Pipe Sizing - Point Load Method (ie manifold)

[Brad.B.]

Hi All,

One school taught me (in G3) that to size the pipes or tubing going from the manifold to the individual loads, you use the correct table for the material/fuel/pressure, and that individual load, along with the length from that load to the gas meter, thus including the length it is travelling in the manifold.

The other school (currently for G2) is saying that the individual length is only from the load to the manifold - which makes more sense to me because I consider the manifold to be the "source" extended out from the meter, and the manifold is sized to deliver a maximum certain amount of gas from anywhere along its length.    And, the only time you include length of the manifold is during the sizing of the manifold using the LMR to the furthest load from the meter. 

How was everyone taught? 

[Porcupinepuffer]

Both schools I went to taught me the same thing you mentioned in paragraph 2. The other way pretty much involves all the other scenarios where T's are spliced in at various places along a main line... You can treat a manifold the same way you would a regulator. The manifold and main line leading up to it must be sized for the BTU's of everything that will be attached to it, and each appliance attached to it can be sized for it's btu's by its length from the manifold to the appliance.
It seems to me you're understanding it pretty well and shouldn't have any issues.

The first school I went to never really went into any great details and used the term "point load". It was only when I went to the other school for G2 I learned that term. And the manifold diagram for sizing gas piping was on the g3 exam I wrote.

Out in the field you won't find too many tech's sizing their branch runs based on the length from the gas meter itself. If that was the case, the 1/2" gas lines I see upwards of 100' from the meter would be grossly undersized.

Sometimes the theory doesn't quite make sense. Here's an example; lets say you got a main line that's oversized with an over abundance of available btu's and you want to add in a gas fireplace that's only 18,000 btu's. If you're only 10' away from that main line that's 1 1/4" or more, but your T location is 40' away from the meter on the 1 1/4". So an LMR of 50' is what you're supposed to go by in the book for your branch size... Do you really think that if you used 10' of 3/8 copper tubing that the fireplace is going to struggle supplying 18,000 btu's? If it was only 10' from the meter it can supply  35,000 btus... But according to the way we need to size it, we'd have to use 1/2" copper... But I can guarantee the pressure drop and functionality of gas to that fireplace will be fine.

It's crazy how many tech's downsize gas supplies when they're no longer using black iron. I've had some try and convince me to use sizes way too small on branch runs to appliances. I flat out refuse and get annoyed having to re-explain things they should know.

[Admin]

Normally if you have one main branch line and you're feeding multiple appliances you would use the distance from the gas meter to the furthest appliance for all the branch line sizing.  See this example,

http://www.hvactechgroup.com/hvacforum/index.php?topic=702.msg1843#msg1843

I agree when you have multiple main branch lines or a manifold you can size things differently.  If you had one main branch supply line 10' long and another main branch supply at 150' you wouldn't want to use 150' for both.

This video helps to explain,

https://youtu.be/ExNosdWfvnI

[Brad.B.]

Thanks all, wow that was fast replies.  I'm wrapping my head around B149.1 /8.2 and 8.3 tonight now.  A new chestnut question every day.