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Branch or longest run method: where to start measuring?

lilfos
lilfos Member Posts: 8
edited January 24 in Plumbing
Hi everyone,

I want make sure my gas pipes can handle a new water heater and gas range I'm considering. I'm good on sizing methodologies, but I have what sounds like a no-brainer question at first. This post got kind of long, so I'll bold the main parts.

Where do you start measuring the length of the longest branch? Right at the meter outlet or at some other place, like where it enters the house? (meter is on the exterior).

I ask for two reasons: the pipe leaving the meter is 3/4" until it rises to a tee, then it's 1" through the wall and into the basement. Do I count that first section of 3/4" in the total length? If so, do I pretend it's 1"? I know this is a common situation, but I don't know how to handle it in the sizing equations.

The other thing I'm hung up on is this: It seems pretty common to branch gas pipes near the meter to allow smaller pipes throughout the building (lower demand per pipe), but doesn't the pipe before the branch still have to serve the full demand of the building? I don't usually see a large trunk that branches into two smaller pipes. In my anecdotal experience, the trunk is the same size as one or both branches (or smaller!). If two branches were needed to split the load, it seems to me that the trunk is undersized if it's not larger than either branch.

My calculations show that I'm right at the edge of the CFH and length that my 1" can handle. I'm actually over the max length if I factor in each elbow and tee. It's fine if I need to branch my pipes or upsize a portion of the run, but I can't quite nail down where to do that.

For example, should I upsize to 1 1/4" right after the initial 3/4" section after the meter? Or maybe right inside the basement wall?

Or, instead should I stay with 1" and branch it just after it enters the house?

I know that you'd need more details to do a proper sizing, but I'm just trying to make sure I'm not missing something basic about how to handle the first few feet of pipe after the meter. If it's explicit in the IFGC, I haven't found it.

Thank you!

Comments

  • GW
    GW Member Posts: 4,232
    Ahhh yes well how much energy do you have? Yes full size to the meter. And yes measure to the meter. Boring the hole for a bigger pipe is a small pain, just “jig” it. Someone got a little lazy with the initial 3/4”. In general I increase to the full size with nipple tray nipples (like 3 or 4”). I like “decreasing 90s”, but reducing couplings are fine too 

    Branch or furthest- take your pick 

    it’s not rocket science, you “may” be ok with the small piece of 3/4”. Design is one thing (sizing charts), pressure drop is another (check pressure drop when you light things off). One is theory, one is actual real life. The pressure in the street (if you’re in a low pressure neighborhood) kicks into this exact process a little bit, if you want to split hairs 

    Good luck
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Zman
    Zman Member Posts: 6,788
    As a double check, you can calc the pressure drop of each section and add them together. This will tell you exactly what your drop is over the initial 3/4" section.
    https://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,678
    If you use the branch method it keeps some branches smaller than using the longest length method

    You measure from the meter to the farthest appliance and size each section of pipe for what it carries dropping load off at each tee. Then go back and size the branches for what they carry measuring from the main to each appliance
  • mattmia2
    mattmia2 Member Posts: 3,668
    From an engineering standpoint you can figure out the pressure drop in the fittings and 3/4" nipple out of the meter at your total combined flow and subtract that from the pressure at the outlet of the meter at that flow and use that as your starting pressure but I'm not sure how you make that fit in to the dumbed down rules in the code.
  • lilfos
    lilfos Member Posts: 8
    edited January 25
    @GW, thanks for your "real world" perspective. I can read theory all day long in code books and how-tos, but nothing beats hearing from a voice of experience. Do you think the gas company can put a larger coupler on the outlet of my AC-250 meter, or is 3/4" all it can handle?

    I considered testing the actual pressure at the furnace, but I don't have the right fittings for my low pressure gauge, and my other gauge won't register anything below 1 PSI. Maybe I'll swing by the plumbing supplier for some brass.

    @Zman, @mattmia2, I think you're right that the engineering route is the way to go. I've submitted an inquiry with the gas company to find out if the 5.5 IWC stamped on the regulator is a reliable figure to use for my actual input pressure. If so, I could have a full inch of drop and still satisfy the farthest appliance.

    @Youngplumber, I read that somewhere, so it's good to hear it said again. On the flip side, a commercial designer told me he just doubles the longest length to account for fittings. That sounded way too conservative to me. The IFGC is surprisingly ambiguous (at least the parts I read). It says something like "a large number of fittings could reduce pressure." What is a "large number"? What do want us to do with "could"? Are they giving me an order or a suggestion? (that's a rhetorical question)

    @EBEBRATT-Ed, I see the flaw in my logic around trunks and branches. If the incoming pipe from the meter isn't large enough, then branching it just inside the basement wall doesn't help the situation. It only helps keep the downstream pipe sizes stay a little more economical ore easier to work with. My observation that at least one of the branches is usually the same size as the trunk section doesn't mean anything.

    Since I'm right at the limit for 1" pipe using longest length and branch methods, the truth is that I probably need 1 1/4" from the meter to the first branch to stay within the tables. I guess I should find out what the pressure drop formula tells me before I go down that road.

    Thanks all!
  • mattmia2
    mattmia2 Member Posts: 3,668
    edited January 25
    If you have the code tables with the footnotes, which are frequently omitted when they are copied in to the code, there is a footnote that the number includes x number of y type fitting.

    Is your gas supplied at 5.5" wc? Here it is supplied at 7" wc with 6.5" wc after the drop in the meter.
  • GW
    GW Member Posts: 4,232
    yes that's all hardware store stuff. I like the more 'rubbery' hose. I used a stiffer vinyl tube years ago and once it (manometer) popped off, my eyes bugged out a little (raw gas spewing near open flame).

