We are having a professional reroute a natural gas line. The existing black iron pipe is 1" and is in a wall we are removing. To re-route, we were advised by one contractor to increase pipe size then reduce back to make the connection to original pipe. Another contractor stated that increasing pipe size would reduce pressure when going back down to smaller pipe. The re-route will require four 90 degree elbows. Is upsize of pipe the best solution or will pressure go down in the larger pipe and not come back up when returning to smaller pipe.

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    Increasing the pipe size and then reducing it back to the original size sounds crazy. Commented Jun 2, 2021 at 12:57
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    Strictly speaking, the size of the pipe has no effect on the gas pressure. The issue you're concerned about is FLOW rate and ensuring that you don't restrict the flow and therefore pressure in the sections of the pipe that are downstream from the bends when gas is being consumed by your appliance(s). Any reputable professional will know how to properly size the pipe to handle the demand and should factor bends and length into their design.
    – jwh20
    Commented Jun 2, 2021 at 13:53
  • Check out cfd-online.com/Forums/main/…. Yes, elbows cause turbulence but I wouldn't consider 4 elbows to be detrimental.
    – MonkeyZeus
    Commented Jun 2, 2021 at 14:16
  • I have had to upsize from 1” to 1-1/4” for high flow devices like tankless water heater it was at the max flow from the 1” pipe and 3 90’s were causing some restriction it was only about 25’ and that was less expensive than trying to get a commercial account 2 psi in my area residential are ~3/4 psi so elbows can make a big difference the 1-1/4” pipe worked.
    – Ed Beal
    Commented Jun 2, 2021 at 16:31

2 Answers 2


Pressure loss (or drop) happens when there is flow. Straight pipe incurs some loss; elbows incur loss too. For a given flow rate larger pipe incurs less loss.

A gas delivery system is designed so that at peak demand, meaning when all appliances that can operate concurrently are operating, there is at least some minimum level of pressure at each appliance. In other words: pipe size is increased until calculations show gas arriving with adequate pressure pushing it into each appliance.

Realize that no residential gas system is truly engineered. They're all built with prescriptive tables and techniques like the "longest branch length" method. These quick-and-easy approaches bake in plenty of extra margin.

The contractor who says that pressure will reduce when going back down to smaller pipe... in a very technical sense he's correct, but in a practical sense he's wrong. He's not evaluating the system as a whole.

The contractor who suggested up-sizing the pipe is neither crazy nor taking advantage. He's ultra-conservative. He probably judged the extra cost of material for up-sizing to be much less than the extra labor cost of working up a calculation for your entire gas system. And he'd be right: the added material cost likely equals an hour or less of his hourly rate. By installing larger-than-original pipe he can easily argue that his new work results in less pressure loss than the pre-existing system had.

The reality is that your system is likely to have enough margin in it already so that adding four elbows is inconsequential. Up-sizing is totally fine too. Choose between the contractors based on some other factor.


Geeze! The upsize is ridiculous. The pressure is going to be the same throughout the piping system even if a section had larger piping than other sections. I think the contractor recommending the increase in size is trying to rip you off or doesn't know what he is doing. Just go with the 90s on the original pipe size, you'll be fine. Gas pressure in homes is usually less than 1 psi.

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    Pressure is only the same throughout the piping when there's no flow. The issue is with pipe friction and gas flow, and the flip side of "pressure is very low in gas pipes" is that "slight loss of pressure from gas flow can be a significant percentage of the very low pressure you're starting with."
    – Ecnerwal
    Commented Jun 2, 2021 at 14:13

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