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is it a problem connecting a larger gas pipe to a smaller gas pipe?

roverrover Member Posts: 2
I have a low pressure residential gas system with a half inch gas pipe out to my patio that I use for a natural gas BBQ.  Works fine.

I would like to add a T to the line and run a another 21 foot line off the BBQ line to use for a gas fire pit or a patio heater.  However I was given 3/4 inch pipe I would like to use for that line.  There are other reasons I would like to use that particular pipe.

I assume connecting larger pipe to smaller pipe results in a decrease in velocity and pressure, but no change in volume.  I am also assuming that is really not a problem.

Right/wrong/maybe?  Any suggestions or thoughts?




  • Plumdog_2Plumdog_2 Member Posts: 873
    The correct answer is "Wrong"

    A gas distribution system must supply adequate pressure (hence volume) for each appliance connected. Therefore it starts out larger and reduces as it branches off. Get a copy of the Gas Pipe Sizing charts from the Code Body authority in your area. Don't get caught installing gas pipe without a permit.
  • roverrover Member Posts: 2
    Same question

    Thank you but it does not really answer my question.  Let me rephrase.

    If one connects a larger gas pipe to a smaller gas pipe will the gas in the larger pipe have the same pressure and volume as the smaller?  Or will it have the same volume but a lower pressure?

  • Paul48Paul48 Member Posts: 4,492

    It will have larger volume, with the same STATIC pressure. Which means absolutely nothing.
  • jonny88jonny88 Member Posts: 1,139
    good ?

    it has the same static pressure when i see a 2" meter set which increases in size to a 3" line what the 3" line in effect reduced to the capacity of the 2 " line feeding it.would this be a correct example?if i drained a 75 gallon water heater through a 3/4 drain line and then increased the drain line to 2" the heater wont drain any quicker.Would a gas line work on the same principle.
  • ZmanZman Member Posts: 6,312
    Pressure Drop

    Gas line should be sized based on the allowable pressure drop in your system. I think the standard chart allows 1/2" total drop.

    As previously posted you start with bigger pipes and reduce the size as the load decreases.

    To do your system right, you need to map out what size and length pipe you have and compare that to the ratings of the attached appliances. It may be that you can easily add the additional heater using 1/2" line and have no problems. If you don't do the math you will likely end up with appliances malfunctioning due to pressure fluctuations.

    As far as increasing the the pipe size, it won't hurt anything. It doesn't make the job look very professional. Is there a reason for not hiring a pro and doing it right?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Paul48Paul48 Member Posts: 4,492

    I misused the term STATIC, as it does not apply to gas. I used it meaning pressure in its unused state. Which is why I stated it meant absolutely nothing as it related to pipe size.
  • heatpro02920heatpro02920 Member Posts: 991
    OK there is a THICK line between rite and wrong.

    You can pipe it how ever you want, its your home, you probably aren't pulling a permit or getting it inspected and even with out seeing the btu ratings and your supply specifications I would guess it will work... After all that is more important than almost all of the other factors that make up a project, second to only safety...

    Now, if it were me....

    You got the 3/4" pipe for free, so use it, but not how you think... Instead run that to the patio, install a 3/4x1/2"x1/2" tee and use up the 1/2" pipe that you have left over to go to the fire pit and the bbq... SURE it will be a more work, BUT it will be more on the side of "Rite"... And this is your house, but more importantly a job you are performing so take pride in it and do it rite.....

    BUT like I said, that is if it were me...

    What are the BTU ratings and overall lengths for each appliance?
  • jonny88jonny88 Member Posts: 1,139
    larger pipes?

    just a follow up to the original thread.If I see a 1" meter bar with a six inch nipple with a 1x11/4 90 on it .rest of run is in 11/ effect is the whole line restricted to 1 inch.This is in the city so there is no regulators on meter,low pressure gas coming in from street.
  • ZmanZman Member Posts: 6,312

    Sort of,

    The system will have the restriction of 1" but only over the 6" of pipe. If you look on the chart, 6" of 1" pipe can carry lots of gas. If you were to restrict it for 60' that would be a different story

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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