    3/4"-- I would just increase to whatever size right out of the meter. At my shop we have a bigger meter, 1" outlet. It then jumps to 2" (no, I'm not using that much gas, I just had big plans back then). 250 is 250, if you surpass that you need a bigger meter.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • mattmia2
    mattmia2 Member Posts: 3,668
    You can also just get a few feet of clear tubing and a board and make a manometer out of it as well although the water freezing can be an issue this time of year.
  • ratio
    ratio Member Posts: 2,871
    The tables are ambiguous because they're mainly guessing about the pressure drop. Well, not mainly guessing, but they have no way to control the install, quality of material, etc. To be sure, a larger pipe will have less pressure drop than a smaller one with the same flow, but there are too many variables to do more than plug in 'this is the result we got the last time we tried this'.

    On top of that, many gas appliances will work on just about any kind of pressure they get. That water heater? Not gonna care if it gets 10" or 7½" or 5". Neither will the range, although you may notice something if the pressure went from higher to lower.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,678
    @lilfos
    A couple of small fittings off the meter shouldn't be an issue. I would blow it up to the correct size as soon as possible.

    I wouldn't use more than (2) 3/4 90s coming off the meter fewer if possible
  • mattmia2
    mattmia2 Member Posts: 3,668
    You can always use a 3/4 x 1 reducing street ell.
  • lilfos
    lilfos Member Posts: 8
    mattmia2 said:

    Is your gas supplied at 5.5" wc? Here it is supplied at 7" wc with 6.5" wc after the drop in the meter.

    I'll find out soon. I called the gas company, but the recording said everyone is working remotely and I should email them instead. An engineer told me it's probably a .75 PSI service in the street given its age, but then I don't know what to do with the 5.5 IWC on the regulator nameplate. It does sound low for an output pressure, but it doesn't really make sense as a pressure drop amount either. 0.75 PSI = 21" and 21-5.5 = 15.5 into the meter. Unless the meter drops it another 5.5, I'd be exceeding the max input pressure to the furnace.

  • lilfos
    lilfos Member Posts: 8
    GW said:


    3/4"-- I would just increase to whatever size right out of the meter. At my shop we have a bigger meter, 1" outlet. It then jumps to 2".

    This seems to be a pretty common thing, so I won't worry about it too much. I guess what folks are saying is that the pressure loss from this amount of pipe is probably within the margin of error built into the sizing tables. Accordingly, there's no easy way to deal with it using rules of thumb, but I could just think of it as a few extra ells' worth of equivalent length.

  • lilfos
    lilfos Member Posts: 8

    I think a large number of fittings would include the time a pipefitter told me he went around all of the joists with 90's becuse someone told him to stay tight to the ceiling. 

    Ha!
    ratio said:

    The tables are ambiguous because they're mainly guessing about the pressure drop. Well, not mainly guessing, but they have no way to control the install, quality of material, etc.

    Makes total sense. I bet questions like mine are the weeds they're trying to stay out of.
  • lilfos
    lilfos Member Posts: 8

    @lilfos
    A couple of small fittings off the meter shouldn't be an issue. I would blow it up to the correct size as soon as possible.

    I wouldn't use more than (2) 3/4 90s coming off the meter fewer if possible

    That wouldn't be difficult. The first elbow inside the house is about 30" from the meter, and all the pipe from there to the first branch is easily accessible. There is just one fitting between the meter and the first interior elbow. The 3/4" pipe rises straight up to a 1-3/4-1 tee before entering the house. The other end of that tee is plugged.
  • Zman
    Zman Member Posts: 6,788
    lilfos said:

    mattmia2 said:

    Is your gas supplied at 5.5" wc? Here it is supplied at 7" wc with 6.5" wc after the drop in the meter.

    I'll find out soon. I called the gas company, but the recording said everyone is working remotely and I should email them instead. An engineer told me it's probably a .75 PSI service in the street given its age, but then I don't know what to do with the 5.5 IWC on the regulator nameplate. It does sound low for an output pressure, but it doesn't really make sense as a pressure drop amount either. 0.75 PSI = 21" and 21-5.5 = 15.5 into the meter. Unless the meter drops it another 5.5, I'd be exceeding the max input pressure to the furnace.

    The 21" is to the meter. They have a regulator at the meter that steps it down to 7" to the house.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    mattmia2
  • lilfos
    lilfos Member Posts: 8
    edited January 26
    Zman said:


    The 21" is to the meter. They have a regulator at the meter that steps it down to 7" to the house.

    That's what I'm hoping the gas company will confirm. Maybe the "5.5 IWC" on the regulator is a red herring.
  • lilfos
    lilfos Member Posts: 8
    edited January 26
    mattmia2 said:

    You can also just get a few feet of clear tubing and a board and make a manometer out of it as well although the water freezing can be an issue this time of year.

    Ironically, I did this a few years back and JUST threw it away a few days ago. I made it to troubleshoot a furnace pressure switch that wasn't closing. Last week I came across it and though, why am I taking up space with this thing? :s
  • mattmia2
    mattmia2 Member Posts: 3,668
    I don't think the pressure drop in a given section of pipe is as ambiguous as implied here. The roughness of the inside of the pipe and the id of the pipe and fittings and the shape of the fittings is well known. The tables and equations may be worst case, but the math will accurately predict if a given system will have adequate pressure at the appliances. There are tables that will give equivalent lengths for just about every fitting imaginable so the pressure drop can be calculated with the formulas in the code if the components in the system are accurately documented.
  • Zman
    Zman Member Posts: 6,788
    lilfos said:

    Zman said:


    The 21" is to the meter. They have a regulator at the meter that steps it down to 7" to the house.

    That's what I'm hoping the gas company will confirm. Maybe the "5.5 IWC" on the regulator is a red herring.
    As long as the curb pressure is not too low, your regulator can be adjusted.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein