BTUs/hr of gas, and pipeing needs to be able to handle it , WHILE other gas loads are on. ( same thing happens when adding back up electric generator engines , ~7kw gen takes ~ 130k BTU/hr, that's about size of many larger house furnaces))
  • FredmFredm Posts: 4Member
    See, the point in time consumption and the service costs are two items that the sellers of these things never talk about.
  • j aj a Posts: 1,795Member
    Fredm said:

    See, the point in time consumption and the service costs are two items that the sellers of these things never talk about.

    The manufacturers do specify the minimum gas pressure, it’s the installers that don’t pay attention...sell sell sell. Jmo
  • ratioratio Posts: 1,563Member
    I think that often it's worse that that—they don't even know that they should be paying attention. And then someone has to go back to the poor customer and tell them that they're going to have to spend as much on gas piping upgrades to make it work as they spent on the original install. :(
  • j aj a Posts: 1,795Member
    And to take it a step further the gas company only guarantees about 4 .5 inch’s in low pressure areas up here in Boston now you have to tell them it won’t never work as advertised...then they think your full of it...I certainly don’t miss that part of the all could be averted with one simple call to the engineering department of national grid...they don’t want issues either
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