Gas line sizing

Discussion in 'HVAC Heating & Cooling' started by plumber69, Oct 20, 2020.

  1. plumber69

    plumber69 In the Trades

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    Prince Rupert, British Columbia
    I just ran 60 feet of 1 1/4 gas line to a heater because 1" was shy of btu. Then only to realize the connection on the unit was 1". Im confused on running 1 1/4 would even benefit if the final connection was 1"
     
  2. LLigetfa

    LLigetfa DIYer, not in the trades

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    Friction loss is calculated as size over distance. Larger pipe over a given distance will have less friction loss.

    EDIT: Volume moved (speed) is a third and important factor.
     
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  4. Reach4

    Reach4 Well-Known Member

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    Yes, it would benefit.
     
  5. Dana

    Dana In the trades

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    That's right. The line friction increases exponentially with velocity. So while that friction doesn't create a pressure drop big enough to matter for a short length of 3/4" or 1" (since near-nothing to the nth-power is still near nothing), but it matters quite a bit over a long run.

    Most 199,000 BTU/hr burner tankless water heaters have 3/4" connections, but is all but guaranteed to run into operational issues due to the pressure drop of trying to gulp gas at that rate through a 60' long 3/4" straw. The same burner would have no problems at all pulling gas through 55' 0f 1-1/4" pipe with the final five feet being only 3/4".
     
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  6. Sylvan

    Sylvan Still learning

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    What about fittings aren't they also considered for friction losses? What about materials use as Black steel would have more friction then plastic or copper does that also come into consideration?
     
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  7. wwhitney

    wwhitney Well-Known Member

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    For the record, that's a power law, not an exponential law. In an exponential law the variable is in the exponent.

    Cheers, Wayne
     
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  8. Dana

    Dana In the trades

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    Fittings are assigned an "equivalent length" in tables to account for the higher turbulence. Adding the linear lengths of the straight pipe to the equivalent length of the fittings is the number that needs to be looked at in the BTU/hr charts for gas (or pumping head charts for hydronic systems) , not just the straight runs.
     
    Last edited: Oct 21, 2020
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  9. Dana

    Dana In the trades

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    Yeah, alright...;)

    IIRC the exponent is velocity to the third power for simple gases, in which case doubling the velocity results 8x the friction.
     
  10. plumber69

    plumber69 In the Trades

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    So basically if it was all 1 inch, it would start burning up most the gas in the last say 20 feet of pipe.
     
  11. Reach4

    Reach4 Well-Known Member

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    Keep your day job. :p
     
  12. Dana

    Dana In the trades

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    Not burning up the gas, but dropping in pressure. For the burner to work correctly there has to be sufficient gas pressure even when flowing at the full BTU rate (pressure at the appliance. that is.)
     
  13. WorthFlorida

    WorthFlorida The wife is still training me.

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    You still need to consider the origin of the gas source. That is the pipe size to the meter, and meter size. A 1.25" pipe will not provide more gas than what can flow through the meter. All you did is reduce back pressure (loss of pressure) for the 1.25" pipe section and it may save the day but you need to get the pressuresure readings at the meter and at the gas appliance to sort it out. How did you come up with "shy of BTU"? You might be able to ask the gas company the pressure in inches of water at the meter and what the meter can provide. At the meter it may be around 6".
     
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