# Gas line sizing

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

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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"

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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.

4. ### Reach4Well-Known Member

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Yes, it would benefit.

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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. ### SylvanStill 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. ### wwhitneyWell-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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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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Yeah, alright...

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

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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. ### Reach4Well-Known Member

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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. ### WorthFloridaThe 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".